ICHNOLOGY OF THE UPPER JURASSIC CURTIS AND STUMP FORMATIONS, CENTRAL AND NORTHEASTERN UTAH: RELATIONSHIPS BETWEEN PALEOENVIRONMENTAL SETTING AND SEQUENCE STRATIGRAPHIC POSITION

The Oxfordian (~161-156 Ma) Curtis and Stump formations of Utah consist of mudstones, siltstones, sandstones, and conglomerates, deposited in what is interpreted as a marginal/shallow marine system of the Jurassic Western Interior Seaway. Observed trace fossil assemblages are consistent with component lithofacies assemblages. In central Utah, incised, tidally influenced channel-form sandstones are extensively burrowed, with only rare occurrences of Skolithos and Planolites discernable. Thinly laminated mudstone, siltstone, and and sandstone interpreted as tidally-influenced shelf deposits above a marine flooding surface, display a high density of Chondrites, Cruziana, and Planolites traces that increase in abundance up-section. Overall trace-fossil abundance reaches a maximum in overlying heterolithic deposits interpreted as subtidal sand-sheet margin deposits. Overlying sand-sheet/sand flat deposits contain ichnofossils similar to those in the tidally-influenced channel deposits at the base of the formation, although their abundance deceases dramatically into what was likely hypersaline mixed- and mud flat deposits that cap the Curtis Fm, in the study area.

In NE Utah, the Curtis Mbr. of the Stump Fm. is heavily bioturbated, containing Cruziana, pelecypod traces, and Planolites similar to those observed in the tidally influenced heterolithic units of the Curtis in central Utah. The lower Redwater Mbr. of the Stump Fm. consists of sandy mudstone lacking sedimentary structures due to extensive bioturbation. Where discernable, both vertical and horizontal burrows, as well as pelecypod traces are present. The middle Redwater displays a variety of horizontal burrows that increase in abundance and diversity upsection. Upper Redwater Mbr. mudstones, sandstones and oolitic limestones are extensively bioturbated and contain abundant Chondrites and Planolites and Skolithos traces, as well as Rhizocorallium.

Collectively, the identified trace fossil assemblages and hosting lithofacies permit paleoenvironmental reconstructions that are in accord with previous sequence stratigraphic interpretations for the Curtis and Stump formations as a single transgressive-regressive sequence.