The Origin of the Tocito Sandstone and its Sequence Stratigraphic Lessons

Abstract

Abstract A detailed and comprehensive study of all Tocito outcrops along the margins of the San Juan basin, New Mexico, combined with an equally comprehensive study of the subsurface Tocito by David Jennette and clive Jones oil-rich, shale-encased sand bodies. The Tocito Sandstone is entirely Coniacian in age. It is separated from the underlying Gallup Sandstone on the Four Corners Platform by a major unconformity that locally is associated with a lacuna up to 3 million years. In the southeast San Juan basin, however, there is no demonstrable biostratigraphic gap. The formation of the Tocito is intimately connected with an episode of Coniacian tectonism that triggered the uplift of the Waterflow Antic Line that trends northwest-southeast across the Four Corners platform and adjacent regions of the San Juan basin. Uplift of this anticline commenced immediately after Gallup Sandstone deposition and gave rise to an elevated seafloor region across which shallow marine tidal (and storm?) currents eroded several hundred feet of Mancos Shale. Concurrently, the stream system that previously had fed the wave-dominated shorelines of the Gallup, the Torrivio system, began delivering its load into the embayment (or strait?) formed behind the uplifted anticline. In that setting of strongly enhanced tides and reduced wave action, the lower Tocito formed as a tide-dominated delta complex. In late Coniacian time regional subsidence set in, probably by reversal of movement on the inferred fault below the flank of the anticline. The ensuing marine transgression truncated much of the tidal delta facies of the lower Tocito, as well as some of the fluvial Torrivio, and formed a set of shelf sand ridges across much of the Four Corners platform Studieson modern continental shelves have revealed that such juxtaposition of transgressive shelf sand ridges directly above anearly Holocene ravinement surface that has truncated underlying regressive deltaics is quite common. Therefore, an accurate documentation of the differences in distribution, geometry, vertical textural trends, and component facies between the lower, regressive member and the upper, transgressive member of the Tocito Sandstone may provide a useful template for a better understanding of the many similar delta/cstuary/shclf sand-stone complexes in the rock record. The Tocito Sandstone has excellent porosity and is the major oil reservoir rock in the San Juan basin. Documentation of the Tocito sand body and shale body geometries, therefore, will help us better understand fluid flow in such reservoirs, of which there probably are many more than currently recognized.