PROGRESSIVE DETERIORATION OF TROPHIC CONDITIONS IN A CARBONATE RAMP ENVIRONMENT: THE LITHOTHAMNION LIMESTONE, MAJELLA MOUNTAIN (TORTONIAN-EARLY MESSINIAN, CENTRAL APENNINES, ITALY)

摘要

The Lithothamnion Limestone constitutes the uppermost carbonate unit of the Bolognano Formation outcropping in the Majella structure (Central Apennines, Italy). It represents the northern extension of the large Apulia Carbonate Platform and preserves an excellent record of the progressive decay of trophic conditions due to the approach of foredeep systems characterized by turbiditic siliciclastic sedimentation during the early Messinian. Sedimentological and compositional analyses were used to reconstruct the depositional model and evolution of platform environmental conditions. The profile is consistent with a homoclinal carbonate ramp, with a wide middle-ramp environment in which coralline algae, mainly forming the maerl facies, dominated carbonate production. This facies was associated with seagrass meadows colonizing the inner ramp. The outer ramp was characterized by bioturbated hemipelagic marl with planktonic foraminifera and pectinids in the aphotic zone. Three main stages of ramp evolution have been identified. During the first stage, the ramp was subjected to high-energy wave-dominated conditions, which favored the development of deep rip channels in which accumulations of vertebrate bones have been identified. In the second stage, maerl facies and seagrass meadows developed, initially in an oligotrophic setting, later followed by a slight reduction in light penetration. The third stage involved a general increase in fine terrigenous sediments, together with a further decrease in light and also by the spread of coralline algal bindstone facies. This elevated terrigenous input was associated with increased trophic conditions, as also shown by the occurrence of abundant plankton and low-oxygenated foraminiferal assemblages.