Marine and Petroleum Geology 68 (2015) 480e491 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo Research paper Geological structures and controls on half-graben inversion in the western Gunsan Basin, Yellow Sea Young Jae Shinn Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea article info abstract Article history: This paper focuses on the structural styles of an inverted half-graben in the western Gunsan Basin Received 11 March 2015 (CretaceousePaleogene) in the Yellow Sea. Detailed seismic interpretation calibrated by well data in- Received in revised form dicates that the half-graben formed by the Cretaceous to Eocene extension and subsequently underwent 26 August 2015 contraction which led to the basin inversion. The inversion occurred during the Oligocene and Middle Accepted 28 September 2015 Miocene. In the Oligocene inversion phase, the hanging-wall strata underwent shortening by inverted Available online 13 October 2015 extensional faults, newly-formed reverse faults, and fault-related folds. The inversion deformation was caused by an NNE-directed regional contraction that is nearly normal to the orientation of earlier Keywords: Inverted half-graben extensional faults. The style and distribution of internal deformation was mainly governed by variations Inversion structures in the dip angle of the pre-inversion fault plane. The Early Miocene inversion phase commenced with the Gunsan Basin development of a broad asymmetric hanging-wall anticline caused by the preferential reactivation of the Northern South Yellow Sea Basin NW-trending bounding fault. This preferential reactivation suggests an orientation change of inversion stress from the Oligocene NNE-directed to the Miocene NE-directed contraction. The bounding fault continued to be reactivated during the Middle Miocene, which caused the growth of a monocline. © 2015 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction from the Bohai Bay, East China Sea, Subei, and Songliao basins (Ma et al., 1989; Allen et al., 1997; Liu et al., 2000; Grimmer et al., 2002; The Cretaceous to Paleogene Gunsan Basin is part of the Northern Su et al., 2009). A few studies have focused on the spatial and South Yellow Sea Basin that formed as a large intracontinental basin temporal variations of inversion structures to understand their between the Tan-Lu fault system and the subduction margin of the geological controls and inversion history, although the Oligocene proto-Pacific plate (Fig. 1). The Gunsan Basin initially formed by an inversion was widely recognized from sedimentary basins in East extension under a transtensional tectonic setting during the Creta- Asia. ceouseEocene and was subsequently deformed by the Oligocene This study further elucidates deformation styles of a low-to- contraction (Ryu et al., 2000; Yi et al., 2003; Park et al., 2005; Shinn moderately inverted half-graben based on seismic reflection data et al., 2010). The contraction was interpreted to be associated with a in the western Gunsan Basin. Restoration and balancing of cross- dramatic change in the stress regime from dextral transtension to sections are used to validate geological interpretation and to eval- sinistral transpression along the Tan-Lu fault (Shinn et al., 2010), uate the timing and amount of deformation through time (e.g., which caused tectonic inversion of the Gunsan Basin in a complex Hossack, 1979; De Paor, 1988; Buchanan, 1996; Coward, 1996; manner and formed an inverted half-graben that was particularly Bulnes and McClay, 1999; Dubey et al., 2001; Zhou et al., 2006). developed in the western Gunsan Basin. This study will help to reveal the main factors responsible for The inverted half-graben in the western Gunsan Basin repre- structural variability of a low-to-moderately inverted half-graben. sents typical inversion structures affected by reverse reactivation of extensional faults and related folds (Shinn et al., 2010), which likely 2. Geologic setting reflects multi-episode deformation with varying degree of inver- sion style. Regional Oligocene inversion has also been documented The Gunsan Basin is the eastern extension of the Northern South Yellow Sea Basin, located in the central Yellow Sea between the Shandong Peninsula and Korea (Fig. 1). The basin is bounded by the E-mail address: [email protected]. Qianliyan Massif to the north and by the Central Massif to the south http://dx.doi.org/10.1016/j.marpetgeo.2015.09.013 0264-8172/© 2015 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 481 Fig. 1. Distribution map of the Cretaceous to Paleogene sedimentary basins around the study area and major structures of the Northern South Yellow Sea Basin and Gunsan Basin (modified from Zhang et al., 1989; Yi et al., 2003; Shinn et al., 2010). Abbrevia- tions: GB ¼ Gunsan Basin; NSYSB ¼ Northern South Yellow Sea Basin; SSYSB ¼ Southern South Yellow Sea Basin; SB ¼ Subei Basin. (Zhang et al., 2007; Wu et al., 2008). The southern margin of the basin is marked by a series of ENE- or NE-striking basement faults with significant throws. To the north of the basin, the Qianliyan fault continues with the north-bounding fault of the Subei Basin, which is part of a wrench fault branching from the Tan-Lu fault system (Fig. 1). The distribution and geometry of the basin were largely controlled by the eastward-fanning bounding faults and the associated NW-striking fault set (Shinn et al., 2010). The formation of the Gunsan Basin was initiated during the Early Cretaceous when sinistral wrench faulting affected the eastern margin of the Eurasian continent (Xu et al., 1987; Lee, 1999; Chough et al., 2000; Shinn et al., 2010). The regional wrench-induced, normal faulting formed fault-bounded depressions and structural highs. The depressions continued to subside until the regional uplift and erosion occurred and the following Miocene succession covered the basin. The basin was infilled by up to 6 km of non- marine successions mostly comprising CretaceouseEocene allu- vial to fluvio-lacustrine deposits with subordinate volcanic rocks (Ryu et al., 2000; Yi et al., 2003). The overlying Lower Miocene successions are marked by a regional unconformity at the base and areas of local subsidence were developed to accommodate the reactivation of faults. The main phase of postrift subsidence began in the Middle Miocene (Ryu and Kim, 2007). The Middle Miocene to present-day succession is characterized by flat-lying configuration of continuous seismic reflectors and basin-wide distribution with uniform thickness. These seismic characteristics are indicative of tectonic quiescence and several events of marine transgression. The Middle Miocene to present-day succession cannot be subdivided Fig. 2. Lithologic column, biostratigraphy, and depositional environment of the due to the lack of geological information from exploration wells. Kachi-1 well (after Ryu et al., 2000; Yi et al., 2003). The basement rock of the Gunsan Basin is unconformably overlain by the Cretaceous to Eocene synrift strata. It varies in age and lithology over the basin, containing sedimentary and meta- JurassiceEarly Cretaceous, Early Cretaceous, and Late Cretaceous morphic rocks of the Paleozoic Yangtze Platform (Kim and Oh, units (Yi et al., 2003; Ryu and Kim, 2007)(Fig. 2). The Late Juras- 2007; Wu et al., 2008). The maximum depth of the basement siceEarly Cretaceous unit mainly contains poorly sorted, arkosic rock in the Gunsan Basin was estimated to range from 6 to 8 km on sandstones with tuffs, resulting from a rapid deposition in alluvial the basis of magnetic and gravity data (Park et al., 2009). environment (Yi et al., 2003). Interbedded reddish mudstones and The Cretaceous succession encountered in the Kachi-1 well sandstones become predominant in the Early Cretaceous unit, drilled in the western Gunsan Basin is up to 2079 m in thickness which reflects depositions in fluvial and floodplain environments (Fig. 2). The Kachi-1 well reached the basement rock of the Triassic (Yi et al., 2003). The overlying Late Cretaceous unit is characterized dolomite (Yi et al., 2003). Based on lithological and micropaleon- by interbeds of dark gray mudstones and limestones deposited in tological data, the Cretaceous succession was subdivided into Late fluvio-lacustrine to deltaic and marginal lacustrine environments 482 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 (Yi et al., 2003; Ryu and Kim, 2007). In the Kachi-1 well, the upper part of the Cretaceous unit is incomplete and the PaleoceneeEarly Miocene unit is missing due to significant uplift and erosion. The Middle Miocene succession rests unconformably on the Cretaceous succession and consists mainly of unconsolidated sands inter- bedded with clays and lignites that are interpreted to be fluvio- deltaic in origin (Yi et al., 2003). 3. Results 3.1. Seismic to well correlation The available seismic reflection data for this study include composite multi-channel seismic reflection profiles acquired by the Korea National Oil Corporation (KNOC) and the Korea Institute of Geoscience and Mineral Resources (KIGAM). A grid of seismic data in the western Gunsan Basin mainly comprise SWeNE trending profiles perpendicular to the major structural trend and subordi- nate NWeSE and EeW trending profiles. The Kachi-1 well was drilled in an anticlinal structure for hydrocarbon exploration but did not encounter hydrocarbons. Well data were obtained from unpublished sedimentological and palynological reports of the KIGAM and KNOC, incorporated with the work of Ryu et al. (2000), Yi and Batten (2002), and Yi et al. (2003). The geological informa- Fig. 4. Time-structure map of acoustic basement showing the major fault distribution tion from the well data provides the calibration for the seismic and structural domain in the western Gunsan Basin.