Abstract: Trace-Fossil Assemblage of Early Cretaceous Sand Flat-Shallow Subtidal Environment

H. Allen Curran

Silty fine-quartz sands of the Englishtown Formation (Upper Cretaceous) exposed along the banks of the Chesapeake and Delaware Canal in Delaware contain a diverse assemblage of trace fossils formed by endobenthic animals that are indicative of the paleodepositional environment. Most prominent are extensive shaft and tunnel Ophiomorpha nodosa dwelling burrows and dense concentrations of Planolites feeding burrows. Also present are several types of Chondrites, Skolithos shafts, and a distinctive radiate structure probably formed by the feeding activity of a sediment-ingesting worm.

Ophiomorpha nodosa systems display the stack configuration and basal maze structure characteristic of burrow systems of the ghost shrimp Callianassa major found today in shoaling intertidal and shallow subtidal environments of the southeast Atlantic coast. Gently meandering, essentially horizontal Planolites burrows are outlined by a rim of dark organic-rich material and probably were formed by sediment-ingesting worms. The several types of delicate Chrondrites consist of simple branching shafts, structures with branching spiral shafts that form interconnected networks, and branching tubes that surround the walls of Ophiomorpha shafts. These structures closely resemble the feeding burrows formed by several species of capitellid polychaetes characteristic of the nearshore zone of the G orgia coast. Delicate Skolithos shafts are similar to dwelling burrows formed by onuphid polychaetes also common to the southeast Atlantic coast.

The trace-fossil assemblage and sedimentary characteristics of the Englishtown Formation indicate deposition in an intertidal sand-flat to shallow subtidal-shoaling environment closely analogous to the sand-flat and shallow subtidal shoaling environments of the embayed, low-energy Sea Isles coast of Georgia. This interpretation is supported by regional geologic mapping and subsurface data which indicate that the Englishtown is a nearshore facies of the Matawan Group.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA