Paleontological and sedimenfological study of the Upper Triassic carbonate of the Antimonio Formation, northwestern Sonora, has documented the details of a diverse shallow water marine biota. Norian exposures of the Antimonio Formation are dominated by lenticular carbonate strata with interbedded argillite, mudstone, and limy sandstone. Constructional elements within these reefoid and biostromal buildups include corals and chambered thalamid sponges. Spongiomorphs and calcareous red algae were active sediment stabilizers. The dweller guild consisted of hydrozoans, brachiopods, bivalves, gastropods, foraminifers and green algae. The baffler guild was filled by both high growing corals and stalked echinoderms. Bivalves acted as bioeroders.

Tabular bodies of immature lime-cemented sandstone implies that a siliciclastic source lay close by, probably to the east. Different exposures of the Antimonio Formation contain varying amounts of clastic sediments. Those with less sand/silt/clay are interpreted to have been deposited in response to high-order changes in sea-level. Cyclic siliciclastic-carbonate strata exhibit distinctive small-scale successional patterns; soft substrates were colonized by bivalves arid sponges which are in turn were colonized by corals. The coincidence of lithofacies and biofacies indicates that biological development of biostromes was controlled by physical processes. Together, paleontological and sedimentological signatures suggest that these units were deposited in normal to restricted marine lagoonal environments at an interface between basinward carbonates and shoreward siliciclastic environments.

The tropical, shallow water fauna of the Antimonio Formation shares affinities with both the western Tethys and inboard terranes of Western North America. Evaluation of sedimentary and paleontological similarities between the Luning Formation, in west-central Nevada, and the Antimonio Formation are consistant with the hypothesis that the Antimonio terrane experienced significant post-depositional tectonic dislocation during middle Jurassic time. Further study is needed to confirm this hypothesis.