Origin of peloids in Early Cretaceous deposits, Dorset, South England

Peloids are ubiquitous components in modern and fossil carbonates. The term peloid is non-genetic because the origin of these grains and the pathways of their formation are not fully understood. Based on Berriasian material originating from Dorset, southern England, we report here on peloids that result from the more or less in-place breakdown of previously micritized bivalve shells. The continuum from shell breakdown to peloids is documented by petrography and observation by scanning electron microscopy. The identical elemental composition of peloids and micritized shells confirms the petrographic observation and interpretation. Bivalve shells that were previously entirely micritized appear to be the preferential source for the formation of peloids. Obviously, the micritization weakened the shells, facilitating their breakdown and abrasion. This result identifies the fragmentation of micritized shells as a process leading to the formation of distinct peloids, adding to the categories of peloids recognized to date. Mold, mud, and microbial peloids observed in the studied sections and documented herein are distinct from peloids derived from bivalve shells.