René Avalos

Field Methods

May 19th, 2015

Geologic History of Rainbow Basin, San Bernardino County, Ca

Abstract

Rainbow Basin, in the Mojave Desert of San Bernardino County, was formed from

convergence, divergence, and uplift. The mapped section contains Miocene rocks

and Quaternary alluvium. These more recent rocks were the result of divergence

and uplifting. Divergence occurred as the North American Plate attached to the

Pacific Plate and produced a rift sequence. The rift sequence created a basin, which

then filled with sediment and water, forming a lake. The sediments deposited during

this time were the Lower, Middle, and Upper Barstow Formations. Lower, Middle,

and Early Upper Barstow represent periods of anoxicity. After deposition of

additional sediment, faulting occurred as shown by the offsetting of Barstow marker

beds. As a result of faulting, folding took place. Evidence of this is shown by zeolites

found in the Barstow Formations. Uplift and erosion then occurred, exposing the

stratigraphic layers that had been buried. Old alluvium (Qoa) was deposited and

then faulting transpired from evidence of Qoa being offset. Middle alluvium (Qa)

deposited and was then cut by water to form a new channel of young alluvium

(Qay). This geologic history was assembled by mapping Rainbow Basin over a 2-day

period and constructing a cross section along strike. Mapping was done with a

Brunton compass to measure strike and dip. These measurements were then plotted

precisely on a map of the area and were used as a tool to create the cross section.