Tectonic Control of Early Cretaceous Carbonate Platforms in the Gulf of Mexico

Abstract

ABSTRACT In the Gulf of Mexico, a review of basin-wide spatial relationships between Early Cretaceous carbonate platform margins and olderstructures, combined with multi channel! seismic reflection data from the northeastern Gulf, indicates that a tectonic hinge zone controlled the margins' initial positions. There is no evidence that this hinge zone influenced pre - Early Cretaceous sedimentation patterns throughout the basin. Thus, we think that it formed during latest Jurassic - earliest Cretaceous time. Such timing suggests that the hinge zone is a new type of geologic structure. During this period, the Gulf of Mexico was undergoing onlythermal subsidence. The main phases of intracrustal extension and sea floor spreading, the stages of basin evolution most commonly associated with tectonic hinge zone formation at other divergent continental margins, had already taken place. We conclude that the hinge zone formed by processes until no w not thought to be associated with hinge zone development: the cessation of sea floorspreading in the central part of the Gulf and associated rapid subsidence of cooling oceanic crust in the basin's center. INTRODUCTION During the Early Cretaceous (144-98 Ma), margins of shallow-water carbonate platforms nearly encircled the Gulf of Mexico (Fig. 1). The platforms have been studied extensively for their hydrocarbon potential1,2.. Nonetheless, one important question concerning these prominent geologic features persists: why didthe margins initially form where they did? An emerging consensus is that a tectonic hinge zone was a controlling factor; however, hinge formation and its role in basin evolution remain very ambiguous. Along Atlantic divergent continental margins, tectonic hinge zones occur widely and are interpreted as fundamental mechanical and thermal boundaries3–6. Commonly, these zonesare marked by abrupt increases in depth-to- [acoustic] basement and characterized seismically by on lapping overlying sequences5–7-8 (Fig. 2). The cause and timing of hinge formation remain poorly understood. Sawyer and Harry have related U.S. Atlantic hinge zone formation to a change in spreadinglocus that occurred halfway through the rift stage of basin evolution. Initially, spreading occurred landward of the future hinge zone then moved seaward of it, resulting in two spatially separated phases of extension and subsidence. Crust landward of the hinge zone was only affected by the first phase, while crust seaward of the zone experienced both phases. Because crust seaward of the hinge subsided more than crust landward of it, the hinge itself was created. When discussing tectonic hinge zones, "basement" needs to be defined. Since in this paper, we are evaluating basement as it is identified seismically, we subscribe to Klitgord and others definition of "acoustic basement" in the Gulf: a prominent reflection that can be identified and traced throughout the basinwhich forms the base of postrift and synrift sedimentary rocks. Buffler and Thomas" have termed this same reflection the "Mid-Jurassic Surface" an unconformity that is overlain and onlapped by Middle Jurassic and younger strata. In the Gulf of Mexico, Smithll was one of the first to suggest that initial margin position was controlled by a tectonic hinge zone.