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Reconstructing Pre-Salt Paleodrainage Pathways and Source Terrains for the Late Triassic Eagle Mills Formation, Northern Gulf of Mexico Basin

Abstract

U-Pb geochronological analyses have been conducted on detrital zircon (DZ) grains extracted from late Triassic Eagle Mills Formation fluvial sandstones as a method for reconstruction of sediment routing and provenance models for the pre-salt Gulf of Mexico (GOM) basin. With continued exploitation success in South-Central US and GOM Mesozoic Era geologic formations, pre-Callovian sedimentary basin dynamics and structural models have recently taken on renewed scientific interest with the potential to both estimate subsalt reservoir quality and constrain overlying Werner and Norphlet Formation structure and distribution. We present a preliminary GOM paleodrainage model for Eagle Mills Formation siliclastics based on the integration of U-Pb geochronological results with more traditional biostratigraphic, sedimentologic, and sequence stratigraphic provenance constraints from geologic cores, cuttings, and geophysical well logs. Eagle Mills data indicate distinctly different source terrains and drainage basin extents for the eastern versus western synrift fluvial deposits. While the eastern Eagle Mills source terrains are rooted predominantly in the late Alleganian and peri-Gondwanan (i.e. Suwannee) orogenic terrains with very little Grenville orogenic signature, the western Eagle Mills core data identifies much more prominent Grenville and Ouachita orogenic signatures with less peri-Gondwanan influence. This along-strike provenance variability in the proto-GOM Eagle Mills provides strong evidence that multiple fluvial depositional systems with distinct drainage basins were routed toward the early Mesozoic tectonic expansion of the NE-SW oriented GOM rift basin. Distinct alluvial bedforms and depositional cross strata characterize the prevalent channel belt and floodplain facies with bioturbated trace fossils only rarely preserved and no evidence of marine incursion. Igneous intrusive lithologies, including diabase, basalt flows, and volcanic ash deposits indicative of dynamic rift tectonics, are also often apparent in subsurface Eagle Mills Formation cuttings, core and petrographic samples. These integrated U-Pb geochronological results continue to improve upon Eagle Mills Formation source to sink models during the synrift deposition of basal GOM continental alluvium in a horst-graben block topography with active associated igneous intrusions, little to no marine influence, in a warm, humid environment of deposition with increasing aridity.