Sedimentation of Lower Toarcian (Lower Jurassic) brackish deposits from the Częstochowa-Wieluń region (SW Poland)

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1 Acta Geologica Polonica, Vol. 61 (2011), No. 2, pp Sedimentation of Lower Toarcian (Lower Jurassic) brackish deposits from the Częstochowa-Wieluń region (SW Poland) PAULINA LEONOWICZ Institute of Geology, University of Warsaw, Al. Żwirki i Wigury 93, PL Warszawa, Poland. Paulina.Leonowicz@uw.edu.pl ABSTRACT: Leonowicz, P Sedimentation of Lower Toarcian (Lower Jurassic) brackish deposits from the Częstochowa- Wieluń region (SW Poland). Acta Geologica Polonica, 61 (2), Warszawa. The Lower Toarcian Ciechocinek Formation from the Częstochowa-Wieluń region is composed of poorly consolidated mudstones, claystones and siltstones with subordinate sandstone intercalations. The succession is represented by twelve lithofacies grouped into seven facies associations of a shallow brackish marine basin with an embayment character. The sedimentation was dominated by clay and silt deposition from low-density suspension flows, spreading out from river mouths, and from suspension clouds whirled up during storms. Sand intercalations mark episodes of storms, removing coarser material from nearshore to more distal settings, migration of elongated sand bars, as well as sedimentation at river mouths and on sandy shoals. The deposition was affected by three main factors: (1) progradation and displacement of river mouths; (2) river discharge fluctuations; and (3) short-term weather changes. Key words: Fine-grained clastic sedimentation; Brackish marine environment; Ciechocinek Formation; Lower Jurassic; Silesian-Cracow Upland. INTRODUCTION The Early Jurassic time in north, west and central Europe is characterized by clastic and calcareous deposition in an extensive epicontinental sea the Central European Basin System (CEBS Pieńkowski, Schudack et al. 2008). The succession records several transgressive-regressive cyclothems (Graciansky et al. 1998), with the maximum transgression noted during the late Early Toarcian (Hesselbo and Jenkyns 1998), characterized by widespread black shales deposition in Western Europe (Jenkyns 1988). The area of Poland (Polish Basin) was situated in a marginal part of the CEBS (Text-fig. 1A), in which the Lower Jurassic succession was deposited mostly in terrestrial and restricted nearshore environments, while the open marine and brackish sedimentation of main transgressive peaks was restricted to an elongated central zone (Polish Trough) and adjacent areas (Karaszewski 1962; Dadlez and Kopik 1975; Deczkowski 1997; Pieńkowski 2004). The maximum transgression in the Early Toarcian (Text-fig. 1B) is recorded by deposition of the mud-silt succession that is widespread throughout the Polish Basin and referred to as the Ciechocinek Formation (Pieńkowski 2004). The main purpose of this paper is a detailed threedimensional reconstruction of the facies architecture of the Ciechocinek Formation in the Silesian-Cracow Upland (SW part of the Polish Basin), where the Lower Jurassic succession crops out or is covered by only a thin layer of younger deposits. The analysis is based on detailed field examination and studies of archive material and includes reconstruction of depo-

2 216 PAULINA LEONOWICZ Text-fig. 1. Lower Jurassic in Poland. A Polish Basin as a part of the Central European Basin System in the Early Jurassic time (after Pieńkowski 2004), B Polish Basin during deposition of the Ciechocinek Formation and the present distribution of Lower Jurassic deposits (after Dadlez 1973, Pieńkowski 2004, simplified) sitional processes, water dynamics and lithofacies distribution. The deposits included in the Ciechocinek Formation (Różycki 1958; Pieńkowski 2004) were mentioned in a number of papers, albeit in most cases only as a part of general lithological descriptions and studies dealing with the entire basin evolution (Dadlez and Kopik 1975; Deczkowski and Franczyk 1988; Dadlez 1987, 1989; Deczkowski 1997). Some papers, in which the Ciechocinek Formation is mentioned (Różycki 1958; Dadlez 1958, 1964a, b, 1969; Feldman-Olszewska 1997; Pieńkowski 1997, 2004), described Jurassic transgressive-regressive cycles and proposed correlations of depositional sequence. The sedimentology of the Ciechocinek Formation from the Silesian Cracow Upland was studied by Pieńkowski (1988, 1997, 2004), while Leonowicz (2009) presented an analysis of its trace fossils. GEOLOGICAL SETTING The study area lies in the northern part of Silesian Cracow Upland, between the towns of Częstochowa and Wieluń (Text-fig. 2). Structurally it consists of two units: (1) the Silesian-Cracow Monocline built of Permian and Mesozoic deposits, which dip gently towards the NE; and (2) underlying them, strongly folded and faulted Palaeozoic rocks (Bukowy and Wielgomas 1981). The monocline formed during the Laramian inversion of the Polish Trough. During the Cenozoic it underwent substantial erosion, so the present-day western range of Jurassic deposits is of an erosional character. The Permian and the Mesozoic rocks of the study area were deposited in the marginal part of the epicontinental Polish Basin, which formed subsequent to the Variscan orogenesis and represents the easternmost arm of the Central European Basin System (Dadlez 1989). In the Early Jurassic the Polish Basin was surrounded from the north, east and south by land (Text-fig. 1), which served as the source area of its sediments (Teofilak 1966). The thickest and most complete Lower Jurassic succession, exceeding 1300 m (Feldman-Olszewska 1997), occurs in the Polish Trough, the NW SE oriented axial zone of the Polish Basin (Text-fig. 1B). Towards the NE and SW, the thickness and completeness of the succession diminishes gradually and terrestrial deposits predominate. Marine deposition in these areas prevailed only during the strongest transgressive events. The maximum thickness of the Lower Jurassic in the Silesian-Cracow region does not exceed 200 m (Deczkowski 1997).

3 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 217 Text-fig. 2. Geological sketch-map of the Częstochowa-Wieluń region and location of the sections studied Lithology and stratigraphy The Lower Jurassic deposits of terrestrial and restricted, nearshore environments are poor in biostratigraphically indicative fauna and consequently no detailed biozonal scheme has been established so far. For this reason, the recent lithostratigraphic division by Pieńkowski (2004) is applied herein. The studied succession (Text-fig. 3) starts with the Pliensbachian Blanowice Formation, which rests with a large stratigraphic hiatus on Upper Triassic strata (Pieńkowski 1997, 2004). It is possible that some terrestrial sediments were deposited here during Hettangian Sinemurian times but were subsequently removed during the Late Sinemurian (Pieńkowski 1997, 2004; Kopik 1998). The Blanowice Formation is composed of sand-mud and sand-gravel deposits of alluvial/floodplain and deltaic/nearshore environments and characterized by a great lateral variability. The upper part of the formation is dominated by alluvial and lacustrine/backswamp sand and coal-bearing sediments (Pieńkowski 1997, 2004). The Blanowice Formation is overlain by the Ciechocinek Formation, composed of a mud-silt succession, consisting of olive-green, grey and willowgreen muds, clays and silts and of poorly consolidated mudstones, claystones and siltstones with lenses and intercalations of fine- and very fine-grained sands and sandstones. These deposits often contain layers and lenses of siderite mudstones and siltstones, as well as siderite and pyrite concretions. Fine plant debris is also common, usually concentrated together with micas on parting and lamination planes. Larger wood fragments, up to 1 m long, can also be found. The maximum thickness of the Ciechocinek Formation in the Częstochowa- Wieluń region is estimated herein to be about 70 m (usually less than 50 m). The succession represents a brackish marine environment of a large, shallow embayment (in the Silesian Cracow region about 15 m deep), with widespread deltaic facies developed in its marginal parts (Pieńkowski 2004). Its brackish character is confirmed by generally impoverished associations of body and trace fossils as well as by its geochemical characteristics, based on iron compounds and boron content (Pieńkowski 2004; Leonowicz 2007). The fauna is represented by conchostraca, rare foraminifers and scarce ostracods, accompanied by occasional undetermined bivalves, gastropods and fish teeth (Kopik and Marcinkiewicz 1997; Kopik 1998). Trace fossils are rel-

4 218 PAULINA LEONOWICZ Text-fig. 3. Lithostratigraphy of the Lower Jurassic in the Cracow- Silesian region (after Kopik 1998, Pieńkowski 2004, Deczkowski and Daniec 1981) and representative section of the Wręczyca 3 borehole. Missing part of the core supplemented after Pieńkowski (2004) atively common, albeit not diverse, and include Planolites, Palaeophycus, Helminthopsis, Gyrochorte, Diplocraterion, Protovirgularia, and Spongeliomorpha, as well as some undetermined grazing, feeding, crawling, dwelling and resting traces (Pieńkowski 1988, 2004; Leonowicz 2009). The occasional appearance of Diplocraterion indicates periodically elevated salinity (Pieńkowski 1997, 2004). Barski and Leonowicz (2002) additionally reported finds of marine dinoflagellate. Based on dinoflagellate cysts, the Ciechocinek Formation of the Silesian-Cracow region is assigned to the late Pliensbachian early Toarcian (margaritatus tenuicostatum zones; Barski and Leonowicz 2002). Dadlez (1958, 1969) and Pieńkowski (1997, 2004) linked the Ciechocinek Formation with the Early Toarcian transgression that is clearly recognized throughout the whole Central European Basin System. Based on megaspore analysis (Marcinkiewicz 1957, 1960, 1964, 1971) and sequence stratigraphical correlation, Pieńkowski (1997, 2004; Hesselbo and Pieńkowski 2011) located the formation within his sequence VIII (subdivided into 6 parasequences), which corresponds to the Pl 8 To 3 sequences of Graciansky et al. (1998) and the 3 rd -order cycle 4.3 of Haq et al. (1987). According to Pieńkowski (2004), the maximum flooding of this transgression occurred in Poland in the VIIIb and VIIIc parasequences, which are correlated with the tenuicostatum and falciferum biochronozones. Recently, Hesselbo and Pieńkowski (2011) presented a study on the carbon isotope chemostratigraphy of the Upper Pliensbachian/Lower Toarcian strata in Poland and Western Europe. According to them the Ciechocinek Formation is entirely of Lower Toarcian age (tenuicostatum falciferum biochronozones) and can be subdivided into five correlatable chemostratigraphic units, which are linked with climate changes, forced by orbital eccentricity. Hesselbo and Pieńkowski (2011) also correlated shifts of carbon isotope curves with shifts in the position of the shoreline. Pieńkowski (2004) defined the lower boundary of the Ciechocinek Formation as a transgressive surface and the upper as an erosional one. Both formation limits coincide with sequence boundaries. The most characteristic features of the Ciechocinek Formation, used in the present study for its identification in outcrops and short core lengths, where the boundaries themselves cannot be observed, are: (1) predominance of mud-silt character, (2) prevalence of greenish colour, (3) occurrence of trace fossils, and (4) the presence of siderite and pyrite mineralization. The Ciechocinek Formation is overlain by mostly alluvial sandy and subordinately muddy deposits of the Borucice Formation (Pieńkowski 2004). The Borucice Formation completes the Lower Jurassic succession in the Silesian-Cracow Upland and is covered by Middle Jurassic marine deposits. MATERIAL AND METHODS Field studies included examination of the successions exposed in two clay-pits of the Cerpol Kozłowice Enterprise in Kozłowice and the Boroszów Brickyard in Boroszów, as well as of 14 fully cored

5 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 219 boreholes from the Lubliniec-Wieruszów area (Text-fig. 2), drilled by the Polish Geological Institute. Deposits in exposed sections and in borehole cores were carefully examined layer by layer taking into account lithology and sedimentary structures. The approximate grain size of fine-grained deposits was determined using the field criteria of Lundegard and Samuels (1980). The lithofacies successions were logged on a 1:10 scale (for exposures) and on a 1:200 scale (for borehole cores) and than generalized. Palaeotransport directions were measured based on cross-bedding and laminations, as well as on the orientation of asymmetric ripple crests, and presented on rose diagrams. In order to recognize the microfabrics of muds and silts, 20 thin sections were prepared from consolidated siderite mudstones and siltstones and examined under the microscope. In the case of incompletely preserved cores, references were made to the detailed descriptions of Kieżel (1990) and Pieńkowski (2004). Additionally, data from about 150 archive boreholes from the Częstochowa-Wieluń region (Text-figs 2, 13), stored in the Central Geological Archive of the Polish Geological Institute in Warsaw, were taken into account in the preparation of cross-sections and in the interpretation of the spatial arrangements of the lithofacies. In lithofacies descriptions, small-, medium- and large-scale sedimentary structures have been distinguished, with the following boundary thicknesses: 6 cm for small/medium and 30 cm for medium/large structures. The terminology of stratification units is after Reineck and Singh (1980). LITHOFACIES DESCRIPTION AND INTERPRE- TATION Twelve lithofacies have been distinguished (Tables 1, 2). These are grouped into seven facies associations, representing different sedimentary conditions. Association 1 fine-grained background sedimentation of distal type This association is represented by massive, laminated and parallel-bedded muds and silts of lithofacies M m, M s and M p 1; Table 1; Text-figs 4, 5), forming packets up to 26 m thick. Trace fossils are relatively common and include mainly Planolites, undeterminable grazing traces and occasional Helminthopsis. The homogeneous texture of some massive beds possibly resulted from complete bioturbation. Deposits of association 1 originated as a result of continuous background sedimentation from suspension, delivered by rivers from neighbouring land, which was interrupted by short episodes of coarser sediment supply. The background sedimentation is marked by massive deposits, which reveal in thin section parallel alignment of plant detritus and mica flakes, as well as faint horizontal lamination of lithofacies M m and M s, visible due to differences in colour. The horizontal lamination reflects subtle changes in component contents and is a common feature of sediments marking slow settling from suspension (O Brien 1996). A similar origin is postulated here for the thicker, regularly alternating mud and silt layers with distinct boundaries of lithofacies M p. These, however, reflect longer intervals with fluctuations of sediment supply, and possibly a greater supply of sediment. The contents and the clay/silt ratio depend mainly on the location of the depositional setting relative to the river mouth and on the variations in river discharge. Other deposits interpreted as a result of slow, low-energy sediment accumulation include strongly bioturbated, homogeneous muds and silts (compare with Schieber 1999), and muds with thin, non-graded silt streaks. The latter resemble the even silt laminae of Schieber (1990) and are interpreted as a result of deposition from suspension clouds (see also O Brien 1996). Event deposits included in this association show very subtle sedimentary structures and it is usually difficult to distinguish them from background deposits in exposures. They include: (1) thin, graded silt-sand streaks of lithofacies M m and M s, (2) sequences: trough cross-laminated silt parallel-laminated silt massive mud of lithofacies M s, and (3) sand-silt lenses and intercalations of lithofacies M p. Thin silt-sand streaks with normal grading and sharp bases are interpreted as deposits of fine-grained suspension flows fed by river floods, and as distal storm layers re-suspended in shallow zones and transported offshore by storm-induced gradient currents (compare with the thin silt-mud couplets of Schieber (1990), the silt-streaked muds of de Raaf et al. (1977) and the D, E, F laminae types of Pedersen (1985). The records of these two processes are very similar and indistinguishable in the deposits studied. Reverse grading occurring in some silt streaks could have resulted either from intensification of the depositional event (flood or storm) or from current reworking of the uppermost part of the bottom sediment (Hiscott 2003; see also Schieber 1994). A similar origin as for silt-sand streaks, but with higher intensity of depositional processes, is postulated here for sequences: trough cross-laminated silt parallel-laminated silt massive mud, resembling the fine-grained turbidites of Stow and Shanmugam (1980), the muddy turbidity currents of Lash (1987) and the thick silt-mud couplets of Schieber (1990). Such sequences can mark deposition

6 220 PAULINA LEONOWICZ Lithofacies Description Interpretation Massive and laminated muds (M m ) (Text-fig. 4) Massive and laminated silts M s ) (Text-fig. 5A-D) Parallel-bedded muds and silts (M p ) (Text-fig. 5E-F) Lenticular-bedded muds and silts (M l ) (Text-fig. 6) Interlaid mud-silt-sand heteroliths (M i ) (Text-fig. 7A-C, F) Wavy-bedded muds and silts (M w ) (Text-fig. 7D, E) In outcrop and core, structureless mud (-stone) and clay (-stone) with subtle horizontal lamination seen in places. Laminae up to 2 mm thick, visible due to the different colour and enrichment in muscovite, plant detritus or coarser quartz grains. In thin section, homogenous texture as well as horizontal lamination arising from the parallel alignment of mica flakes and plant detritus or the presence of thin, up to 2 mm thick, quartz silt streaks. Silt streaks show normal grading and sharp, undulating bases or reverse grading and indistinct bases and sharp tops. Streaks without grain-size gradation also present In outcrop and core, structureless and horizontal-laminated clayey silt (-stone). Laminae up to 1 mm thick, perceptible due to the colour differences and enrichment in muscovite, plant detritus or very fine sand. Rarely sequences with trough cross-lamination passing upwards into horizontal lamination and massive mud occur in 2-3 cm thick layers. In thin sections, homogenous texture and horizontal lamination, developed similarly as in massive and laminated muds (M m ) Alternating layers of mud/clay (-stone) and silt (-stone), 1 mm to 5 cm thick, forming horizontally bedded packets which are up to 1 m thick. Intercalations of sandy silt and isolated, up to 5 mm thick, lenses of silty sand present. Silts and silty sands are massive, horizontal or occasionally cross-laminated, have sharp bases and sharp or gradational tops. Sequences: sand-silt-mud or sand-silt-sand common but irregularly distributed in the section. Mud/clay (-stone) and silt (-stone) with lenses and streaks of very fine sand. Mud/sand ratio varying from isolated streaks and flat sand lenses to thick, abundant, sometimes connected sand ripples. Lenses are up to 1 cm thick, usually asymmetric and often cross-laminated; laterally pass into streaks, up to 3 mm thick and several centimeters long. Bases and tops of lenses and streaks distinct, lenses often loaded. Levels of connected ripples display undulatory and lingoid crests of asymmetric current ripples, indicating variable transport directions. On the lower surfaces groove marks present. Mud/clay (-stone) and silt (-stone) with thin silt (-stone) and very fine sand (-stone) intercalations and abundant sand lenses and streaks. Sand streaks up to 1 mm thick with sharp bases and tops; sand lenses up to 2.5 mm thick, displaying cross-lamination. Asymmetric and symmetric, current and wave ripple crests seen on exposed upper surfaces of connected lenses. Sand layers up to 3 mm thick with sharp bases and normal or inverse grading at the top, sometimes cross-laminated. Silt intercalations with horizontal lamination up to 5 mm thick, tops sharp or gradational. Grain size gradation typical for wave reworking, restricted to the upper part of the layer, the base of overlying bed sharp. Coarsening or fining upward sequences: sand-silt-mud or silt-mud common. Mud (-stone) and silt (-stone), often enriched in sand, with up to 5 mm thick intercalations of very fine sand and sandy silt, displaying lenticular and wavy bedding. Sandsilt lenses symmetric or asymmetric, faintly cross-laminated; sand layers massive or cross-laminated. Asymmetric wave ripples with asymmetry direction changing within one layer observed. Thin sand streaks with sharp bases and tops common. Mud/sand ratio varying from muds with thin, isolated streaks of sandy silt to sand-mud deposits with approximately equal content. Continuous background sedimentation from suspension interrupted by episodes of more abundant silt supply. Silt streaks deposited from clouds of suspension and finegrained, low-density suspension currents, moving near the bottom or in the water column. Bottom currents often scoured the sea bed. Deposition far from the source area, in quiet water - below the storm wave base and above it, during periods of calm water. Continuous background sedimentation from suspension interrupted by episodes of more abundant silt and sand supply. Event deposition from fine-grained, low-density suspension currents and near-bottom muddy density flows often scouring the sea bed. Deposition in quiet water, below and above the storm wave base, closer to the source area than M m lithofacies. Higher intensity of derivation processes also possible. Continuous background sedimentation from suspension with regular variations in sediment supplied, interrupted by episodes of more abundant silt and sand supply. Event deposits transported by fine-grained, low-density suspension currents and occasionally by traction currents, scouring and reworking the sea bed. Deposition in quiet water, below and above the storm wave base, in similar situation as M s lithofacies but affected by regular changes in sediment supplied. Migration of starved ripples on the cohesional mud-silt bottom. Continuous background sedimentation from suspension interrupted by episodes of fine sand supply; sand transported by traction currents of variable directions. Deposition at different depths, below and above the storm wave base, still far from the source area but closer than lithofacies Mm, M s and M p. Gravitational settlement from suspension flows, moving near the sea-bottom; these scoured and modeled the bottom surface. After deposition sediment displaced by traction currents and/or reworked by waves. Short depositional episodes in quiet environment, above and maybe below the storm wave base, far from the source area. Sedimentation from suspension and complete reworking of bottom sediment by the wave action. Deposition in shallow settings, above the fair weather wave base, in similar distance from the source area as lithofacies M l. Table 1. Mud-silt lithofacies identified in the study

7 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 221 Lithofacies Description Interpretation Wavy-bedded sands (Sw) (Text-fig. 8, 9) Parallel and cross-bedded sand lenses (Sl) (Text-fig. 10) Parallel-laminated sands (Sp) (Text-fig. 11A, B) Flaser-laminated sands (Sf) (Text-fig. 11C, D) Cross-bedded sands (Sc) (Text-fig. 12A-C) Small-ripple cross-bedded sands (Sr) (Text-fig. 12D-F) Fine sand (-stone) with wavy intercalations of mud and silt. Sand layers are 1-6 cm thick, flaser-, cross-, parallel- and trough cross-laminated, sometimes massive, displaying morphology of ripple trains. Occasionally hummocky cross-stratification was observed. Ripples symmetric and asymmetric, with undulatory, sometimes bifurcated crests; variations of asymmetry direction within particular ripple trains observed. Internal structure indicative of wave origin (Text-fig. 9); small mud intraclasts common; groove marks occasional. The sand/mud ratio varies from muds with single wavy sand layers to sands with thin intercalations of mud. Fine or medium sand (-stone) lenses, up to 3 m wide and 75 cm thick, with flat parallel and cross-bedding; in the upper part small ripple cross-bedding sometimes occur. Boundaries of bedding sets erosional; original bedding sometimes strongly deformed. Upper surfaces of lenses flat or slightly concave, with undulatory and lingoid crests of asymmetric current ripples; lower surfaces concave, with different load casts. Wood fragments and muddy intraclasts common. Fine sand (-stone) beds, few centimetres to few decimetres thick, with horizontal lamination. Lamination perceptible due to the different coherence of laminae or presence of faint clay-organic-mica streaks (type 1); sometimes consists of distinct, up to 1 mm thick, organic-mica laminae (type 2). Fine sand (-stone) beds, cm thick, with subtle flaser-lamination; undulating upper boundaries display outlines of symmetric and rarely asymmetric ripple trains. In the top of bed, small-ripple cross-bedding with variable directions of laminae set inclination occurs sporadically. Flasers and single wavy mud intercalations sometimes present. Fine plant debris and wood fragments arranged parallel or slightly oblique to the bedding common Fine sand (-stone) forming decimetres thick single beds with planar cross-bedding or packets over 2 m thick with tabular and wedge-shaped sets of planar cross-bedding. Foreset laminae form a sharp angle at the base of lee slope. Set boundaries are flat or undulating, the latter displaying outlines of ripple trains; usually covered by thin lamina of mud. Fine sand (-stone) with preserved trains of small ripples; up to few decimetres thick sets composed of 1-5 cm thick layers with undulating tops. Ripples symmetric and asymmetric, showing constant or variable asymmetry direction within particular trains. Cross-lamination unidirectional within particular layers. Periods of sand derivation alternating with mud and silt sedimentation from suspension; sand transported by bottom currents and in the suspension. Sand transportation accompanied by erosion of muddy bottom; deposition on the inclined or undulating bottom surface likely. After deposition reworking of sand by wave action. Short, high energy episodes in a quiet part of basin, above the storm wave base as well as in higher energy environment, above the fair weather wave base. Settings proximal relatively to the sand source. Isolated casts of small erosional channels (gutter casts) scoured in a muddy bottom and filled with sand transported in suspension or in form of small and medium current ripples. Short, high energy episodes in a quiet part of basin, through which sand was bypassed from the nearshore to deeper zones. Deposition by the bottom currents in the stage of upper plane bed (type 1) or from suspension clouds, spreading out in the water column with slight, periodical changes of the flow velocities (type 1, 2). Deposition at different depths, proximally relatively to the sand source. Relatively abundant sand sedimentation; complete reworking by wave agitation. Deposition below and above the fair weather wave base, in the vicinity of river mouths or sandy shoals. Migration of sand megaripples; modeling of the upper surface in periods of increased wave agitation. Deposition at different depths, on the main current paths, in the vicinity of the sand source. Migration of small, current and wave-current ripples, the latter resulting from imposition of wave and current activity. Deposition in the same location as lithofacies Sc. Table 2. Sandy lithofacies identified in the study

8 222 PAULINA LEONOWICZ Text-fig. 4. Mud-silt lithofacies: massive and laminated muds (M m ). A macroscopically structureless mud, Kozłowice section, B silt streaks showing internal grading with grains fining upwards (the lower one) or coarsening upwards (the upper one); sharp base is visible in the lower streak (microphotograph), Nowa Wieś 12 borehole, depth 86.5 m, C thin streaks with indistinct boundaries enriched in a quartz silt (microphotograph), Kozłowice section from muddy density flows, generated either by major river floods or by stronger storms. The presence of crosslamination indicates that the bottom in this generally quiet part of the basin was periodically affected and reworked by bottom currents. Isolated intercalations of homogenous silt, a few centimetres thick, as well as homogeneous, parallel-laminated and graded silt/sand layers of lithofacies M p, are interpreted as storm deposits, similar to the B, C and M laminae types of Pedersen (1985). Association 1 marks deposition in quiet settings, below the storm wave-base, and relatively distant from the source of sediment - far from river mouths, sandy shoals and main current paths. Association 2 fine-grained background sedimentation of proximal type This association is composed predominantly of lenticular-bedded muds and silts (lithofacies M l ) (Table 1; Text-fig. 6) with intercalations of massive, laminated and parallel-bedded muds and silts of lithofacies M m, M s and M p (Table 1). Association 2 is the main component of the whole succession. Trace fossils include common Planolites and some undeterminable grazing traces, accompanied by rarer Gyrochorte, Protovirgularia and Spongeliomorpha. Similarly as association 1, association 2 resulted from slow background sedimentation from suspension interrupted by short episodes of supply of coarser sediment. Event deposits include mainly lenses and streaks of very fine sand, marking migration of isolated, starved ripples on a cohesive muddy bottom, due to reworking by weak traction currents, which is confirmed by common cross-lamination. Groove marks on lower surfaces of sand lenses indicate that deposition of sand was preceded by erosion. Similar structures, but developed in finer-grained mud-silt deposits, were reported by Schieber (1990) and Pedersen (1985), who interpreted them as distal storm deposits. In the case of the deposits studied, episodes of sand supply could be linked either with storms, causing off-

9 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 223 shore redeposition of coarser material or with stronger river floods, generating density currents moving near the bottom. Reworking of bottom sediment by currents could take place already during episodes of sand derivation by offshore currents, or result from sediment reworking by wind-driven and wave-drift currents after sand deposition. The latter interpretation provides an explanation of the high variability of the palaeotransport directions, measured from ripple crestlines. Weaker storms and floods resulted in the formation of event layers resembling those from association 1. Association 2 records similar processes as described for association 1, but in more proximal set- Text-fig. 5. Mud-silt lithofacies: (A-D) massive and laminated silts (M s ), (E-F) parallel-bedded muds and silts (M p ). A subtle horizontal lamination visible owing to enrichment of light laminae in very fine sand, Wichrów 50 borehole, depth 65.5 m, B a quartz silt streak with indistinct boundaries in the massive sideritic clayey siltstone (microphotograph), Kozłowice section, C a sequence: trough cross-laminated silt horizontally laminated silt - massive mud, Wręczyca 3 borehole, depth m, D faint trough cross-lamination in silt, Przystajń 25 borehole, depth 55.1 m, E alternating thin layers of massive and parallel laminated mud and silt; in the upper part of picture thin lenses of very fine sand occur, Wręczyca 3 borehole, depth m, F sand-silt-mud sequences, Praszka 1 borehole, depth 55.8 m. Lithology of alternating layers in M p lithofacies is marked on the right side of pictures (m mud, st silt, sd sand)

10 224 PAULINA LEONOWICZ tings relative to the sediment source. It could originate in deeper parts of the basin, below the storm wave base, where the bottom was only periodically affected by storm-generated currents, as well as in the shallower environment, in which bottom sediment was reworked by wind-driven currents. Association 3 distal fine-grained tempestites This association forms layers a few centimetres thick of interlaid mud-silt-sand heteroliths, representing lithofacies M i (Table 1; Text-fig. 7A C, F), occurring within fine-grained deposits of association Text-fig. 6. Mud-silt lithofacies: lenticular-bedded muds and silts (M l ). A mud with connected sandy lenses, B single, cross-laminated lens of sandstone, C thin groove marks on the lower surface of sandstone lens, D-F asymmetric ripple crests visible on the upper surfaces of connected sandy lenses: D straight and slightly undulating ripple crests (scale in cm), E lingoid ripple crests (arrows), F undulatory ripple crests. A, B, D-F Kozłowice section, C Przystajń 2 borehole, depth m

11 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 225 Text-fig. 7. Mud-silt lithofacies: (A-C, F) interlaid mud-silt-sand heteroliths (M i ) and (D-E) wavy-bedded muds and silts (M w ). A fragment of mud-silt-sand heterolith, B detail of A: sequence starting with lens of silty sand, passing upwards into horizontally laminated silt with inverse grading and horizontally laminated mud at the top, C mud-sand heterolith on polished surface; cross-lamination in sand layer and load cast at the base of sand lens are visible, D wavy-bedded mudstone; thin sand layer in the lower part of picture reveals faint cross-lamination (white arrow), black arrows indicate small burrows, E detail of D: symmetric (white arrow) and asymmetric (black arrow) ripples are visible, F detail of C: microphotograph of mud-silt-sand heterolith; in the lower sand layer cross-lamination is visible, in the upper one gradational, upward fining of grain size occurs; note the distinct boundary between silt and mud in both layers. A-C, F Kozłowice section, D, E Gorzów Śląski 15 borehole, depth 24.8 m

12 226 PAULINA LEONOWICZ 2 and rarely association 1. Lithofacies S p is represented by layers a few centimetres thick of very fine grained sands and sandstones, revealing horizontal lamination of type 2 (Table 2). In deposits of lithofacies M i various undeterminable small burrows occur. Deposits of association 3 mark short-lived events of sand and silt supply to those parts of the basin in which association 2 was normally deposited. Rarely these events also affected more distal parts of the sea, dominated by deposits of association 1. Sedimentary structures indicate transport by traction currents, erosion, settlement from suspension and wave reworking. Parallel-laminated silts and sands, silt and sand layers with gradational tops, as well as graded sand-silt-mud sequences mark deposition from waning suspension flows (Stow and Shanmugam 1980). Grain size distribution in the graded layers is often bimodal, with gradation occurring mainly in their upper part (Textfig. 7F). Such a fabric points to very good sorting of grains by wave reworking and is characteristic of tempestites (Einsele and Seilacher 1991). Reverse grading observed in these sequences can result either from energy increase, modifying the suspension content, or from the reworking of the bottom layer by an overriding current (Hiscott 2003; see also Schieber 1994). The activity of traction currents is marked by crosslamination observed in thin layers and lenses of sand as well as by thin sand streaks with distinct boundaries. The upper surfaces of connected sand lenses show, however, both current and wave ripple crests, which point to wave reworking of sand after its deposition. Similar sand and silt layers, revealing the sedimentary structures described above, are known from modern and ancient muddy shelf successions, where they represent distal storm deposits (Reineck and Singh 1980; Dott and Bourgeois 1982; Aigner 1985; Pedersen 1985). Thus, association 3 is interpreted as distal fine-grained tempestites, consisting of sediment whirled up by wave turbulence and transported in suspension offshore to a relatively quiet part of the basin by gradient currents, scouring and modelling the seabottom. If the depositional setting was situated above the storm wave base, the sediment was additionally reworked by waves and displaced by wave-drift and wind-driven currents. Association 4 proximal sandy tempestites This association comprises mainly wavy-bedded sands (lithofacies S w ), parallel and cross-bedded sand lenses (lithofacies S l ) and massive sand beds (Table 2; Text-figs 8, 9, 10), with less common beds of flaserlaminated, cross-bedded and parallel-laminated sands (lithofacies S f, S c and S p ) (Table 2; Text-figs 11A, C D, 12B). They form layers several centimetres thick or, less commonly, packets a few decimetres thick, occurring within mud-silt deposits of associations 2 and rarely 1. Some sandstone beds contain an admixture of coarse grains or even thin insets of fine gravel. Wavybedded sands (S w ), which are the most typical constituent of this association, laterally merge into sand lenses of lithofacies S l and into deposits of association 3 (lithofacies M i and M s ). Commonly, deposits of association 4 contain wood fragments several centimetres long (Text-fig. 11D) as well as small, usually sideritized, muddy intraclasts. Various load casts and syndepositional deformation structures occur in sandstone beds and lenses of lithofacies S w and S l (Textfig. 10D). The bases of sandstone beds of lithofacies S w also reveal occasional groove marks and common trace fossils, including frequent Planolites and rarer Palaeophycus, Helminthopsis and Gyrochorte. Sandy layers and lenses lack visible bioturbation. Deposits of association 4 mark short episodes of fine sand supply to those parts of the basin in which the deposits of associations 1 and 2 were continuously accumulated. All the processes responsible for the formation of association 3, i.e. erosion, transportation by traction currents, settlement from suspension and wave reworking, also operated here; however, the higher intensity of these processes and the abundance of sand resulted in the formation of thicker sand beds and more distinct sedimentary structures. Sand was transported mainly by traction currents, in the form of small and medium ripples, marked by cross-stratified beds. Variable directions of the inclination of laminae in successive beds point to different flow directions. When the current transport was accompanied by wave oscillation, stirring bottom sediment into suspension, parallel-lamination and hummocky cross-stratification formed (Reineck and Singh 1972, 1980; Dott and Bourgeois 1982; Myrow and Southard 1996). In the case of rapid deposition of suspended load, massive sand beds formed. Sand derivation was accompanied by erosion of the muddy bottom, which is indicated by the presence of small muddy intraclasts, groove marks and gutter casts. Gutter casts, distinguished as lithofacies S l, represent small, elongated erosional channels cut into the muddy bottom and filled with sand when the flow velocity decreased (compare with Myrow 1992). These structures mark the highest-energy conditions, occurring in the offshore bypass zone, through which the sediment was carried out from the nearshore to deeper parts of the basin.

13 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 227 Sedimentary structures recording current activity are preserved in thicker sand beds, which were reworked by waves only in their uppermost part and in places in which the bottom lay below the storm wave base. In most cases, the whole sandy cover was intensively reworked by waves, which resulted in remodelling of the bottom surface, the formation of reworked ripples with a complex internal structure, or in a complete obliteration of primary structures and the formation of flaser lamination, displaying features Text-fig 8. Sandy lithofacies: wavy-bedded sands (S w ). A wavy-bedded sands with changing sand/mud ratio: in the lower part sand with thin intercalations of mud occurs, above it mud with thin layers of sand appears, B wavy sandstone beds with hummocky cross-stratification, C wavy-bedded sand in which original ripple trains covered by mud can be observed; asymmetric ripples show variable asymmetry direction along the ripple train (arrows), D wavy-bedded sand with flat symmetric ripples in the top (arrows), E fragment of wavy sand layer, in which trough cross-lamination occurs, F fragment of wavy sand layer, with few sets of cross-lamination, displaying reverse inclination of laminae, G fragment of wavy sand layer, in which flaser bedding (white arrow) and faint flaser lamination (black arrow) can be observed; scale in cm, A-D, G Kozłowice section, E, F Wręczyca 3 borehole, depth m. (Photo B G. Pieńkowski)

14 228 PAULINA LEONOWICZ river mouths or sandy shoals, which served as the source of sand. The distance from the source areas is reflected by the thickness of sand accumulations. Association 5 sandy barforms Text-fig 9. Internal geometry of ripples from wavy-bedded sands (S w ) from Kozłowice section indicative of wave action. A intricately arranged cross-lamination sets, topped by bundle, draping adjacent ripples, B swollen set draping ripple crest, C bundled upbuilding forming chevron-type structure, composed of oppositely dipping bundles indicative of wavy origin (Text-fig. 9; compare with de Raaf et al. 1977). Moreover, the arrangement of sediments that arose from rapid deposition of sand on a muddy bottom was often unstable, due to reversed density gradient, and could lead to deformation of sand lenses. Similar deformations of sand filling erosional channels were described by Pieńkowski (1991) from the Hettangian of Southern Sweden. The sedimentary structures described above, as well as syndepositional deformations of rapidly deposited sand, are typical features of storm layers, which are common components of muddy shelf succession (Reineck and Singh 1980; Dott and Bourgeois 1982; Aigner 1985; Pedersen 1985; Seilacher and Aigner 1991; Myrow and Southard 1996). Summing up, the deposits of association 4 are interpreted as sandy tempestites, accumulated in relatively quiet parts of the basin, but more proximal, in relation to the source of sediment, than association 3. Sand whirled up by wave turbulence during storms and carried out offshore by seaward returning gradient currents could be deposited at various depths, below and above the storm wave base, but in settings located in front of This association is composed of cross-bedded, parallel-laminated and small-ripple cross-bedded sands (lithofacies S c, S p and S r ) (Table 2; Text figs 11B, 12A, C F), which form lenses a few metres thick and few hundred metres wide within mud-silt deposits. Lithofacies S p is represented by sands and sandstones with horizontal lamination of type 1 (Table 2). Deposits of this association lack visible bioturbation. Relatively thick, cross-bedded sand packets, which are the main component of association 5, mark the migration of sand megaripples along the paths of the prevalent bottom currents. The high variability of particular set inclinations in the S c lithofacies points to frequent changes of flow direction. The acute angle contact between cross beds and the bottoms of sets indicate that the flow velocity was relatively low (Reineck and Singh 1980). However, alternations of parallel-laminated, cross- and small-ripple cross-bedded sets show that the velocity changed through time. Sometimes, wave superposition on a current flow led to the formation of wave- and mixed wave-current structures. The periods dominated by wave agitation are marked by the S r lithofacies, which developed as a result of the migration of small, wave-current ripples. Wave action could be also responsible for the origin of parallel-laminated sands, if the water agitation was intensive enough to whirl up sediment into suspension (Reineck and Singh 1980). Other features marking wave action include undulating tops of crossbedded sets, resulting from reworking of the bottom surface after sand deposition. After waning of the waves, dispersed fine sediment fell out from the suspension, draping the upper surfaces of sand beds. Sandy bedforms dunes, sand waves and bars are common elements on modern ocean shelves, as well as in ancient marine successions. Most commonly they are linked with tidal circulation; however, sand bodies formed and moved by storm-generated and semipermanent oceanic currents have also been reported (Johnson 1979). Taking into account the storm-dominated character of sedimentation ascertained for the previously described associations, a significant influence of tidal processes during formation of association 5 is unlikely. In the case of deposits occurring within shallow water sequences (i.e. associations 6 and 7), transport from the river mouth by semipermanent longshore currents is more probable.

15 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 229 Text-fig. 10. Sandy lithofacies: cross-bedded sand lenses (S l ). A small lens of ferruginous sandstone, in which two sets of indistinct crossbedding are visible (photo and line drawing), B lens of ferruginous sandstone, consisting of two sets of cross-bedding, separated by erosion surface (photo and line drawing), C upper surface of sandstone lens, on which undulatory crests of asymmetric current ripples occur; arrow indicates the direction of flow, D sand lens with strongly deformed parallel lamination. Kozłowice section In the case of isolated sand bodies surrounded by mudsilt deposits of deeper associations, transport by stormgenerated geostrophic currents is possible, followed by reworking by semipermanent bottom currents. In both cases, relatively large thicknesses of sand bodies suggest the proximity of a sand source. In summary, association 5 is interpreted as sandy barforms formed by semipermanent longshore currents and storm-generated offshore flows, which carried them out to zones of permanent mud sedimentation. Deposition took place in the vicinity of sand sources, in settings lying on the main current paths. Multiple reworking of their surfaces by wave oscillation indicates accumulation above the storm wave base and possibly also above the fair weather wave base. Association 6 wave-dominated muddy shoals This association forms packets several centimetres to a few metres thick composed of wavy-bedded muds and silts of lithofacies M w (Table 1; Text-fig. 7D-E) with intercalations of massive, laminated, parallel-bedded and lenticular-bedded muds and silts of lithofacies M m, M s, M p and M l (Table 1). The common feature of these deposits is the occurrence of small burrows filled with light grey, fine sand and sandy silt (Text-fig. 7D). Deposits of association 6 originated in the shallow part of the basin, above the fair weather wave base, but away from the sand source. The sedimentation was mainly from suspension; however, the common presence of typical wave-ripples and the lack of graded sequences indicate that the bottom was permanently

16 230 PAULINA LEONOWICZ Text-fig 11. Sandy lithofacies: (A B) parallel-laminated sands (S p ) and (C D) flaser-laminated sands (S f ). A distinct horizontal lamination in sand, enhanced by continuous organic-mica laminae (type 2); scale in cm, B indistinctly parallel-laminated sand; lamination accentuated by faint clay-organic-mica streaks (type 1), C two layers of flaser-laminated sands with undulating tops, displaying outlines of symmetric ripple trains (arrows), D irregular flaser-lamination, enhanced by thin clay and organic-mica laminae; in the middle coalified fragment of wood is visible. A, C, D Kozłowice section, B Boroszów section influenced by the wave action, resulting in the formation of wavy bedding. As is shown by experimental studies and analyses of modern deposit (McCave 1971, 1985; Rine and Ginsburg 1985; Schieber et al. 2007), such a dynamic environment does not preclude sedimentation of fine-grained deposits. If the suspension concentration is high, mud can be also deposited in agitated water. During periods with low wave activity quiet deposition of other mud-silt lithofacies prevailed. In summary, association 6 represents sedimentation on muddy shoals and in shallow embayments, above the fair weather wave base, but far from sandy shoals and river mouths and outside main current paths. Association 7 wave-dominated sandy shoals This association forms packets few metres thick composed predominantly of flaser-laminated, wavybedded and parallel-laminated sands (lithofacies S f, S w and S p ) (Table 2) with intercalations of massive sand and various mud-silt deposits, representing lithofacies M m, M s, M l, M i and M w (Table 1). The association contains common fine plant debris and lacks visible bioturbation. Deposits of association 7 mark deposition in shallow, nearshore parts of the basin, close to the river mouths, in areas with high sand supply and with the bottom above the fair-weather wave base. The sand was permanently reworked by waves, which resulted in a complete obliteration of primary sedimentary structures and the formation of flaser-lamination. During periods of more intensive wave activity (storms) the bottom sediment was whirled up into suspension and deposited as massive or parallel-laminated beds. In fair weather periods, during which the waves weakened, thin mud layers were deposited, leading to the

17 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 231 Text-fig. 12. Sandy lithofacies: (A C) cross-bedded sands (S c ) and (D F) ripple-bedded sands (S r ). A cross-bedding in a sand bed, accentuated by different coherence of sand laminae; hammer as a scale, B two sets of cross-bedding, displaying different directions of inclination (black arrows); cross-bedding is accentuated by sideritization of laminae. Cross-bedded layer is overlain by another thin, wavy layer of sand (white arrow); hammer as a scale, C thick sandy packet showing few sets of indistinct cross-bedding, accentuated by Fe-hydroxides mineralization; opposite inclination of cross-bedding is visible (arrows), D thick packet of sand, in which cross-bedding (lower part) and ripple-bedding (upper part) occur; rusty colour of sand results from the presence of Fe hydroxides, E sand layer with a few sets of ripple cross-lamination; lamination is accentuated by different coherence of sand laminae; scale in cm, F detail of D: cross-laminated ripples, showing variable direction of asymmetry along the ripple train (arrows); ripple crests often covered by thin clay-fe hydroxides laminae. A, E Boroszów section, B D, F Kozłowice section

18 232 PAULINA LEONOWICZ formation of wavy bedding in sand bodies. In periods when the sand derivation declined, due to changes of river discharge, thicker mud-silt intercalations were formed. Sandy deposits with similar association of sedimentary structures, i.e. parallel lamination, flaser stratification and sand/mud intercalations were described from modern nearshore marine environments (e.g. Reineck and Singh 1972, 1980). Thus, association 7 is interpreted as a record of sedimentation on wave reworked sandy shoals. LITHOFACIES DISTRIBUTION The analysis of lithofacies distribution is based on two sections (Kozłowice and Boroszów) and 14 borehole cores (Text-fig. 2). Kozłowice section (Text-fig. 13A, B) The main part of the Kozłowice section (Text-fig. 13A) consists of olive-green and grey mud-silt deposits of association 2 alternating with and merging laterally into association 1. The mud-silt deposits contain thin intercalations of sand and mud-silt-sand heteroliths, representing associations 3 and 4. Sand layers are variable in thickness and often wedge out or change laterally into interlaid mud-silt-sand heteroliths (lithofacies M i ) and sideritic mudstones. Where sand layers thicken, intercalations of mud or silt sometimes appear, resulting in doubling or even trebling of them. Sand lenses representing lithofacies S l occur only sporadically. They appear directly in the mud or within siderite beds that pass laterally into sand layers. In the middle and upper parts of the section two sandy complexes appear. The lower one (LSC on Text-fig. 13A, B) is composed of a few, relatively thick sand beds displaying flaser and cross-lamination. Hesselbo and Pieńkowski (2011) interpret them as deltaic lobes (delta front/distributary channels), recording rapid delta progradation. The upper complex (USC on Text-fig. 13A, B), present only in the western part of the outcrop, consists mostly of rippleand cross-bedded sands. Hesselbo and Pieńkowski (2011) interpreted it as a delta front/shoal/barrier. Correlation of boreholes has revealed that none of the sand beds observed in the sections formed a continuous sand sheet, but instead were elongated, hundred metres long lenses, wedging out laterally. According to this, sands of the lower complex (LSC) are interpreted here as storm patches (association 4), accumulated in front of a prograding delta. The upper complex (USC) is assigned to association 5, representing sandy barforms, which agrees with the earlier interpretation of Hesselbo and Pieńkowski (2011). The sand bodies of the LSC are aligned N S and NNW SSE (Text-fig. 13B) while the sand bodies of the USC are aligned NNE SSW. Palaeotransport directions measured in the sandstones show high variability, but W E, NW SE and NNW SSE directions prevail (Text-fig. 13A), marking offshore flows parallel and oblique to the shoreline. A characteristic feature of deposits from the Kozłowice section is the occurrence of trace fossils including Planolites, Palaeophycus, Helminthopsis, Gyrochorte, Protovirgularia and Spongeliomorpha, as well as some unidentified pascichnia (Text-fig. 13A). Boroszów section (Text-fig. 13C) The lower part of the Boroszów section is represented by massive sands of unclear origin, overlain by olive-green muds and silts of association 2. The upper part is more diversified and starts with grey deposits of associations 1 and 2, with lenses and layers of associations 3 and 4. In the uppermost part of the section there is a c. 5 m thick sandy complex, assigned to associations 5 and 7. As the Jurassic strata in Boroszów are glaciotectonically displaced and rest on the Quaternary (Rylko 1954; Sobkiewicz and Swoboda 1979), the observed cross-lamination and cross-bedding inclinations cannot be interpreted in terms of palaeotransport directions. However, the high variability of these structures is similar to that of the structures noted in the Kozłowice section. Borehole sections (Text-figs 14, 15) The successions of the Ciechocinek Formation known from boreholes resemble those from outcrop sections and consist mainly of mud-silt deposits of associations 2 and 1 with subordinate intercalations of other lithofacies. The muds and silts are mostly olive-green with less common grey intercalations, while in the uppermost part of the succession the colour commonly changes to willow green. The most monotonous succession is that in the Przystajń 2 borehole (Text-fig. 14), with only sporadic intercalations of sandstones of association 4. More abundant sandy and heterolithic layers of associations 3 and 4 appear in the Wręczyca 3, Przystajń 20 boreholes (Text-fig. 14) and in the Nowa Wieś 12 borehole (Text-fig. 15). In the first two, they are concentrated in the lower parts of the succession, while in the last one sand beds are more abundant in

19 ACTA GEOLOGICA POLONICA, VOL. 61 PAULINA LEONOWICZ, FIG. 13 Text-fig. 13. Logs of the Ciechocinek Formation deposits from the Kozłowice (A) and Boroszów (C) sections, showing distribution of facies associations, trace fossils, Fe mineralization and plant remains. Transport directions measured in the Kozłowice clay pit are presented on rose diagrams; n numbers of measurements. On B, selected section of the Kozłowice deposit, demonstrating lateral relations of sand and mud-silt lithofacies, and inferred extent of sand bodies from the lower sandy complex (LSC) are presented. Data from archival core descriptions by Jaczynowski (1959, 1960) were used. Location of dinoflagellate cyst Luehndea spinosa after Barski and Leonowicz (2002)

20 ACTA GEOLOGICA POLONICA, VOL. 61 PAULINA LEONOWICZ, FIG. 14 Text-fig. 14. Representative logs of the Ciechocinek Formation: boreholes from the north and central parts of the study area. For localities see Text-fig. 2, for other explanations see Text-figs 13 and 15. Missing parts of cores supplemented after Kieżel (1990) and Pieńkowski (2004), location of dinoflagellate cyst Luehndea spinosa and foraminiferal linings after Barski (personal communication)

21 ACTA GEOLOGICA POLONICA, VOL. 61 PAULINA LEONOWICZ, FIG. 15 Text-fig. 15. Representative logs of the Ciechocinek Formation: boreholes from the south and central parts of the study area. For localities see Text-fig. 2, for other explanations see Text-figs 13 and 14. Lacking parts of cores supplemented after Kieżel (1990) and Pieńkowski (2004)

22 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 233 the upper half of the succession. Other sections, i.e. Wichrów 50, Przystajń 25, Dąbrowa 46 (Text-fig. 14), Praszka 1, Gorzów Śląski 16 and Pawłowice 40 (Text-fig. 15), in addition to storm beds, also contain relatively thick sandy packets, representing association 7, which are thicker and more common in the lower part of the succession. Correlation of boreholes has revealed that the vertical arrangement of all types of sandy intercalations is random and that these intercalations usually do not correlate laterally with each other, which means that instead they represent extensive, flat lenses (Text-fig. 16). A common feature of the Ciechocinek Formation in all the boreholes studied is siderite and pyrite mineralization as well as the occurrence of trace fossils, represented by the ichnogenera Planolites, Gyrochorte, Diplocraterion and some unidentified pascichnia (Text-figs 14, 15). In some cases, the upper part of the succession contains phyllopods of Estheria sp., as well as horizons with plant roots. The position of the Ciechocinek Formation boundaries is evident only in some sections. The lower boundary is distinct in the Wręczyca 3 and Dąbrowa 46 boreholes (Text-fig. 14), in which mudstones of associations 1 and 2 rest directly on medium- and coarse-grained sandstones of the Blanowice Formation. The upper boundary is easy recognizable in the Wręczyca 3, Przystajń 20 (Text-fig. 14), Nowa Wieś 12 and Gorzów Śląski 16 (Text-fig. 15) boreholes, in which the contact between sandstones of the Borucice Formation and mudstones of association 1 is erosional and commonly marked by the occurrence of intraclasts. In other sections, the boundaries lie within intervals consisting of mud-silt and sand intercalations and are therefore less evident. They can be established approximately, based on features other than lithology, i.e. trace fossils, siderite and pyrite mineralization, as well as by spatial relationships with neighbouring boreholes, in which the boundaries are evident. The successions in two boreholes, Bór Zajaciński 45 and Pogorzałki 36, lack deposits typical of the Ciechocinek Formation. The whole succession is developed here similarly to the Blanowice Formation, as poorly sorted sandstones with gravel, alternating with grey mudstones and siltstones. However, thick mudsilt intervals in the middle parts of these sections reveal features pointing to marine influence (i.e. pyrite and siderite mineralization, trace fossils). Moreover, borehole correlation shows that the successions of both boreholes merge laterally into grey and olive green muds of the Ciechocinek Formation (Bór Zajaciński 45 on Text-fig. 16 B) and thus can be considered as their stratigraphical counterparts. EARLY TOARCIAN SEDIMENTATION IN THE CZĘSTOCHOWA-WIELUŃ REGION Sedimentation during marine transgression: the Ciechocinek Formation (Text-fig. 17) The predominant mud-silt character of the Ciechocinek Formation points to its deposition in a quiet sedimentary basin with a continuous influx of clay and fine silt, to which sand was brought by occasional events. Suspended sediment spread out into the basin from the river mouths as fine-grained, low-density suspension currents and detached suspension clouds. The amount and grain size of suspended sediment depended on the site location in relation to the position of the river mouths and on the discharge from the rivers. Both factors changed with time. The location of the river mouths shifted because of the progradation of deltas, avulsion of river channels and the long-lasting sea-level changes. The river discharge varied according to alternating low- and high-water seasons. The other important mechanism for mud and silt transportation was re-suspension of sediment by wave turbulence during storms and its offshore transport by returning gradient currents. The role of these processes is confirmed by the presence of erosional microstructures and cross lamination in the mud-silt lithofacies. This type of sedimentation, recorded by association 1, prevailed in quiet settings, below the storm wave base and far from river mouths, including both deeper, distal parts of the basin, as well as shallow but sheltered locations. Exceptionally strong storms affecting these areas led to the formation of thin intercalations of association 3. The same processes, acting in areas closer to the sediment source, are marked by proximal background sediments of association 2, the most common deposit type in the whole succession. River floods and storm episodes supplied to these areas small amounts of finegrained sand, which was transported in the form of isolated starved ripples by weak traction currents. The high variability of palaeotransport directions indicates that the main transporting agents were meteorologically induced flows; however, this association could form also below the storm wave base. Stronger storms are marked here as intercalations of associations 3 and 4, representing distal and proximal tempestites. Distal tempestites formed in more distant areas or during the last phase of the storm in proximal settings. Proximal tempestites were deposited in areas lying in front of the river mouths and sandy shoals. Generally, the more proximal the depositional setting, the thicker and coarser-grained were the storm deposits, and the more

23 234 PAULINA LEONOWICZ Text-fig 16. Interpretation of facies relationships in representative sections of the Lower Jurassic deposits from Częstochowa Wieluń region: A central part and B south part of the study area

24 LOWER JURASSIC BRACKISH DEPOSITS FROM SW POLAND 235 Text-fig 17. Schematic depositional model of the Ciechocinek Formation in the Częstochowa Wieluń region. Dashed line marks boundary between Ciechocinek and Blanowice Formations; arrow indicates north distinct and diverse were the sedimentary structures (Text-fig. 18). In shallow, nearshore parts of the basin sedimentation took place in two subenvironments: sand- and mud-dominated. In both cases wave agitation acted as an important factor influencing depositional processes. In areas located far from sand supplying river mouths, deposits of association 6 formed, resembling in places the floodplain sediments of the Blanowice Formation. They mark prevalent deposition from suspension in sites lying above the fair weather wave base and permanently affected by waves. In areas located closer to the river mouths, sandy deposits of association 7 formed. They probably represent subaqueous parts of deltas, mouth bars or sandy shoals intensively reworked by wave turbulence, especially during storms. During periods of low river discharge, mud-silt deposits alternating with wavy-bedded sands were deposited. The few metres thick sand bodies of association 5 represent elongated sand bars, formed and displaced by semipermanent and storm generated currents, parallel and oblique to the coastline. Superimposition of relatively constant longshore currents and short-lived but strong seaward returning flows that operated during storms resulted in high variability of the inclination of co-sets. Intensive wave activity during storms led to multiple reworking of sandbar surfaces and to the formation of wave and complex wave-current generated structures. The source of sand for all the identified associations were rivers and, in the case of the lowermost part of the marine succession, probably also reworked older alluvial deposits. All types of sand bodies occurring in the succession form elongated lenses, up to a few metres thick and a few hundred metres long. They form a random spatial pattern and rarely continue in neighbouring boreholes. It indicates that their appearance in the succession was not linked with general sea-level changes but rather with changes of the river system configuration, especially with displacements of river mouths. The beginning of the marine transgression The marine transgression that led to the deposition of the mud-silt succession of the Ciechocinek Formation studied here entered the Polish Basin from the west, along the Polish Trough (Dadlez and Kopik