Integrated foraminiferal biostratigraphy and chemostratigraphy of the Querecual Formation (Cretaceous), Eastern Venezuela

An integrated foraminiferal biostratigraphy and chemostratigraphy is presen ted for the Lower to Upper Cretaceous Querecual Formation exposed on Chiman a Grande Island, Eastern Venezuela. The formation consists of >450 m altern ating foraminiferal and organic-rich carbonates and laminated mudrocks, and is considered the main hydrocarbon source rock for the eastern Venezuela B asin. Biostratigraphic resolution within the Querecual Formation is poor, d ue to a paucity of keeled planktonic foraminifera and impoverished benthic faunas, Deposition occurred in a bathyal environment, with dysaerobic or an oxic bottom waters resulting from high rates of surface productivity associ ated with an upwelling environment. Biostratigraphic evidence indicates that the Querecual Formation ranges fro m the upper Albian Rotalipora ticinensis Zone to the Santonian Dicarinella asymetrica Zone. Iron and Al contents fall through the Albian-Cenomanian in dicating a progressive decrease in the detrital supply, driven by rising eu static sea level. A Ca profile demonstrates variations in carbonate product ion and dissolution. High total organic carbon (TOC) intervals occur in the upper Albian to mid-Cenomanian and Turonian, and high Ba/Al and Si/Al rati os characterize mid-Cenomanian and younger sediments. Variations in these e lements primarily reflect changes in marine productivity, but are also affe cted by diagenetic processes, A stable carbon isotope curve established from analysis of organic matter ( delta(13)C(org)) correlates well with published delta(13)C curves for carbo nates from England and Italy. The Cenomanian/Turonian boundary cannot be id entified using planktonic foraminifera, because key taxa are absent, but th e base of the Turonian is clearly indicated by a sharp fall in delta(13)C i mmediately above a major positive excursion. The bottom of the Coniacian is placed below a delta(13)C minimum, towards the base of the Dicarinella con cavata Zone. Combined with the foraminiferal data, the isotopic data enable much improved stratigraphic resolution compared to previous investigations of the formation.