Summary

An unusually large trace fossil assemblage from the Upper Cretaceous-Lower Tertiary of central-western Patagonia, Chubut Province, Argentina, is described and considered to be the result of sea level changes during the deposition of the lower part of the Lefipan Formation, in shoreface facies. The sedimentary succession, represented by the Paso del Sapo and Lefipan formations, is interpreted as an estuarine system that evolved to open marine environments. The trace fossil assemblage, located towards the base of the Lefipan Formation and top of a retrogradational systems tract, consists of the ichnogenera Ophiomorpha and Rhizocorallium which are found in three very thick (1.7 m, 1.0 m and 0.6 m) sandstone beds that can be followed regionally. In each bed, trace fossils are arranged in a vertical succession composed from bottom to top by a lower, Rhizocorallium-rich sediment, and an upper Ophiomorpha-rich level. Ophiomorpha suspension-feeder trace makers colonized the foreshore and the shoreface of the estuarine beaches, while deposit-feeder organisms represented by Rhizocorallium lived in a deeper position, from lower shoreface to offshore. During a temporary sea level fall there was erosion of the beach zone and deposition on the shoreface. The trace makers of Ophiomorpha moved to the new coastal line where they developed over the older Rhizocorallium zone, while a mixture zone was also formed. In the same way Rhizocorallium is developing in seaward position. Continued relative sea level fall resulted in the generation of the observed of shallowing up trace fossil arrangement. A comparison between this trace fossil sequence and a classic progradational parasequence set allows us to introduce the concept of trace fossil stacking pattern, an useful tool for the recognition of eustatically-driven high-order bioturbated sequences.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah