Abstract: Holocene Ichnofacies from Northern Gulf of California and Possible Analogs in Cretaceous of Utah

A. A. Ekdale

A large variety of biogenic sedimentary structures are present in the sandy intertidal zone of the northern Gulf of California, especially in the vicinity of Puerto Penasco, Mexico, where spring tidal ranges exceed 7 m. Burrows of decapod crustaceans, bivalves, and polychaetes, as well as tracks and trails of many other animals, are common. These modern organism traces are significant as future trace fossils and potential facies indicators. In fact, many trace fossils in marine Cretaceous sedimentary rocks of Utah appear to be identical with the modern structures.

Callianassa and Upogebia are two modern decapod crustaceans ("ghost shrimp") that are very similar in appearance but build very different kinds of burrows. Callianassa inhabits clean sand in the high intertidal zone and constructs a vertical shaft that extends 20 to 50 cm into the sediment and then bends laterally into a subhorizontal tunnel that commonly terminates in a slightly enlarged "room," from which several other tunnels may branch off at right angles. Although the burrow wall rarely is pelleted in the northern gulf, Callianassa burrows on many other coastlines commonly are pelleted. Burrows similar to those of Callianassa are known as Ophiomorpha and are common in nearshore sand environments preserved in the Cretaceous of Utah.

Upogebia lives higher in the intertidal zone and in muddier sediment than Callianassa. In the northern Gulf of California it builds a smooth-walled burrow system with many straight tunnels branching off at 120° angles. This burrow type is virtually identical with the trace fossil Thalassinoides, which is abundant in Cretaceous rocks of Utah's Book Cliffs and Wasatch Plateau.

Other modern biogenic structures that have common fossil counterparts in the Cretaceous include various types of gastropod trails. Bivalve burrows, crab trackways, and ray pits are uncommon in the Book Cliffs and Wasatch Plateau. Agglutinated worm tubes and bird tracks, which are common on the modern tidal flats, have not been identified in the marine Cretaceous of Utah.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah