Dense accumulations of dinosaur eggshell fragments and occasional whole eggs were encountered in two beds of the Allen Formation, Lamargue Group, Rio Negro Province of Patagonia, Argentina. Eggshell fragments and whole eggs were transported in what were probably storm surge sequences from an as yet, unidentified nesting site, and deposited in an Upper Cretaceous marine embayment.

A lower carbonate mudstone bed (1.5 m thick), underlain by a calcareous sandstone and capped by a siltstone, contains numerous eggshell fragments in continuous outcrop over an area of at least 12 sq.km. A 2 sq.m. grid was excavated through the mudstone bed to document the orientation and distribution of the eggshell fragments. The random distributions and orientations of the fragments provide structural evidence that the mudstone was rapidly deposited during what was most probably a storm surge.

Whole eggs were found at a single site in an upper sandstone unit 6.5 m above the fragment-filled mudstone bed. The lower portion of this cross-bedded sand deposit con- tains mud lenses and whole eggs. The sediments lack organics and root casts, and the mud lenses show evidence of soft-sediment deformation. The cross-bedded structure showed no evidence of post- depositional disturbance which would be expected if nesting had occurred in situ. The spatial distribution of whole eggs varied.

Presence of eggs and eggshell fragments may not indicate the presence of in situ nesting areas. Sedimentological investigations should be conducted concurrently with paleontological excavations where nesting interpretations are expected.

Published in: Geological Society of America, Abstracts with Programs, November 1995, A-318