PALEOCEANOGRAPHIC CHANGES ACROSS THE CENOMANIAN-TURONIAN BOUNDARY (MID-CRETACEOUS) IN CARBONATES OF SOUTHERN MEXICO

The Albian-Turonian Morelos Formation (>800 m) was deposited along a westward-deepening carbonate platform in southern Mexico (Guerrero state) and is composed of subtidal through peritidal skeletal wackestone-grainstones. It is abruptly overlain by the Turonian Mexcala Formation (1000⁺m), which is composed of pelagic carbonates and siliciclastic turbidites. The higher sediment accumulation rates of platform carbonates (vs. pelagic rates) permits previously unattainable high-resolution data on the Cenomanian-Turonian (C-T) boundary and the relationship between nearshore physical/biologic processes (sea-level and benthic biota changes) and geochemical processes (anoxia, stable isotope, TOC, trace elements). In addition, these C-T boundary deposits provide a much-needed record of Pacific/Caribbean ocean influences; much of our present knowledge comes from C-T sections in the proto-Atlantic or Western Interior seaway.

Only minor evidence of high-frequency sea-level changes is observed (meter-scale upward shallowing cycles) and systematic facies changes related to M.y.-scale sea-level change (depositional sequences) are absent except for abrupt deepening just above the C-T boundary. Three sections show similar d ¹³C _carb trends over a 60-100 m-thick interval (~0.5 m sampling interval): relatively uniform values positive values (over ~30 m), two abrupt 2-3‰ negative shifts (<10 m), an abrupt 5-6‰ positive shift (<3 m), plateau of uniformly higher values (~10 m), a gradual 2-6‰ decrease (~10 m), followed by relatively uniform values (10⁺m). The large positive shift correlates to the positive excursion observed globally (OAE-2). d¹³C _org values show significantly more variability, but some of the d¹³C _carb curve trends are observed. TOC values are low (<0.2 wt%) and show no obvious stratigraphic trends. Trace metal abundance anomalies (Co, Ni, Cr, Pb, Zn, Cs, W, Sb, Mo, Ag, Se) across the Mexican C-T boundary are explained by magma outgassing and hydrothermal exchange with seawater during volcanic activity that built the Caribbean oceanic plateau at ~90-94 Ma, supporting a possible link between the global C-T isotope excursion (OAE-2) and large oceanic plateau formation.