MIDCRETACEOUS BASIN MARGIN CARBONATES, EAST CENTRAL MEXICO

Isolated, high relief carbonate platforms developed in the intracraton ic basin of east-central Mexico during Albian-Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep a s 20-43-degrees. Basin-margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the e astern margin of the Valles-San Luis Potosi Platform, an exceptionally thick (1380 m) progradational basin to platform sequence of the Tamab ra Formation can be divided into six lithological units.Basinal carbon ate deposition that preceded deposition of the Tamabra Formation was e mphatically punctuated by an allochthonous reef block 1 km long by 0-5 km wide with a stratigraphic thickness of 95 m. It is encased in Tama bra Formation unit A, approximately 360 m of peloidal-skeletal wackest one and lithoclastic-skeletal packstone that includes some graded beds . Unit B is 73 m of massive dolomite with sparse skeletal fragments an d intraclasts. Unit C, 114 m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlamina ted lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or la minated mudstone. The breccias contain a spectrum of platform-derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to c ompletely dolomitized mudstone or skeletal debris. Beds are up to seve ral metres thick. Unit E is 206 m of massive, sucrosic dolomite that r eplaced breccias. Unit F is approximately 500 m of thick bedded to mas sive skeletal packstone with abundant rudists and a few mudstone intra clasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, con sisting of massive beds of caprinid packstone and grainstone with many whole valves. Depositional processes within this sequence shift from basinal pelagic or peri-platform sedimentation to distal, platform-der ived, muddy turbidity currents with a large slump block (Unit A); thro ugh more proximal (coarser and cleaner) turbidity currents (Unit B?, C ); to debris flows incorporating platform margin and slope debris (Uni ts D, E). Finally, a talus of coarse, reef-derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic t o include influxes of dilute turbidity currents. Units containing turb idites with platform-derived bioclasts reflect flooding of the adjacen t platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, blac k, pelagic carbonates, locally cherty, are interbedded with both brecc ias and turbidites. At least those interbedded with turbidites may hav e been deposited within an expanded mid-water oxygen minimum zone duri ng relative highstands of sea level. They are in part coeval with mid- Cretaceous black shales of the Atlantic Ocean.