DEPOSITIONAL AND SEQUENCE STRATIGRAPHIC MODEL OF TRANSGRESSIVE SHELF SANDSTONE: THE LATE CRETACEOUS TOCITO SANDSTONE, SAN JUAN BAIN, NEW MEXICO, USA

There has long been debate about the origin of long, linear, isolated shallow marine sandstones. In the 1980s these were routinely interpreted as wave- or tide-modulated shelf sandstones. With the advent of sequence stratigraphy, many of these sandstones were reinterpreted as top-truncated forced-regressive to lowstand delta or shoreface, inviting the question regarding whether the concept of a shelf sandstones remain a viable depositional model. More recent syntheses demonstrate that shelf sands tend to be derived by reworking of underlying sediment through transgressive erosion and show varying levels of detachment and reworking into discrete ridges and bars by wave and tidal processes as they become more evolved. The Late Cretaceous Tocito Sandstone in the Western Interior Seaway, which overlies the regressive Gallup Formation, is a candidate for a transgressive shelf sandstone. In this study, we present detailed facies analysis, based on sedimentological sections measured from well-exposed outcrops of the Tocito Sandstone and its related stratigraphic units in northwest New Mexico, USA. With high-resolution sequence stratigraphic control, facies analysis, detrital-zircon anatomy, and petrographic analysis are integrated to investigate the sequence stratigraphic evolution and depositional processes of the Tocito Sandstone. Detrital zircons indicate that the Tocito, Torrivio, and Gallup deposits are derived from different source areas, and therefore imply separate sequence stratigraphic origins. The Tocito Sandstone is interpreted to have been deposited as shelf sand ridges in retrogradational estuarine to drowned-barrier-system environments with tidal and wave influence, characterized by intense bioturbation. The Tocito was deposited in an overall transgressive systems tract and sourced from reworking of the underlying Torrivio and Gallup units through processes of transgressive erosion.