Lab 8: Facies Analysis and Correlations: Sequence Stratigraphy in the Book Cliffs, Utah

Transcription

1 Geology 109L Lab 8: Facies Analysis and Correlations: Sequence Stratigraphy in the Book Cliffs, Utah Goal: In this lab, you will put together your knowledge of near-shore facies and sequence stratigraphy to correlate sections and interpret facies changes in the Cretaceous stratigraphy of the Book Cliffs. Introduction Facies analysis is an important part of any stratigraphic study. The basic idea is to recognize how different depositional environments are expressed by the development of characteristic structures, lithologies and organic components in sedimentary rocks. The usual way to reconstruct the depositional history of an area is to group strata into facies assemblages; that is, measuring numerous stratigraphic sections, examining the rocks, and assigning those rocks a depositional environment based on sedimentary structures, paleontological evidence and lithology. When this has been accomplished, the measured sections are correlated using the facies assemblages and sequence stratigraphic concepts, and three dimensional models of deposition are constructed. When doing this, the stratigrapher must not only be familiar with sedimentary rocks and structures, but also bear in mind the regional history of the area. It is important to consider facies changes when correlating measured sections. A facies change is the natural gradation from one environment to another. For example, at a certain point of sea level, the coastal plain of Hot-As-Heck, Texas grades into a tidal flat, which grades along the shoreline into a beach, both of which grade out to the deeper waters of the Gulf of Mexico. The sediments in these environments differ, but there are no clean breaks dividing the environments; the sediments change continuously. Add to this a stream that flows out to the Gulf, and you add more environments. When sea level rises or falls, those environments migrate, producing a vertical stacking of the sediments deposited in these laterally contiguous environments. Stratigraphy in the Book Cliffs During Late Cretaceous time, a time of world-wide high sea level, the stable cratonic area of North America was inundated by marine waters. The invasive body of water is called the Western Interior Seaway, or the Great Cretaceous Seaway. It was approximately 4800 km long and up to 1600 km wide. At this time, the western side of the seaway was bordered by a foreland basin formed by thrusting of the Columbian-Sevier orogeny (which runs through present day Nevada), and on the east by a stable exposed platform. Several basins formed due to the weight of the thrust sheets loading the crust and causing subsidence. The stratigraphy and lithofacies differ in the different basins, but in all, coal and natural gas deposits are an important economic consideration. (Did you know that the Denver area has abundant petroleum resources? They are from organics deposited in this environment during Cretaceous time.) Understanding the depositional environments and stratigraphy in these basins is very important for exploration for these resources. They also provide information on the timing and influences of thrust sheet movement; by dating sediments in these basins, the timing and location of tectonic activity along the western paleomargin of North America can be accurately constrained. ASSIGNMENT 1) Identify the facies assemblages on the measured sections that will be provided in lab and color the units different colors. Remember to consider Walther s Law: Neighboring environments should appear next to each other vertically unless an unconformity is present. 2) Identify and correlate the unconformities between sections. Some do not appear in all sections. Think about why and how to deal with the correlations. Book Cliffs, Page 1

2 3) Correlate the facies and facies changes between sections considering the temporal significance of the unconformities. Color the areas between sections with the facies you predict. 4) On the far right of the stratigraphic columns, construct an approximate sea level curve for the area based on the location of unconformities and shifts in facies distributions. 5) Answer the questions at the bottom of the stratigraphic columns. 6) Turn in your correlations. Important Note: The stratigraphic columns are taken from Yoshida (2000), who provided a correlation similar to the one I am asking you to do. However, Yoshida (2000) also mapped facies changes and unconformities along cliffs between sections. Thus, Yoshida (2000) had much more information than you do, and the correlations in the paper show much more detail. Also, I slightly changed the facies I am asking you to map out, because it was unclear to me how Yoshida (2000) identified some of the facies. Why this matters to you: You are welcome to look up Yoshida s paper and look at the published correlations, but this is neither required nor particularly encouraged (or discouraged). The paper may or may not help. It contains complex terminology, making it difficult to read, and you should be able to do this lab without that additional information. If you do choose to look up the paper, you still need to work through your own correlations based on the information provided in the lab; they should not be the same as the published ones. I did not get the published correlation when I tried the correlations myself. Please reference it by answering the questions on the bottom of the strat section page. Please note that if you use the paper and do not reference it, you will be guilty of plagerism, which broadly consists of using other people s work without acknowledging them. Your answers to the questions will NOT affect your grade, unless you use the paper and do not say that you did. Facies Assemblages for the Upper Blackhawk Formation and Lower Castlegate Sandstone The following descriptions of facies are taken from Yoshida (2000). Use these descriptions and the facies tables that follow them to interpret the depositional environments represented in the stratigraphic columns that you will correlate for this lab. Yoshida, S Sequence and facies architecture of the upper Blackhawk Formation and the Lower Castlegate Sandstone (Upper Cretaceous), Book Cliffs, Utah, USA. Sedimentary Geology, v. 136, p Lithofacies assemblages This study divides the strata of the uppermost Blackhawk Formation and the Lower Castlegate Sandstone between Horse Canyon and Coal Canyon into nine lithofacies assemblages (Table 1) Assemblage A: braided-fluvial facies Assemblage A dominates the lower part of the Lower Castlegate Sandstone, and comprises superimposed sandstone sheets bounded by laterally extensive erosional surfaces (Figs. 3 and 4A). On a single outcrop (e.g. hundreds of metres wide), thick sandstone sheets (e.g m) commonly comprise a few, laterally extensive sand bodies. Paleocurrents measured from trough cross-bedding within each sheet sandstone are unimodal, oriented to the south-east and east. Many sand bodies within each sheet sandstone contain large-scale cross-bedding (2 13 m high), dipping in a direction parallel, oblique or normal to local flow. Log impressions, wood/ plant fragments, and rare dinosaur/reptile bones occur at the base of sandstone sheets. Sandstone beds within the sand bodies are thin (,20 cm) to thick (3 4 m), are mostly fine- to medium-grained, and typically have a pale orange to brown colour. Abundant trough cross-stratification and convolute bedding with minor current ripples occur within beds. Minor associated lithofacies include light-grey shale and Book Cliffs, Page 2

3 siltstone beds. Each sandstone sheet shows an incomplete fining-upward profile. Assemblage A is interpreted to be of braided fluvial origin Assemblage B: tidally influenced sandy fluvial facies Assemblage B is characterized by lenticular to tabular sand bodies 2 11 m thick, which commonly contain large-scale cross-bedding ( m high) and mud drapes. Some of these can be classified as inclined heterolithic strata (hereafter called IHS; Thomas et al., 1987) (Figs. 3, 5 and 6), but some lack the diagnostic mud/sand couplets. Sandstones exhibit a white to very light grey/brown colour. They are mostly very fine- to fine-grained, but medium-grained sandstone with abundant lag deposits (e.g. wood fragments and reptile bones) occurs near the erosional base of some sand bodies (Fig. 4B). In many localities these sand bodies laterally coalesce to form a sheet geometry in the lower part of the assemblage (Fig. 4A). Each sheet sandstone has an upward-fining profile (Figs. 3 and 5). Trough cross-bedding and convolute bedding are the most common sedimentary structures in this assemblage. Log impressions are common at the base of sheet sandstones. Siltstones typically have a light grey to greenish grey colour. Shales are dark grey and carbonaceous, and contain abundant vascular plant fragments. In addition to IHS, this assemblage contains many sedimentary structures indicative of tidal influence (e.g. Shanley et al., 1992), such as sigmoidal bedding (Fig. 7A), fine organic detritus along the cross-stratification (Fig. 7B), oscillation/bidirectional ripples (Fig. 7C), flaser/wavy/lenticular bedding and multiple reactivation surfaces. Trace fossils including Teredolites and various crawling/resting traces occur in the sandstones, which also contain rare dinosaur footprints (Fig. 5). Paleocurrents measured from the trough cross-bedding are commonly unimodal, oriented to the east and southeast. Dispersion of the paleocurrent readings in this assemblage is, however, higher than those in Assemblage A, and subordinate northwestward flow is indicated in cross-bedding in some localities. This assemblage changes facies updip to braided-fluvial deposits of Assemblage A and downdip to muddy estuarine deposits of Assemblage D (Fig. 3). Assemblage B is interpreted as tidally-influenced fluvial to upper estuarine deposits Assemblage C: alluvial plain facies Assemblage C, together with Assemblage D, comprises the Upper Mudstone Member of the Blackhawk Formation (Figs. 3 and 8). This assemblage consists of isolated channel sand bodies encased in mudstones and siltstones. The sandstone fill of the channels is mostly fine-grained, commonly has red ferric silica cement, and displays the geometry of lateral accretion. Sandstone beds in the channels are characterized by trough cross-bedding and convolute bedding with minor current ripples, and are typically interbedded with thin mudstones or siltstones. Inter-channel deposits comprise dominantly siltstones and shales. Siltstones are light grey to grey, massive to thick-bedded, and often have parallel laminations and minor current ripples. Shales are light to dark grey, and carbonaceous, with abundant vascular plant fragments. Rootlets, thin coals and minor sideritic nodules occur within the shales and siltstones. Assemblage C is interpreted as fresh-water alluvial plain deposits with meandering river systems Assemblage D: muddy marginal-marine facies Assemblage D consists of interbedded siltstones and mudstones with subordinate sandstones and thin coals (Figs. 3, 5, 6, and 9). Sandstones are very fine- to fine-grained, thin (,50 cm) to thickbedded (1 2 m). Thick sandstones occur with thin mud drapes either as channel-fill deposits (Figs. 5 and 10A), or within upward-coarsening and upward-thickening clinoform units (Fig. 11) or as lenticular sand bars with mud drapes (Fig. 12). Channel fills are commonly made of distinct, concave-up IHS (Fig. 10A). Thin sandstone beds have horizontal to wavy parallel lamination and contain abundant oscillation and current ripples and minor trough cross-stratification. Tidal indicators such as wavy/flaser/lenticular bedding, sigmoidal bedding and bidirectional ripples are common. Trace fossils Ophiomorpha, Thalassinoides, Teredolites, Skolithos, Chondrites, and various crawling traces and log impressions are present in the sandstones and siltstones. Book Cliffs, Page 3

4 Siltstones have a distinct greenish to light grey colour, or dark grey where carbonaceous, and are commonly bioturbated to varying degrees. They are massive to laminated, contain abundant vascular plant fragments and organic material. Shales are dark- to brownish-grey and carbonaceous, have horizontal to wavy lamination with abundant vascular plant fragments, minor sideritic nodules and large (,15 cm long) intraformational clasts. Body fossils of marine and brackish water fauna (e.g. gastropods, pelecypods) are rare and only locally preserved in the muddy part of the thicker intervals of this assemblage (e.g. Fisher et al., 1960; Maberry, 1971) (Figs. 3 and 9). Assemblage D is interpreted to have been deposited in a wide range of low-energy, marginal marine environments. Interpretation of depositional environments of Assemblage D (e.g. lagoon vs. estuary) requires a regional context, including spatial relationships with other facies assemblages, and a sequence stratigraphic framework (Fig. 3) Assemblage E: upper-estuarine clinoform and channel facies Assemblage E infills deep and narrow incised valleys (up to 18 m deep) associated with the Desert sequence boundary (Fig. 3). This assemblage is characterized by stacked scoop-shaped channels and/or sand bars (Miall, 1993) (Fig. 13A), and large-scale (,18 mthick), down-streamdipping clinoforms (Fig. 13B). Assemblage E grades upward to muddy deposits of Assemblage D interpreted to be of lagoonal origin (Fig. 3). Some clinoforms are traceable for several hundred meters or more. In paleo-slope section of the incised valleys, this facies becomes thinner toward the head of the incised valley system, and grades updip into alluvial plain deposits (Assemblage C) (Fig. 3). Assemblage E is dominated by inclined, medium- to fine-grained sandstones with thin (typically,20 cm thick) carbonaceous mud drapes. These sandstone/ mudstone beds form sandy IHS at some localities. Upward-fining trends occur to varying degrees, both as individual sandstone mudstone couplets of IHS and in overall successions of this assemblage. Sandstone units have abundant trough cross-bedding, current/ bidirectional ripples, convolute bedding, low-angle/ flat parallel laminations and rare sigmoidal bedding. Fine organic detritus is commonly present along the crossstrata of sandstone beds. The occurrence of mud drapes, bidirectional ripples and rare Teredolites and Skolithos trace fossils may indicate tidal influence (Shanley et al., 1992). However, paleocurrents measured from cross-bedding indicate dominantly unimodal flow in the SE direction, and brackish-water/marine fossils have not been found by this study fromthis assemblage west of Tusher Canyon. Assemblage E is interpreted as channel deposits of a large, sandy bayhead delta in the upper estuary Assemblage F: backshore/back barrier sandstone facies Assemblage F commonly has a white flaggy with subtle to moderate upward-coarsening profiles (Fig. 14A), or thin to thick (1 5 m) tabular and massive sandstone (Fig. 14B). This assemblage pinches out updip into lagoonal deposits of Assemblage D, and grades downdip into shallow open marine sandstones of Assemblage H. Sandstones are very fine- to fine-grained and well to moderately sorted, and contain horizontal to low-angle parallel lamination with subordinate trough cross-stratification and current ripples. Organic detritus and plant fragments are common. Rootlets may be present, typically near the top of the facies. Paleocurrent data measured from cross bedding and current ripples indicate a dominant updip (westerly) flow direction. Assemblage F is interpreted as a complex of back barrier environments composed of backshore, wash-over fan and flood tidal delta Assemblage G: tidal inlet channel facies Assemblage G occurs as isolated channels in the uppermost part of the Lower Castlegate Sandstone in some localities east of the Little Park Wash South section (Figs. 3, 9 and 10B). These channels are filled with medium-fine grained sandstone, ripped-up siltstone flakes, and local bioclasts including coquina. Paleocurrents measured from the axis of trough cross-bedding show a dominant basinward (south-easterly) flow direction. Assemblage G is interpreted as having been Book Cliffs, Page 4

5 formed in tidal inlets (ebb channels) in the lower estuary, and subsequently eroded at its upper part by a marine ravinement surface associated with the transgressing Buck Tongue shoreface Assemblage H: shoreface to foreshore facies Assemblage H is dominated by sandstone, which coarsens upward from very fine to fine grain size (Figs. 3 and 13). The lower part of the succession comprises interbedded thin sandstone, siltstone and ripples, and hummocky cross-stratification. This interval changes upward to thickbedded sandstone with hummocky cross-stratification, which is then overlain by thick amalgamated sandstone with swaley cross-stratification near the base, trough cross-bedding and inclined parallel lamination in the middle, and flat to low-angle parallel lamination near the top. Various marine trace fossils including Thalassinoides, Ophiomorpha and Skolithos occur, comprising the Skolithos and proximal Cruziana ichnofacies (e.g. Pemberton and MacEachern, 1995). Assemblage H is interpreted as a shoreface foreshore deposit Assemblage I: muddy open marine facies Assemblage I comprises most of the Buck Tongue (Fig. 3), and is made up of thick mudstones and thin sandstones and siltstones. The mudstones are typically dark grey, carbonaceous, and often laminated and silty. The sandstones and siltstones are light to moderate grey, and horizontal to wavy laminated with rare oscillation ripples, but some are structureless due to intense bioturbation. The upper part of the Buck Tongue contains thin (,50 cm) sandstones with hummocky crossstratification (Fig. 3). Trace fossils including Ophiomorpha, Diplocraterion, Terebellina, Teichichnus, and various crawling traces occur. Assemblage I represents deposition on the open marine shelf and in the transition zone between the lower shoreface and the shelf. Book Cliffs, Page 5