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Growth and Demise of a Paleogene Isolated Carbonate Platform of the Offshore Indus Basin, Pakistan: Effects of Regional and Local Controlling Factors

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Growth and Demise of a Paleogene Isolated Carbonate Platform of the Offshore Indus Basin, Pakistan: Effects of Regional and Local Controlling Factors Published in "Marine Micropaleontology 140: 33–45, 2018" which should be cited to refer to this work. Growth and demise of a Paleogene isolated carbonate platform of the Offshore Indus Basin, Pakistan: effects of regional and local controlling factors Khurram Shahzad1 · Christian Betzler1 · Nadeem Ahmed2 · Farrukh Qayyum1 · Silvia Spezzaferri3 · Anwar Qadir4 Abstract Based on high-resolution seismic and well depositional trend of the platform, controlled by the con- datasets, this paper examines the evolution and drown- tinuing thermal subsidence associated with the cooling of ing history of a Paleocene–Eocene carbonate platform in volcanic margin lithosphere, was the major contributor of the Offshore Indus Basin of Pakistan. This study uses the the accommodation space which supported the vertical internal seismic architecture, well log data as well as the accumulation of shallow water carbonate succession. Other microfauna to reconstruct factors that governed the car- factors such as eustatic changes and changes in the carbon- bonate platform growth and demise. Carbonates domi- ate producers as a response to the Paleogene climatic per- nated by larger benthic foraminifera assemblages permit turbations played secondary roles in the development and constraining the ages of the major evolutionary steps and drowning of these buildups. show that the depositional environment was tropical within oligotrophic conditions. With the aid of seismic stratigra- Keywords Paleogene carbonate platform · Seismic phy, the carbonate platform edifice is resolved into seven sequence stratigraphy · Drowning · Offshore Indus Basin · seismic units which in turn are grouped into three packages Larger benthic foraminifera · Biostratigraphy that reflect its evolution from platform initiation, aggrada- tion with escarpment formation and platform drowning. The carbonate factory initiated as mounds and patches on Introduction a Cretaceous–Paleocene volcanic complex. Further, the growth history of the platform includes distinct phases of Paleogene carbonate platforms have been intensively intraplatform progradation, aggradation, backstepping and studied over the last few decades with respect to sea level partial drownings. The youngest succession as late-stage changes, shallow benthic biostratigraphy (Serra-Kiel http://doc.rero.ch buildup records a shift from benthic to pelagic deposition et al. 1998; Scheibner and Speijer 2009; Courgeon et al. and marks the final drowning in the Early Eocene. The 2016), paleoclimate (Scheibner et al. 2005, 2007; Scheib- ner and Speijer 2008a; Robinson 2011), environmental conditions (Hallock et al. 1991), reconstruction of depo- sitional systems and lithofacies of economic importance * Khurram Shahzad (Mresah 1993; Jorry et al. 2003; Scheibner et al. 2007; [email protected] Zamagni et al. 2008; Höntzsch et al. 2011). The Paleo- 1 Institut für Geologie, Universität Hamburg, gene was a time of major reorganizations of continents Bundesstraße 55, 20146 Hamburg, Germany and significant climate fluctuations such as the Pale- 2 MOL Pakistan Oil and Gas Co, B.V, Plot No. 5/A, ocene–Eocene thermal maximum (PETM) which culmi- Crown Plaza, F-7 Markaz, Islamabad, Pakistan nated with the Early Eocene Climatic Optimum (EECO). 3 Department of Geosciences, University of Fribourg, These events were marked by a prominent negative car- Chemin Du Musée 6, Fribourg, Switzerland bon isotope excursion (CIE) and carbonate dissolution in 4 Department of Geology, University of Haripur, Hattar Road, deep-sea sediment records (Zachos 2001). Shallow water Haripur, Pakistan carbonate platforms are acutely sensitive to such climatic 1 fluctuations and are fundamental archives encoding these On the northwestern periphery of the Indian Plate, cov- environmental changes (Schlager 1981; Hallock and ering an area of 3500 km2, the Paleogene carbonate system Schlager 1986; Schlager and Camber 1986; Betzler et al. represents the largest isolated carbonate platform system 2000, 2009, 2013). of the region (Fig. 1). In the past, regional studies on tec- Carbonate platforms develop as a result of different tonic and stratigraphic framework of these buildups have factors including tectonic, climate, eustatic variations, been performed. However, they did not address the internal siliciclastic and nutrient availability, and environmental stratigraphic architecture of these carbonate platforms and conditions (Schlager 2005). These factors determine the buildups (Carmichael et al. 2009), which is a key for under- relationship between carbonate production and accommo- standing the evolution of these platform. dation space. Hence, they control the internal sedimen- Considering the current research gaps, this study inves- tary architecture and facies distribution of the carbonate tigates the architectural evolution and drowning history of buildups (Sarg 1988; Sarg et al. 1999). Seismic interpre- one of the Paleogene carbonate platform of the Offshore tation becomes challenging when the carbonate deposi- Indus Basin, Pakistan, through a comprehensive analysis tional cycles operate over a magnitude of a few meters. of regional 2D seismic profiles and well data. In the light This problem leaves behind a poorly resolved strati- of seismic stratigraphic concepts and facies analysis, this graphic pattern (e.g., progradation) on a seismic scale. In study focuses on internal reflection geometry to trace the this situation, seismic data show an overall aggradational development phases and assess the intrinsic and extrinsic section, which can mistakenly be interpreted as a high- factors that governed the carbonate growth and demise. stand or a transgressive systems tract. Herein, the ques- Furthermore, this study documents the biotic assemblages tion of sequence internal stratification is elucidated by and constrains the ages of the major evolutionary stages of applying the HorizonCube method (Qayyum et al. 2012, platform growth and final drowning based on the biostrati- 2013). graphic analysis. Thus, the outcomes have significant Fig. 1 Geographical position of the Offshore Indus Basin. Bathymetry after Ryan et al. (2009). The study area is marked with the black rectangle and its position relative to the Indian Ocean is shown in the bottom right corner http://doc.rero.ch 2 implications for the understanding of the evolution of car- Calvès et al. 2011). The crustal thickness in the region var- bonate platforms, which indirectly helps better in predict- ies between 9 and 11.5 km (Calvès et al. 2008). Geohistory ing the depositional system and lithofacies of economic analyses, performed by various researchers, indicate a ther- importance. The depositional model as reconstructed mal subsidence phase with cooling of the Pakistan volcanic through this study may also serve as an analog to the sub- margin, followed by the induced flexuring responsible for surface carbonate platforms elsewhere. the basin development through the Cenozoic (Mohan 1985; Agrawal and Rogers 1992; Whiting et al. 1994). The tectonic elements of the Indus Basin encompass Geological background over 200 million years of geological history (Gaedicke et al. 2002; Chatterjee et al. 2013). The development of The Offshore Indus Basin is located in the eastern part of the western Indian margins started with the Early Jurassic the Arabian Sea, on the northwestern margin of the Indian breakup of the supercontinent Gondwana into East Gond- Plate. Tectonically, the basin is bounded by the Murray wana, comprising Antarctica, Madagascar, India, and Aus- Ridge—Owen Fracture Zone in the northwest (Fig. 2a), tralia, and West Gondwana, comprising Africa. During the which is a major plate boundary between the Indian and Early Cretaceous, the Indian Plate started to detach from Arabian Plate (Edwards et al. 2000). To the north, it is fac- the Antarctic and the Australian Plates and drifted north- ing the present-day continental shelf which is cut by the ward (Chatterjee et al. 2013). Further rifting resulted in the Indus Canyon and to the south it bounded by the Carlsberg separation of the Madagascar and the Indian Plate (Edwards Ridge. A subsurface buried structure is identified as the et al. 2000). This rifting phase was followed by the rapid Laxmi Ridge in the southeast of the basin and assumed to northward drift of the Indian Plate coupled with a counter- be a continental product of the early stages of Indian Plate clockwise rotation (Copley et al. 2010). During this time, drift (Naini and Talwani 1982; Talwani and Reif 1998). Two the Seychelles and India passed over the Réunion hotspot buried shield volcanoes, the Somnath Ridge and the Sau- and extensive volcanic activity occurred with the extrusion rashtra High, which form prolific structural features of the of the Deccan lava during the Late Cretaceous and Early basins, are hosting carbonate platforms (Malod et al. 1997; Paleocene (Duncan 1990; Todal and Edholm 1998). As a http://doc.rero.ch Fig. 2 a Map of tectonic and structural elements of the northwestern OFZ Owen-Fracture Zone, SR Somnath Ridge, SH Saurashtra High, margin of the Indian Plate and surroundings with the free air grav- BH Bombay High. b Late Paleocene reconstruction (~56 Ma) and ity (Sandwell et al. 2014). The bold black line indicates the position paleogeography of Indian Plate to show the tropical environment with of the cross section in Fig. 3. The dotted line labeled with “COB” ocean circulation and prevailing wind direction (Haq 1981; Aubert marks the continent-ocean boundary (after Carmichael et
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