Sedimentary evolution and distribution of benthic fauna of an Aptian protected bay (Oliete Subbasin, Spain)

Detailed facies and sequence stratigraphic analysis of an Aptian shallow-marine succession illustrates the evolution of sedimentary environments and their benthic faunal associations in a marginal bay developed in northeastern Iberia (Maestrazgo Basin, Oliete Subbasin). In this bay, marine circulation was hampered by wide stationary shoals located in the low-subsidence transitional areas between subbasins. The Aptian sedimentary record of this bay (Josa and Oliete formations) is composed of four transgressive–regressive (T–R) sequences bounded by major unconformities generated during relative sea-level falls. The Josa Fm (formally defined herein) encompasses an asymmetrical T–R cycle (sequence J). It comprises transgressive prodelta clays with episodic orbitolinid blooms, grading upwards to regressive freshwater-influenced littoral and delta-front sandstones and sandy limestones which yield diverse bivalve communities dominated by trigoniids. The Oliete Fm encompasses three T–R cycles (sequences O1–O3), mostly deposited in a protected lagoon. The transgressive hemicycle of each sequence is dominated by marls containing low-diversity faunal associations composed of ostracods and mm-sized gastropods and bivalves, indicating stressed (probably dysoxic) conditions in the deepest parts of the lagoon, related to a lack of water circulation. The regressive hemicycle of sequences O1 and O2 is characterised by low-energy intertidal to shallow subtidal deposits including sponge spiculites, mud-dominated accumulations of articulated bivalves and gastropods with low-diversity microfaunal associations composed mainly of Choffatella and few other textulariids, and marginal oyster-serpulid patches. The uppermost part of sequence O2 is characterised by the development of siliciclastic tide-influenced coastal plains. Sequence O3 consists of massive intertidal to shallow subtidal glauconite accumulations related to the transgressive reworking of siliciclastic deposits. Foraminiferal associations in sequence O3 show an increase in diversity (including miliolids, lageniids and diverse textulariids), which was probably fostered by a more stable connection with open seas, caused by tectonic activity in the bounding shoal areas. The analysis of these four sequences represents a comprehensive and detailed characterisation of shallow-to-marginal marine benthic communities integrated in a sequence stratigraphic framework.