PRECAMBRIAN HIGH-ENERGY, MARINE SILICICLASTIC SHORELINE DEPOSITS IN THE SAN JUAN MOUNTAINS, SW COLORADO (USA)

In southwest Colorado, Precambrian basement rocks include the 1.7 Ga Twilight Gneiss and related intrusives, and the 1.4 Ga Uncompahgre Formation and related metasedimentary rock units. By Late Cambrian time, Precambrian basement in this region had been eroded to form a paleotopographic surface with at least 65 m relief. During the Paleozoic, this paleotopographic surface was successively onlapped by the Cambrian Ignacio Formation and Devonian Elbert Formation, and finally covered by the Devonian Ouray Limestone.

Below the Paleozoic rocks, the Precambrian basement rocks are covered in places by a pebble-to-boulder conglomerate composed mostly of quartzite and vein quartz clasts. There has been a controversy about either interpreting the conglomerate as Precambrian or as part of the Cambrian Ignacio Formation ("Weasel Skin Conglomerate Member"). This study suggests that the conglomerate is a separate lithostratigraphic unit of probable Precambrian age, based on compositional and textural differences between the sandstone matrix of the unit versus the sandstones of the overlying Ignacio Formation or Elbert Formation.

The unnamed conglomerate unit ranges from 0-23 m thick in the western San Juan Mountains. The unit consists of stratified, poorly sorted, clast-supported pebble-boulder conglomerate, massive pebbly sandstone, low-angle stratified sandstone, and trough cross-bedded sandstone. The conglomerates show normal grading and inverse-to-normal grading. There is an abundance of discoid clasts, and imbrication is common. Pocket structures and scours in the coarser conglomerates are infilled by pebble conglomerate or sandstone. Clasts at the top of stratified conglomerate sheets commonly protrude into the overlying conglomerate sheet. In places the conglomerate is buttressed against Precambrian basement rocks.

The conglomerate unit is interpreted as a high-energy clastic shoreline deposit. Clast long-axis orientation, imbrication, and cross-bedding consistently show north-to-south transport toward a paleo-shoreline consisting of bluffs and vertical sea cliffs of Precambrian basement rocks. Rounded quartzite clasts presumably moved by waves are up to 1.4 m in long dimension, up to 0.5 m3 in volume, and had calculated mass of up to 1.1 metric tons. Eroded clasts from the conglomerate are found in overlying Paleozoic units.