FLAKED LITHIC MATERIAL SOURCES IN SOUTHWESTERN COLORADO

The physical geology of southwestern Colorado facilitated the hunt for rock types suitable for making flaked lithic tools for prehistoric peoples living in this region. Four key geologic formations supplied most of these materials. The first three are widespread in Canyons of the Ancients National Monument (CANM). The fourth is restricted to Durango.

The green and purple “silicified siltstones” of the Morrison are well-known lithic sources quarried in the lower canyons of CANM. Recent work by the USGS reveal that these lithologies originated as volcanic ash that was altered to zeolites and feldspar in a broad, shallow, highly alkaline lake. Zoned water chemistry resulted in lithologies that vary systematically within the paleolake boundaries. Archeologists may be able to use these predictable geographic trends to identify sources of Morrison artifacts.

Fluvial sandstones of the Burro Canyon and Dakota formations are heavily silica- cemented forming a white, flakable quartzite. Cementation of the Burro Canyon typically occurs in the basal sandstone in irregular nodules and patches. This style of silicification may be due to groundwater mixing in the Cretaceous and the final quartzite is usually lower quality. Good-quality laminated chert altered from algal lacustrine limestone occurs near the top of the Burro Canyon. This material was quarried along mesa tops in the southwestern corner of CANM.

Quartzites in the sandstones of the Dakota formation formed as silcretes and appear in two zones: the basal sandstone at or just above the Burro Canyon contact, and within thin, gray, fine-grained sandstones interbedded with coaly shales in the upper unit. Both are excellent quality. The former is associated with several known quarries on upper mesa slopes in CANM.

The McDermott Fm. is a syn-tectonic unit deposited at the beginning of the Laramide orogeny. It records the uplift of the LaPlata Mountains as a series of purple, andesitic debris flows. At the base of these flows is an anomalous conglomerate comprising siliceous cobbles interbedded with the more typical andesitic ones. The siliceous cobbles include Precambrian metaquartzites, fossiliferous Paleozoic cherts and Cretaceous petrified wood. These siliceous materials were extensively quarried along the outcrop trend southeast of Durango.