New invertebrate sites and marine ingressions in the Romualdo Formation, Aptian-Albian, Araripe sedimentary basin, NE Brazil

Abstract

Fossil invertebrates are relevant to the understanding of past environmental phenomena due to their sensitivity to abiotic conditions. This work identified fossiliferous associations in four localities of the Romualdo Formation, seeking to better characterize the depositional paleoenvironment of the unit during the Aptian-Albian, in the south-central portion of the Araripe Basin, NE Brazil. All the stratigraphic sections analyzed are located in mines in the Pernambuco state: Vale do Silício and Gregório (municipality of Araripina), VMB and Serrolândia (municipality of Ipubi). In total, 69 samples were collected, 2156 invertebrate specimens were analyzed, and 13 taxa were identified (Shrimp: Paleomattea deliciosa, Kellnerius jamacaruensis; Spinicaudata: Cyzicus brauni, Cyzicus pricei, Martinestheria codoensis, Estheriina costai; Gastropoda: Tylostoma ranchariensis, Paraglaugonia sp., Pseudomesalia (‘Pseudomesalia’) santanensis, indeterminate cassiopids, cerithids and naticids; and Bivalvia: Brachidontes araripensis). Using PAST v 4.04 software the specimens were clustered, based on taxa diversity and abundance, into three fossil associations (Cyzicus sp., Paleomattea deliciosa and Cassiopidae) whose paleoecological characteristics suggest a transitional paleoenvironment affected by transgressive events (marine incursion I and II). Spinicaudata characterize a transitional environment. The isolated presence of the decapods, in turn, reflects the first marine ingression. Finally, the Cassipiodae association, present in shell bed deposits, represents the second ingression. It can be noted that the invertebrate fauna presents variation in terms of temporal distribution along the analyzed sections, reflecting environmental instability in the paleoenvironment. The taphonomic characteristics are consistent with a shallow marine sedimentation environment, occasionally subject to storm action. The tethyan affinity of part of the fossils during the Aptian-Aptian, points to a marine ingression through the Parnaíba.