LATE TRIASSIC HEXACTINELLID SPONGES FROM THE PONGO DE LOROCACHE LIMESTONES, PERU: NEW EVIDENCE FOR SILICEOUS SPONGE DOMINANCE ON SHALLOW CARBONATE PLATFORMS

Abstract

Upper Triassic lyssacinosan hexactinellid sponges are described from the late Triassic Pongo de Lorocache limestones, Santiago basin, Peru. The sponges are found in the uppermost levels of the Chambará Formation. This unit exhibits sequences of the Upper Norian to Lower Rhaetian showing an unconformable contact with Jurassic sequences. A great number of complete cup-shaped sponge body fossils occur in life position showing gregarious habits, and forming biostrome accumulations. Preservation prevents definite identification. However, the specimens have the characteristics of hexactinellid hexactinosans, very closely related to the Superfamily Crepospongiodea. The Triassic-Jurassic marine crises produced an unexpected resurgence of siliceous sponges in detriment of carbonate skeletal fauna. Previous studies in Nevada (USA), Peru, Morocco, Austria and France show that this sponge event can be considered a global phenomenon, apparently associated with an important change in atmospheric pCO2. This finding in the Norian- Rhaetian levels slightly predates the main peak of siliceous sponge dominance recorded in Lower Jurassic units. This implies that at least previous appearances of hexactinellid sponges dominated shallow water carbonate boundstone intervals before the maximum peak of the extinction event.