Sequence stratigraphy of the Punta Ballena Member of the Jama Formation (Early Pleistocene, Ecuador): insights from integrated sedimentologic, taphonomic and paleoecologic analysis of molluscan shell concentrations.

Early Pleistocene strata are well exposed along the uplifted eastern margin of the Esmeraldas–Caraquez Basin, particularly on the coastal cliff west of the city of Jama (a type locality for shallow marine Quaternary sedimentary exposures on northcentral Ecuador). Here, at least 180 m of sediments accumulated in inner-shelf to subaerial settings are present (Jama Formation). This paper is concerned with the middle stratigraphic unit of the Jama Formation (the Punta Ballena Member, PBM). Integrated sedimentologic, paleontologic, and stratigraphic observations reveal that the PBM is composed of eight, disconformity-bounded depositional sequences. Hiatal shell concentrations, which formed as a consequence of landward and seaward stratal attenuation, are associated with sequence bounding and intrasequence surfaces. Together with sedimentologic characteristics, these fossiliferous levels are important indicators of facies architecture and, therefore, are useful tools for interpreting sequence stratigraphy. According to their taphonomic features and taxonomic composition, the hiatal shell concentrations occurring within the PBM may be grouped into two main types, each of which occupies a predictable stratigraphic position within the sequences: current/wave-winnowed beds at the cycle base and community shell concentrations in a roughly mid-cycle position. Current/wave-winnowed beds, commonly underlain by Thalassinoides burrows that penetrate to depths of as much as 1.5 m below ravinement surfaces, are transported skeletal concentrations and formed through physical processes in shallow, highenergy environments. They comprise a time-averaged mixture of specimens having different ecologies and states of preservation and accumulated mainly under conditions of total passing of siliciclastic sediment (i.e. terrigenous sediments are delivered to the site as suspended load but, owing to the high environmental energy, failed to accumulate). Community shell concentrations occur in sparsely fossiliferous intervals and result from reduced net sedimentation within low-energy, relatively deeper settings. They are composed of matrix-supported, loosely clustered, life-oriented and little-disturbed fossil assemblages, but are packed in excess of likely living densities and are visually dominated by successive generations of the infaunal bivalve Raeta undulata, which colonized a slowly aggrading muddy seafloor. The sedimentological and paleontological evidence presented here not only supports the results of previous studies, which demonstrate the value of condensed shell concentrations in the identification of stratigraphically significant surfaces, but also provides adds to the distinctive features of these shell beds.