Sedimentology and Controls on Caddisfly Case-Dominated Lacustrine Carbonate Mounds in the Paleocene Second Member, Funing Formation, Subei Basin, East China

The Paleocene second member, Funing Formation, in the Jinhu Depression, Subei Basin of East China contains lacustrine carbonate successions with exquisitely preserved caddisfly case-bearing microbialites. These understudied deposits provide a unique opportunity to establish diagnostic criteria and elucidate the physico-chemical and biological controls on their formation. The study shows that the fossil caddisfly cases are single-walled or nested tubes, appearing curved and tapered, and ranging from agglutinated to non-agglutinated. They occur in concentrated clusters alternating with micro-laminated carbonate layers. Two facies associations (FA1 and FA2) comprising six discrete facies (F1-F6) are defined. FA1, representing wave-dominated, high-energy carbonate shorelines, is widespread along the western depression. It includes littoral-zone mixed carbonate shoals (F1), caddisfly case-dominated spring mounds (F2) and sandstone (F5), alongside littoral-to-profundal, low-energy carbonate microbial mounds (F3), sediment-gravity flow deposits (F4) and shale (F6). FA2, found in localised eastern areas, corresponds to sheltered, low-energy shorelines dominated by littoral microbial mounds overlying sublittoral-to-profundal facies similar to FA1. These successions are interpreted as deposits of a hydrologically open, freshwater to brackish-water lake. The western margin was influenced by spring/groundwater discharge, evidenced by gregarious caddisfly case accumulations, a dominant freshwater fauna with subordinate brackish-water indicators, absence of evaporites, negative delta(1)& sup3;C and delta O-18 values with poor covariance and crystalline dendritic to fan-shaped calcite fabrics. Microbial mound growth was primarily controlled by microbial processes (e.g. calcification, metabolism and extracellular polymeric substances/cell surface functions). In contrast, the distribution of caddisfly case-dominated spring mounds reflects the larval habit of annually attaching pupal cases to substrates in photic, high-energy littoral zones with concomitant spring discharge.