Abstract

Triassic and Jurassic rocks of the Dinosaur Triangle of Colorado and Utah are mostly continental and marginal-marine red beds with a few thin gray to green marine units. They are present at depth throughout most of the Triangle, and crop out in belts along the southern and northern margins.

Triassic strata, formed between 225 and 188 million years ago (mya), represent only a partial record of Triassic events. In Early Triassic time a shallow sea briefly invaded the western edge of the Triangle. Resulting marine and tidal-flat deposits now constitute the Moenkopi Formation. Erosion prevailed during Middle Triassic time; but, in the Late Triassic, fluvial and lacustrine deposits containing much volcanic ash covered the area to form the Chinle Formation. The Moenkopi has yielded ammonites and vertebrate tracks, whereas the Chinle is a source of reptile and amphibian remains, freshwater mollusks, dinosaur tracks, and many plant fossils.

Jurassic rocks, deposited between 188 and 138 mya, also give an incomplete story of that period. The Early Jurassic was dominated by strong northwest winds. The first onslaught of wind produced a thick layer of sand (Wingate Sandstone) that covered the Triangle. After a brief respite that saw streams flow southwestward across the Wingate dunes and leave behind a thin fluvial unit (Kayenta Formation), northwest winds again spread sands across the area to form the Navajo (Nugget) Sandstone.

In Middle Jurassic time, a shallow sea twice inundated the Triangle. The first advance resulted in the marine and marginal marine deposits of the Carmel Formation. After the sea had withdrawn briefly toward the north, winds from the southwest spread another blanket of sand (Entrada-Preuss Sandstone) across the Triangle. During the second advance and ultimate retreat of the Jurassic sea, the marine Curtis Formation and the marginal marine Summerville Formation were formed.

The principal event of the Late Jurassic was the deposition of the fluvial-lacustrine Morrison Formation. Large volumes of volcanic ash present in the upper part (Brushy Basin Member) of this unit indicate intense volcanic activity in the Sevier orogenic belt to the west.

Invertebrate fossils are common in the Carmel, Curtis and Morrison Formations, but of most interest are evidences of vertebrates. Dinosaur tracks are known from the Entrada and Morrison, and numerous dinosaur and other reptilian remains have been found in the Morrison of this area. Plant fossils are also common in the Morrison.