PALEOZOIC TECTONIC EVOLUTION OF THE SAN JUAN MOUNTAINS, SOUTHWEST COLORADO

Episodic reactivation of existing basement faults occurred throughout Paleozoic time in the central San Juan Mountains, extending into the Paradox basin and adjacent areas. The earliest recorded movement along the basement wrench fault system was between 1.8 and 1.4 Ga and probably occurred around 1.6-1.7 Ga. Two sub-parallel fault blocks form curved patterns in the Grenadier Range, and the Mt. Sneffels massif. In both cases upturned quartzite of the Precambrian Uncompahgre Formation is juxtaposed against gneissic rocks of the older Twilight Granite.

Movement along the faults in Paleozoic time began in the Late Cambrian during deposition of the Ignacio Formation. Conglomerate consisting entirely of boulders from the Uncompahgre Formation abut the ancient faults, grading rapidly away from the faults to fine-grained sedimentary rocks of marine origin. The Late Devonian Elbert Formation rests disconformably on the Late Cambrian rocks regionally, and with angular unconformity on the Precambrian Uncompahgre Formation across the fault blocks. Facies relationships in the latest Devonian Ouray Formation suggest a reversal of vertical fault movement at this time. The Grenadier fault block was again elevated and eroded in Mississippian time.

An angular diastem within the Hermosa Formation indicates Middle Pennsylvanian movement along the Snowdon fault north of Coal Bank Pass. An angular unconformity of low relief at the Hermosa-Cutler contact occurs north of the Coal Bank Pass fault. Limestone-pebble conglomerate beds occur in lower Cutler rocks, Early Permian(?), south of the Coal Bank Pass fault on Engineer Mountain. The entire post-Ismay zone of the Honaker Trail Formation of the Hermosa Group is missing due to pre-Cutler erosion.

An anomalous 800-foot-thick section of fine-grained sandstone occurs below the Paradox Formation in Hermosa Mountain. Rare fusulinids suggest an Atokan (early Middle Pennsylvanian) age. The isolated deposits have been interpreted as deltaic, with sediments derived from the Grenadier and Sneffels fault blocks. It is now believed that the Grenadier and Sneffels fault blocks constitute the San Luis uplift, the source area for the Atokan Sandia Formation of northern New Mexico.