M. Royhan Gani

An overview of the sedimentary geology of the Bengal Basin in relation to the regional tectonic framework and basin-fill history

Abstract The Bengal Basin in the northeastern part of Indian subcontinent, between the Indian Shield and Indo-Burman Ranges, comprises three geo-tectonic provinces: (1) The Stable Shelf; (2) The Central Deep Basin (extending from the Sylhet Trough in the northeast towards the Hatia Trough in the south); and (3) The Chittagong–Tripura Fold Belt. Due to location of the basin at the juncture of three interacting plates, viz., the Indian, Burma and Tibetan (Eurasian) Plates, the basin-fill history of these geo-tectonic provinces varied considerably. Precambrian metasediments and Permian–Carboniferous rocks have been encountered only in drill holes in the stable shelf province. After Precambrian peneplanation of the Indian Shield, sedimentation in the Bengal Basin started in isolated graben-controlled basins on the basement. With the breakup of Gondwanaland in the Jurassic and Cretaceous, and northward movement of the Indian Plate, the basin started downwarping in the Early Cretaceous and sedimentation started on the stable shelf and deep basin; and since then sedimentation has been continuous for most of the basin. Subsidence of the basin can be attributed to differential adjustments of the crust, collision with the various elements of south Asia, and uplift of the eastern Himalayas and the Indo-Burman Ranges. Movements along several well-established faults were initiated following the breakup of Gondwanaland and during downwarping in the Cretaceous. By Eocene, because of a major marine transgression, the stable shelf came under a carbonate regime, whereas the deep basinal area was dominated by deep-water sedimentation. A major switch in sedimentation pattern over the Bengal Basin occurred during the Middle Eocene to Early Miocene as a result of collision of India with the Burma and Tibetan Blocks. The influx of clastic sediment into the basin from the Himalayas to the north and the Indo-Burman Ranges to the east rapidly increased at this time; and this was followed by an increase in the rate of subsidence of the basin. At this stage, deep marine sedimentation dominated in the deep basinal part, while deep to shallow marine conditions prevailed in the eastern part of the basin. By Middle Miocene, with continuing collision events between the plates and uplift in the Himalayas and Indo-Burman Ranges, a huge influx of clastic sediments came into the basin from the northeast and east. Throughout the Miocene, the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin. From Pliocene onwards, large amounts of sediment were filling the Bengal Basin from the west and northwest; and major delta building processes continued to develop the present-day delta morphology. Since the Cretaceous, architecture of the Bengal Basin has been changing due to the collision pattern and movements of the major plates in the region. However, three notable changes in basin configuration can be recognized that occurred during Early Eocene, Middle Miocene and Plio-Pleistocene times, when both the paleogeographic settings and source areas changed. The present basin configuration with the Ganges–Brahmaputra delta system on the north and the Bengal Deep Sea Fan on the south was established during the later part of Pliocene and Pleistocene; and delta progradation since then has been strongly affected by orogeny in the eastern Himalayas. Pleistocene glacial activities in the north accompanied sea level changes in the Bay of Bengal.

Blue Nile incision on the Ethiopian Plateau: Pulsed plateau growth, Pliocene uplift, and hominin evolution

The 1.6-km-deep Gorge of the Nile, a rival of the Grand Canyon, resulted from the deep incision of the Blue Nile drainage into the uplifted Ethiopian Plateau. Understanding the incision history of the plateau is crucial to unraveling the Cenozoic tectonoclimatic evolution of the region, particularly because the region has long been used as a natural laboratory to understand the geodynamics of continental rifting and the evolution of hominins. We undertake a quantitative geomorphologic approach integrating field, geographic information system (GIS), and digital elevation model (DEM) data to analyze incision (volume, long-term rates, and spatiotemporal variability) and river longitudinal profiles of the Blue Nile drainage. Previously published isotopic ages of the Cenozoic volcanic rocks are used to constrain long-term incision rates through geologic time. Our data argue that (1) the Blue Nile drainage has removed at least 93,200 km 3 of rocks from the northwestern Ethiopian Plateau since ca. 29 Ma (early Oligocene) through a three-phase (ca. 29–10 Ma, ca. 10–6 Ma, and ca. 6 Ma to present) incision, where long-term incision rates increased rapidly and episodically in the late Miocene (ca. 10 Ma and ca. 6 Ma); (2) being out-ofphase with the past climatic events and in-phase with the main volcanic episodes of the region, this episodic increase of incision rate is suggestive of episodic growth of the plateau; (3) of the ~2-km rock uplift of the plateau since ca. 30 Ma, 0.3 km was due to isostatic uplift related to erosional unloading, and the rest was due to other tectonic activities; (4) the extremely rapid long-term incision rate increase, thus a rapid uplift of the plateau, ca. 6 Ma might be related to lithospheric foundering, caused by ponded plume material beneath the Ethiopian Plateau and aided by huge tectonic stresses related to the Messinian salinity crisis of the Mediterranean Sea. These events could have caused the plateau to rise >1 km within a few m.y. in the early Pliocene. This uplift history of the Ethiopian Plateau can shed critical light on the geodynamics of the Afar mantle plume and the evolution of the East African hominins via climate change.

Trench-slope controlled deep-sea clastics in the exposed lower Surma Group in the southeastern fold belt of the Bengal Basin, Bangladesh

Abstract Deltaic to shallow marine (neritic) depositional settings have until now been the accepted interpretation for the Neogene Surma Group of rocks exposed throughout the southeastern fold belt of the Bengal Basin. The present study revises the earlier views and proposes some new insights. On the basis of detailed field studies carried out in the Sitapahar anticline, Rangamati area, and in the Mirinja anticline, Lama area, it is here proposed that the Surma Group succession represents an overall basinward progradation from deep marine to coastal marine depositional settings. Sedimentological evidence strongly suggests that the lower part of the Surma Group represents a slope apron, the growth of which is thought to have been governed by a westward-migrating accretionary prism complex within the active margin setting of the Indo–Burmese plate convergence. Thin packages of distinct turbidity-current-generated deposits together with some slump and debris-flow deposits contained within thicker intervals of mudstone characterize the lowermost exposed unit of the Surma Group in the Sitapahar anticline. The overlying slope deposits are essentially mudstone-dominated. Comparable deep-sea clastics with thicker intervals of sandstone turbidites, contained within a submarine canyon, are present in the Mirinja anticline. This new model suggests that it may be timely to re-evaluate the existing stratigraphic and tectonic framework of the eastern Bengal Basin.

Sedimentation and basin-fill history of the Neogene clastic succession exposed in the southeastern fold belt of the Bengal Basin, Bangladesh: a high-resolution sequence stratigraphic approach

Abstract The Tertiary basin-fill history of the Bengal Basin suffers from oversimplification. The interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo-tectonic provinces, namely, western, northeastern and eastern. Each province has its own basin generation and sediment-fill history related mainly to the Indo-Burmese and subordinately to the Indo-Tibetan plate convergence. This paper is mainly concerned with facies and facies sequence analysis of the Neogene clastic succession within the subduction-related active margin setting (oblique convergence) in the southeastern fold belt of the Bengal Basin. Detailed fieldwork was carried out in the Sitapahar anticline of the Rangamati area and the Mirinja anticline of the Lama area. The study shows that the exposed Neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental–fluvial environments. Based on regionally correlatable erosion surfaces the entire succession (3000+ m thick) has been grouped into three composite sequences C, B and A, from oldest to youngest. Composite sequence C begins with deep-water base-of-slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics. Composite sequence B characteristically depicts tide-dominated open-marine to coastal depositional systems with evidence of cyclic marine regression and transgression. Repetitive occurrence of incised channel, tidal inlet, tidal ridge/shoal, tidal flat and other tidal deposits is separated by shelfal mudstone. Most of the sandbodies contain a full spectrum of tide-generated structures (e.g. herringbone cross-bedding, bundle structure, mud couplet, bipolar cross-lamination with reactivation surfaces, ‘tidal’ bedding). Storm activities appear to have played a subordinate role in the mid and inner shelf region. Rizocorallium , Rosselia , Planolites and Zoophycos are the dominant ichnofacies within the shelfal mudstone. This paralic sedimentation of Neogene succession in the study area can serve as a good point of reference for tide-dominated regressive shelf depositional systems. The top of the composite sequence B is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental–fluvial depositional systems represented by composite sequence A. In this composite sequence, stacked channel bars of low-sinuosity braided rivers gradually pass upsequence into high-sinuosity meandering river deposits. A sequence stratigraphic approach has been adopted to interpret the basin-fill history with respect to relative sea-level changes; and to subdivide the rock record into several sequences and units (systems tracts and parasequences) based on identified bounding discontinuities, such as transgressive erosion surface (TES), regressive erosion surface (RES), marine flooding surface (MFS), and incised valley floor (IVF). This approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the Bengal Basin. It is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin, which is based on the Assam stratigraphy, be abandoned or at least revised. A tentative allostratigraphic scheme is presented, and it is suggested that to formalize the scheme further study, both surface and subsurface, is needed.

Three-dimensional facies architecture and three-dimensional calcite concretion distributions in a tide-influenced delta front, Wall Creek Member, Frontier Formation, Wyoming

Ground-penetrating radar (GPR) has been used to image the three-dimensional (3-D) internal structure (and, thus, the 3-D facies architecture) of a top-truncated delta front in the topmost parasequence in the Wall Creek Sandstone Member of the Frontier Formation in Wyoming and to estimate the distribution of low-permeability concretions throughout the 3-D GPR volume. The interpretation of the 3-D GPR data is based both on correlations with outcrop and on calibration with core data from holes within the survey grid. Two main radar facies (RF) are identified. Radar facies 1 corresponds to tide-influenced mouth bars formed by a unidirectional flow during delta progradation or bidirectional flow during tides, whereas RF2 is correlated with laterally migrating channels developed on previous bar deposits. The delta-front foreset beds dip in the same direction as the dominant paleocurrent indicators. The GPR interpretation is consistent with the outcrop interpretation that, following a regressive period, bars and channels were developed at the Raptor Ridge site before subsequent transgressive ravinement. The individual 3-D deltaic facies architectures were reconstructed from the 3-D GPR volume and indicate that the depositional units are larger than the survey grid. Cluster analysis of the GPR attributes (instantaneous amplitudes and wave numbers) calibrated with the cores and the outcrop was used to predict the distribution of near-zero permeability concretions throughout the 3-D GPR volume; clusters of predictive attributes were defined and applied separately in the bars and channels. The predicted concretions in the bars and the channels are 14.7 and 10.2% by volume, respectively, which is consistent with those observed in the cores (14.7 and 10.5%, respectively), and their shape and thickness are also generally in consonance with those in the outcrop and cores. The estimated concretions are distributed in an aggregate pattern with irregularly shaped branches within the 3-D GPR volume, indicating that the cementation does not follow a traditional center-to-margin pattern. The concretions and 3-D geological solid model provide cemented flow baffles and a 3-D structural framework for 3-D reservoir modeling, respectively.

On the Environment of Aramis: Concerning Reply of White to Cerling et al. in August 2014

White (2014) made several comments on our paper (Gani and Gani 2011) while replying to Cerling et al. (2014). His comments contradict our field descriptions presented in Gani and Gani (2011). Based on our fieldwork at the Aramis site, Ethiopia, where a near-complete skeleton of Ardipithecus ramidus was excavated, we presented our field data with their analysis and interpretation in Gani and Gani (2011). We interpreted our data to indicate the presence of rivers and associated mixed vegetation (grasses and trees) in adjacent floodplains, which constitute the environmental context of the very place and time where/whenA. ramidus likely lived and died. It is a well-known fact that scientists can have different interpretations while scrutinizing the same data set, but their field descriptions (that constitute the data set) should not be contradicting. Therefore, in this reply, we focus on the basic field observations and descriptions (rather than the interpretations) of Gani and Gani (2011) that White questioned. White (2014) states that Gani and Gani (2011) “mistook a nearby tufa with uncemented mudstone pellets deposited in standing shallow water as a crossbedded fluvial sandstone” (471). Tufa is a nonclastic limestone formed by the in situ precipitation of carbonate minerals in fluvial or lake environments, whereas sandstone is a clastic sedimentary rock composed of sand-sized (0.063–2 mm) detrital (i.e., transported) grains like quartz, feldspar, and lithoclast. Therefore, it should not be difficult to differentiate tufa from sandstone in the field. Although we cannot discredit the presence of tufa in nearby localities, we observed the sedimentary rock in question with a hand lens (20#) and a grain-size card and determined it to be a fine-grained (0.125–0.25 mm) sandstone composed of various detrital grains including quartz. In these sandstone beds, we found numerous scattered lithoclasts of igneous origin that

Applying modern interpretation techniques to old hydrocarbon fields to find new reserves: A case study in the onshore Gulf of Mexico, USA

AbstractThis study shows how the use of current geological investigative techniques, such as sequence stratigraphy and modern seismic interpretation methods, can potentially discover additional hydrocarbons in old fields that were previously considered depleted. Specifically, we examine the White Castle Field in South Louisiana, which has produced over 84.1 million barrels of oil and 63.1 billion cubic feet of gas but retains additional recoverable hydrocarbons. The field has pay sections ranging from late Oligocene to late Miocene. The upper Oligocene to early Miocene package, which was underexploited and understudied during the previous exploitation phase, contains three primary reservoirs (Cib Haz, MW, and MR). During most of the late Oligocene, the White Castle Salt Dome was located in a minibasin on the continental slope. The Cib Haz and MW reservoirs were deposited in this minibasin and offer great exploitation potential. The Cib Haz interval is an amalgamation of slumped shelfal limestones, sandsto...