Sequence stratigraphy and depositional history of the upper Cañon del Tule, Las Imagenes, and Lower Cerro Grande Formations, central Parras Basin, northeastern Mexico
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This dissertation describes the results of a field-oriented sequence stratigraphic study of a 900-m-thick Maastrichtian interval in the middle portion of the Upper Cretaceous Difunta Group. Basic analysis of rock features and their stratigraphic arrangement were used to subdivide the interval into transgressive (TST), highstand (HST), and falling stage (FSST) systems tracts and to recognize their bounding surfaces. The progradational offshore to lower shoreface highstand (HST) deposits of the upper Cañón del Tule Formation and the underlying transgressive (TST) deposits, contain gastropods and bivalves common in shallow marine Lower Maastrichtian strata. However, the occurrence of the ammonite Coahuilites sheltoni restricts these rocks to the lower Upper Maastrichtian. In the Late Maastrichtian, the final stage of shallow marine sedimentation of the upper Cañón del Tule Formation was characterized by an abrupt change. Accommodation space was reduced in proximal settings, and the shoreline and coarse-grained facies belts migrated basinward in response to a relative sea level fall (forced regression). Wave scouring of the seafloor, and rip and longshore currents produced a regionally extended forced regression surface and sequence boundary characterized by meter-scale gutter casts. The FSST upper shoreface to foreshore deposits of the uppermost Cañón del Tule Formation include a ridge-forming sandstone of variable thickness. Its internal stratigraphic architecture suggests that high-frequency pulses of sea level risings punctuated the forced regression. During this process, thick, aggradational red deposits of the lower Las Imágenes Formation (FSST) accumulated over the adjacent coastal plain until an intermittent marine transgression began. The initial transgressive pulses occurred across a rapidly subsiding coastal plain, which favored the development of charophytes in fresh and brackish water environments, associated with benthonic foraminifers, ostracods, oysters, Ophiomorpha and Thalassinoides. Mixed-load probably meandering rivers drained the coastal plain bounded by lagoons and extensive shoreface and offshore environments for this part of the Late Maastrichtian Gulf Coast of México. After this stage of intermittent marine transgression, shallow seas and transgressive (TST) deposits of the lower Cerro Grande Formation covered the region.