ABSTRACT Two-dimensional depth-migrated seismic data were used to interpret and analyze extension and salt deposition in the ocean-continent transition (OCT) along 720 km of the southern Gulf of Mexico rifted margin. The OCT is characterized by alternating areas of salt-filled, fault-bounded outer troughs overlying a shallow Moho and salt perched at a level above the top of oceanic crust. Normal faults and the limit of oceanic crust are both offset by two sets of transfer faults and paleo–transform faults, respectively, that trend NNW-SSE and N-S. The patterns define five OCT segments that show propagation of both rifting and spreading to the NE, an abrupt jump in pole location, and rifting/spreading nuclei that link up laterally. Salt was deposited during outer trough formation to the SW but prior to it in the NE, where salt consequently flowed from proximal locations into the growing trough during decoupled thick-skinned extension. The salt was deposited at least 0.5–1.5 km below global sea level, with precipitation initially confined to the oldest troughs (in the west) and subsequently spreading to cover the entire basin in a deep brine over a period of at least 5 m.y. Possible siliciclastic strata interbedded with the salt were likely sourced from the south and southeast, and hypersaline conditions waned gradually during punctuated marine flooding over another 5–10 m.y. The Gulf of Mexico was thus a giant evaporite basin formed in a deep depression during late-synrift mantle exhumation in a magma-poor setting, analogous to the South Atlantic salt basins and possibly the Red Sea and southern Moroccan/Scotian margins.

Abstract This is the first of two chapters in which the data presented in the first four chapters of this Memoir are reviewed and interpreted. A summary cross-section across the basin from the Saudi outcrop to Abu Dhabi and the Tethyan margin is used to summarize the regional setting and sequences as interpreted in this Memoir. The key points illustrated by the summary cross-section are stated and discussed in the first part of this chapter. Two different interpretations for the eastern margin of the intrashelf basin are reviewed. In the first of these interpretations, the margin with the deeper waters of the Tethys Ocean shelf is adjacent to the intrashelf basin rim in Abu Dhabi, whereas in the second, preferred in this Memoir, a broad, shallow Tethyan shelf platform (200–300 km wide) extends from the intrashelf basin rim to the Tethys continental shelf edge. The implications of Late Jurassic exposure and erosion on the adjacent Tethyan shelf are discussed. The development of the Arabian Intrashelf Basin during the Callovian–late Oxfordian (the Tuwaiq Mountain Formation, the source rock and the Hanifa intervals and associated sequences) is interpreted and discussed with illustrations, including step-by-step facies maps. The interpretation integrates depositional, eustatic and tectonic factors in the evolution of the intrashelf basin. The interpretation in this Memoir is that the Arabian Intrashelf Basin began with isostatic and extensional subsidence on top of a broad Dhruma Atash Member platform, which had largely filled the accommodation space up to the wave base and to near sea-level. It developed fully into an intrashelf basin during the deposition of the Callovian Tuwaiq sequence, with rising sea-levels coincident with a productive shallow water carbonate factory resulting in a rim of shallow water carbonate. An end-Callovian to early Oxfordian lowstand terminated the Tuwaiq sequence on the basin rim. During the lowstand, restricted conditions in the basin deposited the rich Lower Hanifa source rock as a lowstand systems tract. As more normal conditions returned in a subsequent sequence, the source rock facies graded upwards into Hanifa reservoir facies, which partially filled the basin. Hanifa progradation was terminated by another lowstand during which a subaqueous gypsum/anhydrite marker bed was deposited in at least part of the remnant basin. Earlier interpretations of these sequences are also discussed.

Microbial carbonates developed in the Middle Triassic (Leikoupo Formation, Anisian Stage) of the western Sichuan Basin. The microbial components have been identified and include Renaclis-resembling , Rivularia lissaviensis , Carpathocodium anae , Hedstroemia moldavica , Bacinellacodium calcareus , and Paraortonella getica . These form stromatolitic, laminitic, thrombolitic, spongiostromata stones, dendrolites, and oncolitic structures. Microbial carbonate reservoirs occur in submember unit (SMU) 3-3 in the Zhongba area of the northern segment and SMU 4-3 in the middle segment of the western Sichuan Basin, both of which are of low porosity and permeability. Core descriptions and thin-section analysis show that reservoir porosity is mostly microbial coelom pores, framework pores, fenestral pores, and inter- and intraclot dissolved pores, within which the pores of ≥200 μm in diameter and throat of (40~50) μm are the most important. The SMU 4-3 microbial carbonate reservoirs are more thoroughly studied because of recent exploration activities, including the identification of three reservoir intervals. The middle reservoir interval, composed of thrombolitic and stromatolitic dolostone, hosts the reservoir of best quality. However, this high-quality interval loses effective porosity and thins to the northeast. It is proposed that extreme geological conditions, dolomitization, and burial dissolution influenced the development and distribution of the microbial carbonate reservoirs. The dolomitization process is thought to be penecontemporaneous to very early postdeposition. This early dolomitization contributed significantly to porosity of the microbial carbonate reservoirs and was likely enhanced through burial dissolution.