Calico Site Geology

The Calico Mountains consist of Mesozoic volcanics intruded and overlain by Tertiary igneous and lacustrine deposits. The Calico Site is located in adjacent dissected alluvial deposits known as the “Calico Hills”. These sediments are separated from their source area by younger alluvial fans, many of which are bounded by apparently active faults. The middle Miocene Barstow Formation, widespread in the Calico area contains a wide variety of lacustrine lithologies including the chalcedonies and cherts that are found as archaeological specimens. The Barstow formation, which underlies the Calico alluvial fan is well exposed by uplift and erosion of the western portion of the fan.

The artifact-bearing Pleistocene mudflows, debris-flows, and fanglomerates overlying the Barstow Formation, are termed the “Yermo deposits.” An angular unconformity between the Barstow and Yermo deposits indicates a regional depositional hiatus. The Yermo deposits (about 3 to 11 m thick) are topographically lower than high-level, dissected fans. Lithologies include weathered tuffs and crystalline igneous clasts in a poorly sorted, silty sand matrix. Lesser amounts of chert and limestone clasts are present. Sequentially younger alluvial deposits nested into older fans flanking the Calico Mountains are termed topographically intermediate, and low-, and active-fan sediments. Their relative ages can be inferred by comparison with deposits dated by soils and stratigraphic techniques, U/Th methods, correlation with Lake Manix shorelines, and regional changes in sedimentation (inferentially correlated to the marine isotope-stage chronology (Bischoff et al. 1981; Shlemon and Budinger 1990). Intermediate-, low-, and active-fan sediments are thus estimated to be about 60,000 to 70,000 years old, 10,000 to 20,000 years old, and less than 10,000 years old, respectively.


The Calico site lies in an area of active neotectonism; the Calico and Manix Faults are nearby and many other smaller faults pass through or very near the site (Shlemon and Budinger 1990). Some Quaternary displacements are expressed geomorphically; others have historical seismicity and are clearly active. The Barstow Formation is significantly deformed, and the Yermo deposits are locally folded.

Faulting and folding has contributed to greater dissection of surfaces than is typically observed elsewhere in the Mojave Desert. This is especially true compared with the eastern Mojave Desert where high-level fans with well preserved geomorphic divides and varnished desert pavements may be more than 700,000 years old (Shlemon and Budinger 1990). The deep dissection of the Calico Hills does not necessarily imply that the artifact-bearing beds must be at least 500,000 years old, as has been proposed. It probably reflects area-wide, high rates of recent uplift, folding, faulting, and erosion.

Haynes (1973:181) divided the Yermo deposits into younger and older units and postulated that the base of the 23-m-deep Control Pit I was in the upper (younger) Yermo deposits, whereas the site’s Master Pits penetrate the artifact-bearing, lower (older) beds. An alternative hypothesis (Shlemon and Budinger 1990), states that the two, high-level fan remnants are stratigraphically reversed. By this view, artifacts from the Master Pits occur in a younger, inset deposit, which have been locally displaced by one or more faults.


Evidence for this hypothesis comes from lithology and grain-size differences between the Master Pits and Control Pit I and from consideration of other climatically controlled fans in this desert. In Control Pit I, reddish-brown arkosic sands, locally interstratified with lenses of pebbles and cobbles, predominate; chalcedony and chert clasts are few. The Master Pits penetrate topographically middle-level fan deposits with mudflows, debris flows, and fanglomerates with numerous chalcedony and chert clasts (Shlemon and Budinger 1990). Comparable nested fans elsewhere in the Mojave Desert are thought to reflect geomorphic response to regional climatic change.

Evidence for the tectonic displacement of the high-level fans includes the apparent 25-degree northwest dip of Yermo deposits exposed in Control Pit I; compared with horizontally bedded sediments in the Master Pits; the site’s location in an active tectonic environment; and the presence of post-Yermo-age faults exposed in Master Pit I and adjacent bulldozer and back hoe cuts (Shlemon and Budinger 1990).

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