Abstract: Abstract: Stratigraphy and Clay Diagenesis of Trinidad Sandstone and Vermejo Formation, Walsenburg Area, Colorado

Lee T. Billingsley

The sequence of Pierre Shale, Trinidad Sandstone, and Vermejo Formation of the Raton basin is one of marine to nonmarine deposits that indicate the final regression of the Cretaceous seaway in southeastern Colorado. The upper Pierre is highly burrowed, carbonaceous siltstone to very fine-grained sandstone that was deposited in a prodelta environment. Transitionally overlying the Pierre, the main body of the Trinidad is clayfilled, burrowed, low-angle cross-stratified, fine-grained sandstone indicative of delta-front deposition. Other evidence supporting a deltaic environment includes penecontemporaneous deformation in the form of ball-and-pillow structures. Also, tongues of the Pierre that extend well into the Trinidad are interpreted as destructional units in the deltaic sequence. The uppermost porous and permeable facies in the Trinidad is interpreted as a distributary channel because of the lack of burrowing, scour contact, and high-angle cross-stratification.

The overlying Vermejo Formation is composed of coals, claystones, and sandstones deposited in channel-margin areas, such as freshwater swamps, marshes, lakes, and crevasse splays. Coal accumulated in poorly drained swamps and marshes until an influx of detritus served to raise the level of the swamp and oxidize carbonaceous material. After the influx of detritus ceased, the channel-margin area continued to subside, reducing conditions were restored, and coal again began to accumulate. Numerous cycles of this nature in the Vermejo are related to crevasse splays.

Clay-sized material in the delta-front facies of the Trinidad probably was deposited along with sand grains by flocculation. Conversely, the clay in the freshwater distributary channels is authigenic. The controlling factor in the formation of authigenic clay appears to be depositional texture and, consequently, the environment of deposition.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado