Lithofacies, biofacies, and ichnoassemblage evolution of a shallow submarine volcaniclastic fan-shelf depositional system (Upper Cretaceous, James Ross Island, Antarctica)

Abstract

The Upper Cretaceous (Santonian-Campanian/lower Maastrichtian) Santa Marta Formation on James Ross Island, Antarctica, represents volcaniclastic shallow marine fan and shelf sedimentation adjacent to an active volcanic arc. A combined analysis of sedimentologic, paleoecologic, and ichnologic data allows for the recognition in this unit of six lithofacies associations, eight biofacies, and five trace fossils assemblages. Lithofacies are dominated by fine, massive, tuffaceous rocks; graded, turbidite-like tuffaceous sandstones; carbonaceous mudstones; resedimented conglomerates; coquinas; sandstones; silty sandstones; and minor stromatolite beds. Biofacies are defined by different composition and relative abundance of elements of the benthic fauna, mainly bivalves, gastropods, and serpulids, with minor elements represented by scaphopods, corals, brachiopods, and echinoids. Trace fossil assemblages include the most common elements of the Skolithos and Cruziana ichnofacies. A striking result of the analysis is that lithofacies, biofacies, and trace fossil assemblages form distinct, non-repetitive, vertically successive horizons, with their distribution boundaries roughly coincident. On this basis, seven major facies groups, showing a distinct combination of lithofacies, biofacies, and trace fossils, are distinguished in the Santa Marta Formation. These non-repetitive, vertically stacked facies groups reveal a one-way evolution of the depositional system during a transgressive-regressive cycle, with a new transgression at the top of the unit. The lower facies groups represent shallow marine settings with a very high rate of volcaniclastic sedimentation within subsiding basin. Shallow, volcaniclastic fan systems were probably formed at the base of delta slope and grew rapidly as a consequence of high sedimentary supply in equilibrium with basin subsidence. The upper facies groups probably represent sedimentation within the marine part of the envisaged deltaic system on a more stable shelf with diminished volcaniclastic sedimentary input.