VERTICAL AND LATERAL DISTRIBUTION OF LACUSTRINE MICROBIAL AND ASSOCIATED CARBONATE LITHOFACIES AT THE PARASEQUENCE TO SEQUENCE SCALE, MIOCENE IDAHO HOT SPRINGS LIMESTONE, USA

The Hot Springs Limestone accumulated during the late Miocene in a series of lakes in extensional basins with volcanic underburden, closely associated with lava flows. Its lacustrine carbonate lithofacies vary systematically at meter to decameter scales and record paleobathymetry, limnologic conditions, and paleogeographic influences. Lithofacies include microbial boundstone (as open-framework bioherms, tightly cemented biostromes, and grain coatings), grainstone with a variety of carbonate and siliciclastic components, wackestone, micrite, and diatomite. Grainstone occurs between and within bioherm complexes.

These lithofacies stack in repeated patterns at the dm to m scale. A typical succession has isolated concave-upward conical arborescent bioherms separated by skeletal-dominated grainstone at base, overlain by upward expanding, increasingly arborescent bioherms with minor surrounding biostromal boundstone, and topped by widely expanded to interlinked arborescent bioherms surrounded by thick involute biostromal boundstone. These parasequence-scale successions are separated by thin bedsets of dominantly horizontal biostromal boundstone with subjacent zones of early diagenetic alteration that we interpret to record lacustrine flooding surfaces.

Lithofacies also vary laterally at the km scale in systematic ways over an area of more than 50 km². Lake plain and supralittoral zones are dominated by poorly sorted siliciclastic lithofacies with abundant root, invertebrate, and vertebrate traces. Littoral to proximal sublittoral zones contain poorly sorted siliciclastics, micrite, and wackestone with widely spaced, short (< 33 cm) bioherms. Bioherms are larger, taller (< 242 cm), and more closely spaced towards the medial sublittoral zone, where skeletal-oncoidal grainstone are more common between bioherm complexes. Farther lakeward, into the distal sublittoral zone, bioherms are shorter (< 20 cm), less complex, and more widely spaced. Profundal zone lithofacies include wackestone and micrite, with various admixtures of diatomite.

The distribution and character of diagenetic cements and pore types are strongly influenced by lithofacies and position with parasequence and parasequence set. Microbialites and grainstones have the best preserved primary porosity.