JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.77, February 2011, pp

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1 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.77, February 2011, pp Peperite Occurrence and its Implications on Origin and Temporal Development of the Proterozoic Dhala Basin, Mohar Area, Shivpuri District, Madhya Pradesh K. K. SINHA 1, PRADEEP PANDEY 1, C. L. BHAIRAM 1 and P. S. PARIHAR 2 Atomic Minerals Directorate for Exploration and Research 1 West Block VII, R.K. Puram, New Delhi AMD Complex, /156, Begumpet, Hyderabad sinhakk63@gmail.com Abstract: In the western part of Bundelkhand massif, a caldera with intra-caldera sediments, known as Dhala Formation, occurs as an outlier in and around Mohar village of Shivpuri district, Madhya Pradesh. For the first time, occurrence of peperite is being reported from the basal part of the Dhala sediment. Two types of peperites have been recognized: blocky and fluidal or globular with variable morphology. In peperitic zones, features like soft sediment deformations, presence of sediment into the rhyolite along cracks, vesiculation of the sediments and other evidences suggestive of sediment fluidization are some definite characteristics of interaction of hot magma with wet sediments forming peperite. The occurrence of peperites reflects the contemporaniety of deposition of the Dhala sediments and volcanism, which is well in accordance to the volcanic origin of Dhala structure. Further, the nature of unconformity between the Dhala and overlying Kaimur which is characterized by merely a few centimeter thick pebbly/conglomeratic bed does not appear to represent a large hiatus as expected between the Semri and Kaimur of Vindhyan Supergroup. So, the contemporaniety of the Dhala Formation (at least the lower part) as reflected by occurrence of peperites, coupled with the available age of the rhyolite and the nature of the unconformity between the Dhala and overlying Kaimur provide convincing evidence to correlate the Dhala Formation with the Lower part of the Kaimur and unlikely with the Semri Group or Bijawar as proposed earlier. Keywords: Caldera, Peperites, Dhala sediment, Bundelkhand, Vindhyans, Madhya Pradesh. INTRODUCTION An oblate structure occupied by sediments in its central part occurring as an isolated outlier in and around Mohar village (N25 18'13": E 78 09'11") of Shivpuri district, Madhya Pradesh in western part of Bundelkhand Gneissic Complex (BGC) was identified as a caldera (Jain et al. 2001). The intra-caldera sediments overlying the felsic volcanics is named as Dhala Formation which, in turn, is overlain by a veneer of Kaimur Group of rocks. The Dhala is considered equivalent to Semri Group of Vindhyan Supergroup by Jain et al. (2001). On the other hand, Basu (2007) preferred to consider it coeval to Bijawar Group. The origin of the Mohar structure has also remained debatable. Jain et al. (2001) opined that it was the result of central Plinian type volcanic explosion, whereas others interpreted it as an impact related structure (Pati and Reimold 2007; Pati et al. 2008). Basu (2007) invoked shear rotation of a basement block for the formation of this complex circular structure. The present work reports newly identified, rarely occurring felsic-peperite (Donaire et al. 2002) from the basal part of the Dhala sediments close to the unconformity between the Dhala and underlying volcanics in core-samples obtained from boreholes drilled in Mohar caldera by Atomic Minerals Directorate for Exploration and Research (AMD). Peperite is a rock formed essentially in-situ by disintegration of magma intruded and mingled with unconsolidated or poorly consolidated, typically wet sediments (White et al. 2000). Since peperites represent the contemporaneity of volcanism and sedimentation (Busby-Spera and White 1987; Brooks, 1995; Donaire et al. 2002; Skilling et al. 2002; Martin and Nemeth, 2007), two important inferences can be drawn from its occurrence in Mohar area: (i) some volcanic activities took place even coeval to sedimentation of the basal part of Dhala Formation and (ii) the age of / /$ 1.00 GEOL. SOC. INDIA

2 184 K. K. SINHA AND OTHERS volcanism can provide age constraint on the sedimentation. So, the occurrence of peperites is significant among the prevailing different views regarding the origin of the Mohar structure as well as stratigraphic position of the Dhala sediments. These new findings will provide an insight to resolve some of the outstanding dilemmas. GEOLOGY The Mohar caldera with intra-caldera sediments occur as a distinct geomorphological feature and is characterized by felsic volcanics and a concentric zone of collapse breccias of about 7.5 km diameter, sheared and brecciated granitoids and ring fractures (Fig.1). The felsic volcanics represented by pink to brick red, vesicular to non-vesicular rhyolites and tuffs/tuff breccias with distinct flow structures are scantly exposed as isolated small outcrops (Fig.1). However, in the central part, it occurs as continuous sheet like body below the sediments with varied thickness of 5 m to >200 m. The intra-caldera sediment comprising essentially of an argillaceousarenaceous sequence named as Dhala Formation is capped with a veneer of arenaceous Kaimur Group of Vindhyan Supergroup (Fig.1). The Dhala and the Kaimur are separated by merely a few centimeters thick polymictic pebbly bed/ conglomerate and are in perfect structural concordance. The contact of Dhala sediment with underlying volcanics/ pyroclasts is sharp and sometimes marked by intense hydrothermal alterations. Sediment- Magma Interaction: Formation of Peperites In the core of a borehole (MCB-16) drilled in Mohar caldera (Fig.1), zones of rhyolite intrusion and its intermingling with sediments have been observed at different levels in the basal part of the Dhala Formation close to the sediment-volcanic interface (Fig.2). Such zones are characterized by layers/tongues/clasts (< 50 cm) of rhyolite within sediments that, in turn, are brittly as well as ductily deformed. Brittle deformations are represented by fractures and minor slips whereas ductile by Fig.1. Geological map of Mohar area (Dhala caldera), Shivpuri district, Madhya Pradesh.

3 PEPERITE OCCURRENCE IN THE PROTEROZOIC DHALA BASIN, MADHYA PRADESH 185 Fig.3. Photographs of cores depicting different features of peperitic zones:(a) A longitudinally cut core-section showing vesicular, fragmented rhyolite clasts dispersed in fine grained sediment. The clasts show jig-saw fit structure (upper triangular fragment), a characteristic of in-situ fragmentation; also observe the fluidal morphology of peperites and necking. (B) Peperites with brittly (in upper part ) as well as ductility(in lower part) deformed sediments (gray) characterised by small scale displacements, fracturing and distorted primary beddings. The structures are suggestive of soft sediment deformations. (C) Differential deformations in sediments along different interfaces with rhyolite: mild to negligible distortion of the sediment along the lower contact of rhyolite (lower part) and highly distorted sediment along upper contact. Also note the intrusion of fluidized sediment into the rhyolite (arrow).diameter of cores: 42 mm and photo top is the sample top for all. distorted primary beddings in sediments (Figs. 3, 4 and 5). On the other hand, magmatic part is represented by vesicular/ non- vesicular rhyolite. The features of the sediment and associated rhyolite in these intermixed zones are suggestive of emplacement of hot magma into unconsolidated wet sediments and their interaction, resulting into distortion of primary depositional structures and in-situ fragmentation of the magma. The mixed rocks formed by intermixing of hot magma (the volcanics) and the sediments with above mentioned characteristics are recognized as peperites following the definition of White (2000) and Skilling et al. (2002). The Fig.2. Litholog of peperitic zone as observed in a borehole core close to interface between Dhala sediment and underlying felsic volcanic, Mohar area. Depths are in metres from borehole collar.

4 186 K. K. SINHA AND OTHERS peperites are confined to ~20 m thick zone in basal part of the Dhala and is termed as peperitic zone (Fig.2). Megascopically and microscopically, two types of peperites are identified from Mohar Caldera: - blocky and globular/ fluidal. The blocky peperites are sub-equant, ragged, irregular polygonal to tabular shapes (Fig.6). Fluidal peperites also exhibit variable morphology like bulbous, globular, lobate, interconnected and wispy (Fig.7). Both types show jig-saw fit structure, but less frequent in globular ones. The microglobular peperites are often found to be partly rimmed with fluidized finer sediments that propagated downwards into the magma along fractures causing in-situ fragmentation of the magma. In the peperitic zone, sediments also occur within the rhyolite as lobes, tongues, slivers, clasts or globules (Fig.9). They are either completely detached from the host rock or still have preserved propagation path. Thin and zig-zag fluidized-pipes are common (Fig.9). There are a few narrow zones with fine and roughly parallel tubelike structures, with different orientation than the original sediment structures (Fig.10). They are enriched in fine sediments and assume a broader convex front gradually away from the contact, displacing and carrying the coarser one as an apron, a characteristic of viscous fluidal movement. The contact between rhyolite and the sediment also shows evidence of ductile shearing on microscopic scale. The peperitic zone gradually grades into coherent rhyolite away from the sediment-rhyolite contact. DISCUSSION Peperite is a genetic term applied for rocks formed essentially in-situ by intermixing and intermingling of unconsolidated sediment and magma and its positive identification must fulfil following criteria (White et al. 2000; Skilling et al. 2002): 1. Evidence of unconsolidated or poorly consolidated wet sediment. 2. Evidence of hot magma and its in-situ disintegration in course of intermingling with the sediment. In case of Mohar peperites, presence of unconsolidated/ poorly consolidated wet sediment is inferred on the basis of ductile deformation of sediments causing distortion of original depositional structures (Figs. 3, 4 and 5) and various sediment-fluidization features such as sediment lobes and tongues into rhyolite, vesiculation of sediments and fluidization-pipes (Figs. 9 and 10) (Kokelaar, 1982; Busby- Spera and White 1987; Skilling et al. 2002). Intrusion of hot magma into wet and unconsolidated sediments is evidenced by localised silicification, represented by larger quartz grains/grain aggregate along the sediment-rhyolite interface (Fig.8) which is interpreted as baking due to thermal effects on the sediment in contact with hot magma (Hunns and McPhie, 1999; Gifkins et al. 2002). It is further supported by downwardly propagated fluidized sediments into magma along cracks causing in-situ fragmentation and yielding globular peperites (Brooks, 1995; Wohltz, 2002). The presence of vesicles in the host sediment (Fig. 9) is another indicative of hot magma. The steam generated from heating of the pore fluids or exsolved gas from the hot magma may have caused vesiculations in the host sediments (Hunns and McPhie, 1999). The fluidization usually involves the finer sediments, preferably the matrix which possesses higher density than a rhyolitic magma (Donaire et al. 2002). In case of Mohar peperites, this fact is observed by rare occurrence of tiny rhyolitic globules floating over fluidized sediment. Such fluidized sediments flow as a viscous fluid and being denser, can lobe into rhyolitic melt (Fig. 9). The structure resembles drilling of Busby-Spera and White (1987) and Skilling et al. (2002) but without steam explosion and can be called passive drilling. Such drilling is a strong evidence of intrusion of hot magma into wet sediments. Presence of localized fines-enriched zones with thin and roughly parallel tubular structures, destructing the original sediment structures (Fig.10) are other evidence suggestive to fluidization of sediments (Kokelaar, 1982; Martin and Nemeth, 2002; Skilling et al. 2002). The blocky peperites are often interpreted as a Figs Photomicrographs. (4) Ductile deformation of primary sedimentary structure in fine sediments in a peperitic zone; arrows mark the deformational planes; 1N. (5) Protruding rhyolite lobes and tongues (brownish red) into sediment deforming its primary structures; also note the vesicles filled with quartz (homogeneous light colour) and fracture-filled sediments (inhomogeneous light colour patches marked as S) within rhyolite;1n. (6) Blocky peperites with tabular morphology (P) and also jig-saw fit structure; 2N. (7) In-situ fragmented globular/fluidal peperites (GP) as evidenced by jig-saw fit structure, also note the wispy nature of peperites; 1N. (8) Silicification (arrow) along the contact between rhyolite and sediment; discernable alignments of crystallites/microlites defining flow-structures in rhyolite; V is quartz-feldspar filled vesicle; 2N. (9) Bulbous lobe of fluidized sediment with concentric wrinkles due to its viscous flow; also note vesicles (V) in sediment and fluidized pipes (small arrows); thicker arrow points sediment-infill into rhyolite along cracks; 1N. (10) Fluidization of finer sediments marked by fine tube (arrows) and fan-like structures displacing coarser fraction that forms an apron around fluidized finer sediments; 1N.

5 PEPERITE OCCURRENCE IN THE PROTEROZOIC DHALA BASIN, MADHYA PRADESH 187

6 188 K. K. SINHA AND OTHERS consequence of quench fragmentation of magma and dynamic stressing of cooling magma on contact with wet sediments. The jig-saw fit structure exhibited by both types of peperites (Figs.3, 6 and 7) is a result of in-situ fragmentation of the magma (Kokelaar, 1982; Wohletz, 2002; Skilling et al. 2002). Gradual gradation of the peperitic zone into coherent rhyolite away from the sediment-rhyolite contact is also a positive evidence of in-situ melt fragmentation (Skilling et al. 2002). All these evidences lead to a definite and conclusive presence of peperite at and close to the interface of rhyolite and Dhala sediment in Mohar area. Significance of Mohar Peperites Peperites demonstrate definite contemporaneity of volcanism and sedimentation (Busby-Spera and White 1987; Brooks, 1995; Donaire et al. 2002; Martin and Nemeth 2007). This fact leads to an important inference that there is a temporal relationship between Dhala sedimentation and felsic volcanism in Mohar area. It implies that some volcanic activities took place even contemporaneous to sedimentation of basal part of the Dhala and the interaction between the intruding hot magma and the unconsolidated sediments resulted in the formation of peperites. So, the occurrence of peperites in Mohar area clearly points to multiple phases of magma eruptions. Subsequent to the main phase of volcanism which is represented by the volcanics underlying the Dhala Formation, collapse into the top of the magma chamber (represented by collapse breccias) took place resulting into the development of a caldera. The formation of the caldera was followed by sedimentation of Dhala Formation into this intra-caldera basin. When the sedimentation commenced in the basin, the volcanic activities were waning out but had not completely ceased. A few minor phases of volcanism took place even contemporaneous to sedimentation and interaction between two resulted in the formation of peperites. Therefore, the presence of peperites close to the rhyolite-sediment interface is a strong evidence for the volcanic origin of the Dhala structure/basin as suggested by Jain et al. (2001) and inconsistent with either its impact related (Pati et al. 2007, 2008) or shear rotation (Basu, 2007). Due to temporal relationship between Dhala sedimentation and felsic volcanism in Mohar area, the age of the volcanics can also provide constraint on the time of commencement of sedimentation in the basin. Rb-Sr systematics on the rhyolite underlying the Dhala has yielded an age of 1024±79 Ma (pers. Comm. D. Bhattacharya, 2009). Further, after critical review of available geochronological data on the Vindhyans, Ray (2006) concluded that the time span of sedimentation for the Lower Vindhyans and the Upper Vindhyans were 1721 Ma to 1600 Ma and ~1100 Ma to ~650 Ma respectively. It has also been noted that the unconformity between Semri and Kaimur basin is wide and a pronounced one. The hiatus lasted over a few hundred million years (Ray, 2006) and is ubiquitously marked by a thick zone of conglomerate. On the other hand, the unconformity between the Dhala and the overlying Kaimur is marked by merely a few centimetre thick pebbly/ conglomeratic horizon (15-20 cm). Also the two stratigraphic units are in perfect structural concordance with similar subhorizontal dips. The hiatus between two units is not of the scale as expected between Semri and Kaimur. Thus, the available age data on the rhyolite, its contemporaneity with the Dhala sediments as indicated by the peperites and the nature of unconformity present between Dhala and the overlying Kaimur, strongly suggest that the Dhala Formation should reasonably be correlated to Kaimur Group and unlikely with the Semri or the Bijawar as suggested by Jain et al. (2001) and Basu (2007). The incidence of basin-wide volcanism during early upper Vindhyan, i.e. at the base of the Kaimur (Bose et al. 2001) which could be represented by Mohar volcanics in the area also corroborate the inference drawn. Thus, the sedimentation in the Dhala Basin started in late Meso- Proterozoic time (~1024 Ma) which is also comparable with available and widely accepted age data of 1140±24 Ma (Crawford and Compston, 1970) and 1067±31 Ma (Kumar et al. 1993) for the Kaimur Group. CONCLUSION Two types of peperites, blocky as well as fluidal/globular with different morphologies have been identified for the first time from basal part of Dhala Formation close to the underlying rhyolite in Mohar caldera. The different features associated with them provide strong evidence of interaction of hot rhyolitic magma with wet and unconsolidated/partly consolidated sediments. As the peperite is a definite reflection of contemporaneity of volcanism and sedimentation, this new finding may provide new insights in understanding of Vindhyan geology as well as tectonism of Bundhelkand craton as a whole. It also provides a tool to correlate and assign stratigraphic position of a sedimentary unit with disputed status like the Dhala. In the light of occurrence of peperites in Mohar area, the origin of the Dhala basin/structure is inconsistent with the impact but volcanism related. In the present paper, characteristics of Mohar peperite vis-a-vis its importance in stratigraphic correlation is highlighted and it has been convincingly shown

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