Clear as Mud: Why Carbonate Muds are Forming on Shallow Non-Tropical Seafloors

Marine carbonate muds are well known from deep-water offshore settings and shallow tropical regions. Shallow cool-water marine carbonates are usually characterized by course-grained sediments. Some of these deposits also contain abundant carbonate muds which have rarely been discussed. This study investigates the origin of these particles within Spencer Gulf, which is a large, shallow bay along the southern Australian margin of the Southern Ocean. Gulf sediments contain mixtures of carbonate gravels, sands, and muds, with some samples containing up to 84% muds volumetrically. These muds are deposited in a variety of shallow benthic environments on waters less than 50 meters depth (seagrass meadows, rhodolith pavements, and open seafloors). Fragments of bivalves, benthic foraminifera, coralline algae, and bryozoans form the sands and gravels within these environments. Morphologies of mud grains (silt- and clay-sized) were investigated using scanning electron microscopy point-counting techniques, and mineralogies were determined using X-ray diffraction analysis. Most of these mud grains are interpreted to have formed within their depositional environments from breakdown of larger skeletal grains via maceration (bivalves, benthic foraminifera, echinoderms, coralline algae, and spirorbids). Other mud components include skeletal particles that are naturally mud-sized including ascidian spicules, diatoms, sponge spicules, coccoliths, and planktonic foraminifera. Mineralogy of these muds corresponds with their biogenic origins, containing combinations of intermediate-Mg calcite, low-Mg calcite, and aragonite. Ancient rocks with marine clasts in lime mud matrices (such as packstones and wackestones) are usually interpreted to be from shallow tropical environments, but workers should be cautious while drawing these conclusions.