PICARD, M. DANE, University of Utah, Salt Lake City, UT

ABSTRACT: Early Mesozoic History and Petroleum Potential of Formations in Wyoming and Northern Utah

During the Triassic and Jurassic, over what is now Wyoming and northern Utah, roughly equal amounts of sediment were being deposited in continental settings-lake, stream, and eolian-and in shallow-marine or deltaic-plain settings-delta, beach, marsh, tidal flat, and shallow shelf. Clastic rocks dominate. In order of decreasing abundance, the rocks are fine-grained clastics (siltstone, claystone, mudstone), sandstone, carbonates, evaporites, and claystone- and carbonate-pebble conglomerate. Approximately four-fifths of the succession contains red beds or variegated layers-purple, maroon, lavender, olive, green.

Unconformities bound Jurassic formations in Wyoming-Nugget (Lower? Jurassic), Gypsum Spring (Middle Jurassic), Sundance (Middle Jurassic), and Morrison (Upper Jurassic). Unconformities also bound the continental Upper Triassic section-unnamed red bed unit, Jelm, Popo Agie-separating it from the underlying shallow-marine formations-Dinwoody (Lower Triassic), Red Peak (Lower Triassic), Alcova, Crow Mountain (Middle? Triassic). Within the marine sequence, an unconformity occurs at the top of the Alcova and, quite likely, shorter periods of erosion took place at the top and below the base of the sandy faces that underlies the Alcova. The postulated duration of the principal unconformities totals about 18 m.y., at least one-sixth of early Mesozoic time. The bulk of the remaining 80-100 m.y may be represented by a large number of smaller unconformities. For the lower Mesozoic, as for most stratigraphic intervals, a few beds contain the story of what has taken place during the abyss of geologic time.

Like other places in the world where evaporites occur in the Triassic, the Wyoming section produces little crude oil. No significant sequence in the early Mesozoic shows source-bed characteristics.

The Crow Mountain Sandstone contains the best reservoirs. The Lower(?) Jurassic Nugget Sandstone produces the most oil and gas in the thrust belt of southwestern Wyoming and northern Utah. Cretaceous claystones below the thrusts contain the source beds.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.