Elements in the Monterey Formation:
The Miocene Monterey Formation is broadly distributed (Figure 2) in the southern San Joaquin Valley, San Francisco Bay area, central Coast Ranges including the Santa Maria Basin as shown below (Figure 4), and Los Angeles Basin. Compared to World Shale Average (WSA) abundance, the Miocene Monterey Formation is enriched in numerous trace elements (Figure 5).
Elements that are highly positively
correlated (r2> 0.75) with organic carbon in these rocks include
chromium, copper, nickel, antimony, selenium (Figure
6), uranium, vanadium, and zinc; elements significantly correlated
(r2= 0.4-0.75) include arsenic, barium, beryllium, cadmium,
lutecium, molybdenum, and ytterbium. Selenium (Se) poses a particular
environmental hazard, but arsenic, cadmium, copper, molybdenum, nickel,
antimony, uranium, and zinc are also of environmental concern. In the
Santa Maria-Santa Barbara area of California, Monterey Formation rocks
contain as much as 70 parts per million (ppm) Se, and a selenium-enriched
(>10 ppm) stratigraphic zone up to 500 ft thick is widespread.
New study indicates that selenium-enriched stratigraphic zones in the Monterey Formation can be reliably identified from the association of selenium with carbon-rich intervals and uranium (U), a natural gamma-ray emitter (Figure 7; Isaacs, et al., 1999). Establishment of correlations with U-concentration would allow bedrock concentration of selenium and other potentially hazardous trace elements to be estimated with the use of a gamma-ray log, a type of geophysical record widely available from petroleum exploration wells drilled in oil-producing areas of California. This technique may be useful for making regional subsurface maps showing the distribution of potentially hazardous trace-element-enriched bedrock in California and other areas of North America where petroleum source rocks, such as the Cretaceous Pierre Shale (known to contain high selenium and uranium [e.g. Tourtelot, 1956; Landis, 1959]) of the mid-continent and Great Plains region, may represent similar potential environmental hazards.
Selenium and other trace elements are thought to be enriched in organic-carbon-rich marine mudstones because of their environment of deposition. Learn more about a depositional model for trace element enrichment.
Contact: Margaret A. Keller (firstname.lastname@example.org)
Western Region Energy Group–Environmental Studies
Modified: July 31, 2002