Correlation and depositional environments of the Mid- and Upper Cretaceous in Northern Venezuela : a biostratigraphic and chemostratigraphic study
A chronostratigraphic framework based on foraminiferal biostratigraphy, integrated with carbon-isotope stratigraphy, has been generated for the Upper Albian - Lower Campanian interval of the eastern and western basins of northern Venezuela. The successions are composed of occasionally phosphate-rich calcareous sandstones and shales to the SSW, and foraminiferal organic-rich mudrocks and limestone in more northern areas (Alpuf-6 and TOT-3 wells in the west; Chimana Grande Island and Querecual River sections in the east). Biostratigraphic resolution is very poor in the SSW areas due to deposition in shallow-water conditions. To the north, a palaeoslope reconstruction of the Cretaceous passive margin, integrated with foraminiferal biofacies and abundances of planktonic foraminifera morphotypes, indicates that bathyal conditions prevailed, with dysaerobic or anoxic bottom-waters being caused by high rates of surface-water productivity. These were associated with an intermittent upwelling regime and palaeobathymetric barriers that restricted circulation, causing stratification and accumulation of vast quantities of organic-rich sediments. These adverse conditions caused a paucity of keeled planktonic foraminifera and impoverished benthonic faunas, resulting in low-resolution biostratigraphy for the Querecual and La Luna formations, which range in age from Late Albian 'Roralipora ticinensis' Zone to Santonian 'Dicarinella asymetrica' Zone. A chemostratigraphic study demonstrates that the highest TiO[sub]2. A1[sub]2O[sub]3 and Fe[sub]2O[sub]3 percentages are observed in the Upper Albian, Cenomanian and Santonian, whereas Sr/Ca ratios, despite diagenetic effects, show increasing trends in the Cenomnanian and decreasing trends from the Santonian up to the Campanian in most sections. Organic-rich facies exhibit high (>20%) TOC values, particularly in the Upper Albian to mid-Cenomanian and in the Turonian. A stable carbon-isotope reference curve for organic matter ([sigma][sup]13C[sub]org) generated for the Upper Albian - Lower Campanian interval, shows the [sigma][sup]13C Albian-Cenomanian Boundary, MCEI, CTB, Holywell, Lulwoth, Ruund Down, Glynde, Pewsey and Hitch Wood isotope events, and other Coniacian and Santonian events identified in England. Despite difficulties arising fromn the use of different biostratigraphic schemes in Boreal and Tethyan sections, the [sigma][sup]13C reference curves from England and Venezuela display remarkably similar trends that confirm the usefulness of [sigma][sup]13C chemostratigraphy, combined with biostratigraphy, as a powerful basin analysis and correlation tool.