A Late Devonian Hydrocarbon-Seep Deposit Dominated By Dimerelloid Brachiopods, Morocco

Abstract

A spectacular carbonate deposit from the western Meseta of Morocco consists of microbial and brachiopod limestones. The small, metersized Famennian limestone blocks are embedded in Carboniferous shales, implying later displacement. Primary deposition of the limestones apparently occurred below the depth limit of phototrophic organisms. The brachiopods belong to the genus Dzieduszyckia within the rhynchonellide superfamily Dimerelloidea. They occur in fantastic abundance, in a low-diversity faunal assemblage. The microbial limestones contain fossilized threadlike microorganisms of unknown affiliation. The enormous accumulation of brachiopods on the Famennian seafloor is best explained by the former existence of a seep. Crude oil was a major component of the seepage fluids, as indicated by the occurrence of pyrobitumen (metamorphosed petroleum) and delta C-13(carbonate) values as low as -12 parts per thousand, akin to the isotopic composition of carbonates forming at modern oil seeps. The lowest delta C-13 values in the Dzieduszyckia deposit are found for a carbonate phase referred to as banded/botryoidal cement. It is a fibrous, low-Mg calcite cement in the form of isopachous rims and botryoids that precipitated directly on and within brachiopod shells. The microbial limestones predominantly consist of this phase and therefore can be classified as cement framestones. Reticulate patterns of botryoidal aggregates and an elevated Sr content agree with an aragonite precursor of the banded/botryoidal cement. The interpretation of the Dzieduszyckia deposit as seep related supports the hypothesis that Paleozoic and Mesozoic members of the rhynchonellide superfamily Dimerelloidea were a lineage long associated with seeps that survived from Late Devonian to Early Cretaceous time.