Occurrence and taphonomy of bivalves from the Níjar reef (Messinian, Late Miocene, SE Spain)

Abstract

Bivalves are conspicuous and important components in the Messinian Níjar reef, a fringing reef which developed on the northern margin of the Almería-Níjar basin in southeastern Spain. Bivalves appear in four distinct taphofacies that coincide with the facies and subfacies belts in which the reef has previously been divided: (1) In the reef core (Taphofacies 1) well-preserved, crevice-inhabitants occur. Sand- to gravel-sized bivalve fragments are included in the matrix that fills the framework voids. (2) Bivalve bioclats appear in gravels surrounding reef-framework blocks in Taphofacies 2. Sheltering among blocks diminished breakage of shells, pointing to a hydraulic origin of shell reworking and fragmentation in this upper part of the slope. The recognisable fragments belong to bissally-attached, cemented, and endobenthic forms. Endolithic, and some cemented and endosedimentary bivalves remain in life position. (3) Bivalves are found in Taphofacies 3 as clasts debris-flow breccias and constitute allochthonous assemblages coming downslope from the reef-talus. They also appear as sand- to gravel-sized particles in the calcarenites intercalating the breccias and only a few unbroken and sometimes articulated examples are considered to be autochtonous/parautochthonous in this environment. (4) Scarce, not fragmented, deep-water oyster are dispersed in the fine-grained sediments of the distal slope. The number of bivalve species is very low compared with the species richness of modern coral reefs, and most of the recorded taxa have a present-day temperate distribution. All this supports the hypothesis of the marginal biogeographical character of the western Mediterranean Messinian reefs.