Hubbard, Stephen M.¹, Michael R. Shultz¹ 
(1) Stanford University, Stanford, CA

ABSTRACT: The Significance of a Deep-Water Glossifungites Surface in the Upper Cretaceous Cerro Toro Formation, Chilean Patagonia

Sediment gravity-flow deposits of the Upper Cretaceous Cerro Toro Formation accumulated in an extensive deep-water channel belt present along the length of the Magallanes Basin foredeep in southern Chile. The formation (> 2 km thick) consists of a 300-400 m thick interval of coarse channel material encased in bathyal mudstone. The stratigraphic architecture of channel deposits consists of alternating packages of sandy- and mud- matrix conglomerate, which correlate over much of the basin. Evidence of turbidity current, traction, and debris flow processes is present. 
A broad diversity of trace fossils has been identified within sandstone and mudstone facies of the Cerro Toro Formation, but they are rare in conglomerates associated with deposition in the channel axis. An exception is a Glossifungites surface present at the top of a conglomeratic package dominated by muddy matrix. This surface is characterized by abundant, robust, deeply-penetrating (up to 7 m), vertical to sub-vertical, passively filled Diplocraterion, Skolithos and Arenicolites, and is the only marker horizon within the conglomeratic member of the Cerro Toro Formation in the study area. The surface represents a time when a pause in sedimentation allowed opportunistic organisms to take advantage of an open niche, possibly related to a rise in sea-level. The temporal significance associated with the transformation of cohesive mud-matrix supported conglomeratic material to a firmground suitable for colonization by Glossifungites burrowers is poorly understood, but is likely related to burial and subsequent exhumation by erosive turbidity currents.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.