ABSTRACT

The breakup of Pangaea and the splitting of South America from North America in the early Mesozoic left a rifted and attenuated trailing margin on the latter plate which became the initial depositional surface for a sedimentary sequence of Late Triassic to Recent age. The Late Triassic Eagle Mills Formation and its equivalents are interpreted as being the initial deposits confined to rift grabens of the attenuated plate margin. Deposition of Jurassic evaporites resulted from sedimentation by the brine mixing process in the restricted circulation of a young and narrow seaway similar to the Red Sea. Late Jurassic and Cretaceous strata represent the transgressive deposits formed as open marine conditions prevailed as the plates diverged and the North American plate margin subsided. Laramide tectonism in the continental interior provided a rejuvenated hinterland source area that supplied the voluminous sediment for the regressive and prograding Cenozoic clastic wedge.

Studies of this entire sedimentary record reveal the influence of the tensional effects of continental splitting and lower crustal creep that established the initial depositional surface that slowly subsided as indicated by crustal thinning and the thermal decay curve of cooling oceanic lithosphere. In addition, these studies also reveal the control and influence of: 1) inherited structures of the rifted margin; 2) hinterland source areas; 3) the timing and amount of differential subsidence between continental and ocean crust; 4) active syndepositional faults; 5) hingelines; and 6) post depositional rejuvenation due to contemporary plate movement.

Not only do these studies add to our understanding of the geologic history of the area, which is most important for development of successful exploration programs, but they provide a guide for the study of sedimentary infills within ancient lithospheric plates, a neglected but important task facing all who are confronted by the complex problem of interpreting the sedimentary record of ancient basins.