THE RECOGNITION AND IMPLICATIONS OF SIPHONICHNIDAE IN LACUSTRINE STRANDLINE DEPOSITS, UPPER YELLOW CAT MEMBER, CEDAR MOUNTAIN FORMATION, EAST-CENTRAL UTAH

Bivalves are a common faunal component in Mesozoic lacustrine systems. Extensive lacustrine deposits have been recognized in the upper Yellow Cat member of the Early Cretaceous Cedar Mountain Formation, north of Moab, Utah. These lacustrine deposits have been informally termed “Lake Madsen” strata. Within Lake Madsen strandline deposits is a quartz sandstone lens that contains vertical-oriented ichnofossils attributable to the dwelling (dominica) behavior of bivalves. The sandstone lens is 2.5 m in length and varies in thickness from 0.15 to 0.30 m and contains vertical burrows of the ichnofamily Siphonichnidae. There is no further assignation into ichnogenera because the burrows may be exomorphologic variants of Siphonichnus and Laevicyclus. Siphonichnus represents vertical bivalve burrows with siphon traces and Laevicyclus represents vertical burrows with scraping features, and both attributes are present in the upper Yellow Cat member burrows. Associated facies indicate wind and wave reworking of marginal lacustrine sediments, and from oldest to youngest include: 1) interbedded mudstone and sandstone of shallow offshore deposits, 2) fish bone, ostracode, and charophyte shallow water/beach conglomerate, 3) tabular sandstones with dinosaur tracks, beach deposits that are lateral equivalent to the ichnofossil-bearing lens, and 4) trough cross-bedded sandstones of shoreline eolian origin. The vertical distribution of facies constrain the Siphonichnidae to the beach environment. Siphonichnidae within beach deposits is consistent with a Skolithos lacustrine ichnofacies (with affinities to the wave-dominated shallow-marine setting), and reflects a moderate- to high-energy shoreline environment of a large open (low salinity) lake system.