Trace Fossils in the Green Zone: Ichnology, Mineralogy and Paleoclimatology in Two Mesozoic Seaways, Utah and Wyoming

Sherie Harding and A. Ekdale

The Jurassic Curtis Formation in Utah and Cretaceous Shannon Sandstone in Wyoming reflect marine transgressions on the continent during warm climates. Trace fossils and green ('glaucony') clay minerals offer clues for understanding paleoclimates and sequence stratigraphy of these epicontinental seaways. The Curtis (Stump) Formation represents a transgressive-regressive sequence deposited in response to relative sea level changes in the Late Jurassic seaway, which transgressed southward leaving a signature of greensands and ichnofabrics. A thin, fine-grained sandy facies in the lower part of the formation is a transgressive systems tract topped by a maximum flooding surface. The paleoclimate was hot and arid at a tropical paleolatitude. The Shannon Sandstone Member (Campanian) of the Cody Shale was deposited in a transgressive-regressive sequence during the Late Cretaceous. Ichnologic observations (Macaronichnus segregatis) indicate sandy shoreface deposition, and overlying bioturbated muddy sediments indicate subsequent transgression. Glauconite (which suggests a fully marine environment) in the Shannon Sandstone is often incorporated in primary sedimentary structures. Mineralogic and sedimentologic data clarify the autochthonous or allochthonous nature of the green minerals. Characterization of true mineralogy and description of associated ichnofabrics and ichnofacies clarify the depositional environment and provide examples of trace fossils and greensands in tropical Mesozoic epicontinental seaways. Greensands or glaucony-rich sedimentary units often are linked with marine transgressions. Trace fossils in greensands may not be produced under the same environmental conditions as those that facilitated the authigenic formation of glaucony. Studies of autochthonous glaucony often indicate an origin in offshore, low-energy, sediment-starved environments below storm wave base. With subsequent regression and drop in sea level, the trace fossils may be produced in the same sediment but at a shallower water depth than that in which the glaucony formed. Percent potassium oxide in glaucony minerals is crucial in making paleoenvironmental interpretations of autochthonous greensand facies. When rounded and unfragmented, the likely origin of the pellets is autochthonous. If glaucony grains are fragmented or incorporated into ripplemarks or crossbeds, then it may be inferred that the glaucony grains originated outside the site of deposition.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012