Fauna and palaeoecology of the Middle Cenomanian Praeactinocamax primus Event at the type locality, Wunstorf quarry, northern Germany

Abstract

A systematic account of the fauna from the early Middle Cenomanian Praeactinocamax primus Event, a 50–60-cm-thick marl bed, at the type locality, Wunstorf quarry, to the west of Hannover (northern Germany), is given. Numerous invertebrate taxa (over 50 in total) have been collected, including two species of belemnites, ten ammonites, at least 12 bivalves, a single scaphopod, five gastropods, at least eight brachiopods, two solitary corals, a single hydrozoan, four echinoids, and ten polychaetes. The benthic community of the primus Event clearly represents a soft-bottom fauna, with hard-bottom elements limited to secondary hard substrates. Most of the macrobenthic elements constitute suspension feeders; shallow-infaunal deposit feeders, grazers and microcarnivores occur as well, while deeper infaunal elements are largely missing. The nekton is represented by fish remains, belemnites, and planispiral and heteromorph ammonites with inferred nektobenthic modes of life. Both biofacies (absence of photic elements) and sedimentological evidence (fine-grained fabric, preservation of delicate faunal elements) suggest that deposition of the primus Event at the type locality occurred in a low-energy setting below the (eu-)photic zone and storm wave base in water depths of ca. 50–100 m. The cyclic and correlative nature of the precession-forced marl-limestone couplets of the interval containing the primus Event and the absence of sedimentological evidence for significant redeposition rules out “snapshot preservation” by obrution. Nor is the faunal richness of the primus Event related to time-averaging, because the bed accumulated with sedimentation rates of ca. 50 m/myr. The abundance of suspension- and deposit-feeding biota, however, indicates enhanced fluxes of organic carbon to the seafloor, probably related to high surface-water productivity. The formation of the primus Event was also linked to transgressive depositional conditions after a pronounced sea-level lowstand across the Lower/Middle Cenomanian boundary. It should be noted that correlation of sections across northwest Europe clearly shows that the initial transgressive onlap onto the basin margins following the lowstand started considerably earlier than the primus Event, at the junction of marl-limestone couplets B40/B41 in the Anglo-Paris Basin cyclostratigraphic scheme. The primus Event (marl bed of couplet C1) thus represents a second transgressive pulse of a high-frequency (100 kyr short eccentricity) cycle within the transgressive systems tract (TST) of a third-order depositional sequence. “Pulse faunas” of northerly affinity (such as the Boreal belemnite P. primus) and published oxygen stable isotope records suggest a cool-water incursion during the “ primus transgression”. These special oceanographic conditions (sea-level rise, incursion of cool waters, high primary productivity, ample food supplies, limited physical disturbance) resulted in a diverse benthic (and nektobenthic) faunal community in the primus Event.