GENESIS OF MULTIPLE-EVENT SHELL CONCENTRATIONS IN EPEIRIC SEAS: BAKEVELLIID-DOMINATED SHELL-BEDS OF THE ROMUALDO FORMATION, CRETACEOUS, ARARIPE BASIN, BRAZIL

Shell-rich concentrations are complex deposits yielding key paleoecological and paleoenvironment data. Herein we describe a 0.9-m-thick bakevelliid-bearing shell concentration in the upper Romualdo Formation, eastern Araripe Basin (Sobradinho geological section). The concentrations internal complexity is expressed by: (1) six single, cm-scale shell-rich units bounded by discontinuity surfaces; (2) mixture of shells with distinct taphonomic signatures/pathways; and (3) mixture of mollusk species from different sub-environments. Based on sedimentologic/petrographic, taphonomic and paleoecologic attributes, at least six microstratigraphic units (MUs) were recognized, each one separated by thin layers of silty shale with irregular boundaries. Development of the bakevelliid-bearing shell concentration started with interbedded bivalve-gastropod rudstones (Rbg) and bivalve-gastropod floatstones (Fbg) from MU-1. The degree of packing varies from densely to loosely packed, and bioclasts (few bakevelliids, cassiopids, epitonids, and undetermined remains) are poorly sorted and fragmented, displayed parallel to the bedding. These are overlain by MU-2, that comprehend coarsening upward successions of ostracode mudstones (Mo), gastropod-bivalve wackstones (Wgb), bivalve-gastropod packstones (Pbg) and gastropod-bivalve grainstones (Ggb). Chaotically arranged, highly fragmented shells with occasional occurrence of J-shaped bioturbation characterize MU-2. Above, the MU-3 is arranged from base to top by Wgb, Mo, and Ggb, displaying chaotically arranged and fragmented bioclasts, namely highly fragmented undetermined bivalves and few poorly fragmented large-size gastropods (cassiopids and undetermined ones). Towards the top, chaotically arranged and highly fragmented bioclasts constituting a Wgb characterize MU-4. The MU-5 comprehend three microfacies, Pbg, Mo and Wgb, that are characterized by abundant terrigenous material, highly fragmented bivalve shells and bivalves with closed articulated valves, at the top. The uppermost concentration (MU-6) is a 15-cm-thick succession of interbedded bakevelliid rudstones (Rb) and bakevelliid floatstones (Fb). The MU-6 is dominated by predominantly disarticulated, non-fragmented bakevelliid shells, oriented parallel to bedding, and at the very top by closed articulated bakevelliids preserved in growth position. The _in situ_ shells are abruptly covered by black shales and sandstones with hummocky cross-stratification, indicating that living bakevelliids were abruptly buried by probably storm-induced event deposits [FAPESP 16/13214-7, 17/20803-1, 18/01750-7; CNPq 401039/2014-5, 300107/2015-3, 152385/2016-9, 304800/2017-1; Petrobras 2014/00519-9].