BASIN-SCALE PROCESSES RECORDED IN A MICRO-PARASEQUENCE, LANEY MEMBER, EOCENE GREEN RIVER FORMATION, WYOMING, USA

Parasequences of the LaClede Bed are critical indicators of depositional processes within lacustrine paleoenvironments of the Laney Member, Eocene Green River Formation. To better understand causes for thickness variation in meter-scale parasequences, we examined a centimeter-scale parasequence that contains transgressive, regressive, and flooding indicators noted at larger scale.

Unweathered slabs were examined by polarizing and fluorescence microscopy, SEM-EDS, XRD, staining, and image-analysis. Thin sections and core subsamples (2 mm diameter) were drilled perpendicular to laminations. Variations in composition and structure were recorded and analyzed.

The 20 cm unit is thinly to thickly laminated with diffuse contacts and local disruption by burrowing or fluid escape. One contact is defined by poorly-developed microstromatolites overlain by a microbreccia. The dominant matrix are crystals of Ca-rich dolomite generally less than 10 um with quartz, feldspar, and clay. UV-induced fluorescence suggests discontinuous algal-containing laminae and scattered carbonized plant debris are present. Calcite ostracods are abundant in the upper half and some patches of ostracods are dolomitized. Of particular interest were contrasting laminasets of high and low intercrystalline porosity. Low porosity laminae contain more clastic material including clay.

Five facies (pebble lag, stromatolitic, finely-laminated dolomicrite, dolomicrite with calcite ostracods, and dolomicrite with dolomitized ostracods) present among meter-scale parasequences were found at a microscale in only 20 cm of accumulation. These record several depositional processes expected in a low-gradient setting and demonstrate the ability of high-frequency basin-scale processes to leave complete or nearly complete depositional records.