Sponge growth on the cenomanian carbonate shelves of the carpathian basin: A record from spicule-rich turbidites

Abstract

The composition and features of numerous bioclasts found in the sandy/silty fraction of turbidite layers in the Middle-Upper Cenomanian deep-water sediments of the Silesian Nappe in the Outer Carpathians, dominated by sponge spicules, suggest that they originated from the growth and destruction of sponge assemblages formed on shelves of the northern margin of the Silesian Basin. The morphological types of spicules, by comparison with fossil and living species, lead to their attribution predominantly to three orders: Lithistida, Hadromerida and Astrophorida. Sponges lived in association with other benthic fauna such as calcareous benthic foraminifers and echinoids. Many sponge spicules were calcified in their primary environment before their redeposition by turbiditic flows. Favourable conditions for development of sponge assemblages occurred during the Cenomanian transgressions, being related to balanced delivery of sediment particles trapped by sponges and waters enriched with nutrients. The amount of nutrients absorbed by sponges is related to the dissolved silica content that originated from two sources: 1) terrestrial chemical weathering on the low-gradient plains of the shelves and 2) opal grains, such as siliceous plankton, suspended in the water column during bloom periods forced by seasonal upwelling.