Global Development of Early Jurassic Reefs: “Lithiotis ” Bivalve Bioherms

N. M. Fraser
University of Southern California, Dept. of Earth Sciences, Los Angeles, CA

In the aftermath of the Triassic-Jurassic mass extinction, Early Jurassic reefs are exceptional because they are rare and primarily constructed by bivalves. The only known intact Early Jurassic carbonate platform with well-developed reefs is exposed in the Middle and High Atlas Mountains of Morocco. Outcrops of smaller reefs occur in both carbonate and siliciclastic environments of the Southern Alps, Peru, and Western North America, particularly those constructed by large aberrant bivalves that collectively form the “Lithiotis” facies: Lithiotis problematica, Gervilleioperna sp., Lithioperna scutata, Cochlearites loppianus and Mytiloperna sp.

“Lithioti” facies bioherms in Oregon, California, Italy, and Morocco have been used in this study. Line-intercept transects, bulk sample collection, and species identification have been completed. Aragonite is preserved in some Italian specimens; d¹⁸O, d¹³C and Mg/Ca analyses are waiting to be performed on these specimens. Based on field and thin section observations, there is a strong zonation of the “Lithiotis” facies bivalves in shallow nearshore environments. Gervilleioperna and Mytiloperna are restricted to the intertidal or tidal flat facies. Lithioperna is found in most lagoonal subtidal facies. While Lithiotis and Cochlearites are found in the lagoonal subtidal facies, these two genera also construct bioherms. These bioherms are small relative to Late Triassic reefs or Cretaceous rudist reefs. Lithiotid reefs in Oregon reach lengths 50–70 m and heights of 5.5 m. Lithiotid bioherms in the carbonate settings of Italy and Morocco are of significantly smaller dimensions, 10–12 m long and 3 m high.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid