Hybrid event beds (HEBs) and the ‘greywacke problem’ revisited

Matrix-rich sandstones (matrix content >15%) have long been recognized in the ancient sedimentary record, most commonly in deep-marine depositional systems. The origin of the matrix component, whether it was detrital or diagenetic, has been widely debated in the literature and the issue became known as the ‘greywacke problem’. However, with increased research focus in clay-rich sediment gravity flows that emplace matrix-rich sandstones (hybrid event beds and related mud-rich strata) of fundamentally primary origin, it is timely to revisit the greywacke problem in the context of current state of knowledge of HEBs and associated transitional flow deposits and matrix-rich sandstone models. In the present study, literature-based datasets for greywackes and HEBs were compiled and assessed for patterns in bed structure, matrix origin, matrix cut-off percentage, depositional environment, and geological age in order to address some key questions pertinent to the greywacke problem. The dataset indicates that the bed structure of numerous ancient greywackes is similar to HEBs and its variants. The majority (61%) of studies in the greywacke database suggested a ‘secondary’ origin of the matrix whereas 39% of the studies advocated a ‘primary’ genesis. In comparison, a ‘primary’ origin of matrix was inferred for all HEBs and related deposits. Nevertheless, a mixed nature of matrix comprising both detrital and diagenetic components cannot be ruled out in both greywackes and HEBs. The majority of greywackes (82%) and HEBs (87%) display similar range of matrix contents (>15%), therefore the matrix cut-off threshold for matrix-poor and matrix-rich sandstones can be confidently placed at 15–20% matrix. This dataset illustrates that both greywackes (86%) and HEBs (82%) occur most commonly in deep-marine environments. Greywackes (48%) and HEBs (37%) of Paleozoic and Cenozoic age respectively dominate in the complied database. Analysis of these factors, integrated with learnings from HEB and related deposits suggest that many ancient deep-water greywackes may be re-interpreted as HEBs with significant original detrital matrix.