Calico Site Stratigraphy
The artifact-yielding Yermo Formation overlies the Barstow Formation, and, consists of two depositional units: a basal mudflow and overlying, crudely intercalated debris flows and fanglomerates; and overlying, reworked fan deposits, primarily arkosic sand, with a strongly developed relict paleosol at the surface (Shlemon and Budinger 1990).
The mudflow and fanglomerate consist of lenticular, poorly stratified layers of sands and angular gravel. There are no buried paleosols or significant unconformities. Deposition probably occurred within one climatic cycle of perhaps a few tens of thousands of years.
The upper, reworked arkosic sand unit (about 1.5-m thick at Master Pit I) contains highly weathered tuff fragments. Based on its lithology, distinctive red color, and other weathering characteristics, the sand was probably derived from nearby, previously weathered fan-deposits. The overall stratigraphic section provides evidence that deposition occurred in response to gradual changes from semiarid to arid climatic conditions.
Little if any of the original fan surface is preserved. A relict paleosol, capping the upper arkosic sands, is strongly developed, with a stone-free, vesicular (Av) horizon below a closely packed desert pavement, a clearly visible argillic (Bt) horizon, and multiple, superimposed calcic (Bk1, Bk2), horizons. Illuvial clay films are apparent. The paleosol is polygenetic with gravelly calcic horizons (stage 11 carbonate development) at depths of approximately 10, 25, and 45 cm. Pedogenic carbonates probably derived from weathered carbonates and windblown dust. The soil was probably subjected to several epochs of carbonate illuviation when the climate was more humid than the present
The alluvium deposited around and below the eroded Yermo Fan may be divided into older and younger subunits. The older alluvium deposited well below the Yermo Fan, following the separation of the latter from the Calico Mountains, is itself an ancient surface. This alluvium stands one to two meters above more modern active channels and forms flat-topped interfluves covered by a well-developed desert pavement. Pavement clasts are coated by black Mn-rich rock varnish, attesting to formation during a Pleistocene humid phase. Studies elsewhere reveal that the formation of such a desert pavement requires about 10,000 years. This accords with the presence of multiple indurated K-horizons in the underlying alluvium–also regarded as an indication of at least 10,000 years of pedogenesis under semiarid conditions following dissection of the Yermo Fan and deposition of the post-Yermo Fan alluvium. The multiple carbonate horizons suggest a complex history of environmental oscillations since the deposition of the older post-Yermo Fan alluvium.