FOREBULGE CONTROL ON DRAINAGE DIVIDES USING DETRITAL ZIRCONS - LATE CRETACEOUS GALLUP AND TORRIVIO SANDSTONES, NEW MEXICO

Moderate-n (n ≈ 200-300) detrital zircon samples acquired above and below a hypothesized sequence boundary between the Late Cretaceous Gallup and Torrivio sandstones in northwestern New Mexico has exposed a dramatic shift in provenance. Petrographic point counts (n = 3768), bentonite dates, UAV orthomosaics and traditional measured sections were integrated with detrital zircon data (n = 815 grains) to document this shift in sediment routing. A consistent sampling procedure of mid-channel sandy bars is used to limit the effects hydrodynamic fractionation have on intersample comparisons. Detrital zircons from the Gallup fluvial unit have a mixed source spectra whereas the Torrivio samples have an overwhelming amount (>60%) of Yavapai-Mazatzal (1.8-1.6 Ga) aged zircons with minor contributions from other sources. Petrographically, the Torrivio samples are more feldspathic, coarser-grained and moderate to poorly sorted as compared to the lithic-rich, medium-fine grained, moderately sorted Gallup fluvial units. The cause of this change in sediment supply is attributed to forebulge migration and associated lithospheric deformation causing a drainage divide isolating the Torrivio rivers from recycled orogenic sediment to the west. Although eustatic processes are prevalent in this epeiric seaway during this greenhouse period, the shift in sediment provenance can’t be explained simply by eustasy, rather it is likely generated due to a tectonic overprinting of a falling stage systems tract.