Lacustrine Tufa Mounds of the Miocene-Pliocene Copper Canyon “Formation,” Death Valley, California: Relationship to Organism Abundance and Distribution
Tufa mounds reflecting active spring deposition are numerous within limestone beds of the Copper Canyon “formation” (CCF). Tufa mounds ranging from 5-30cm in diameter and height occur in over 10 stratigraphically spaced limestone beds within the CCF. Many of the tufa mounds have a central micritic pipe surrounded by porous calcium carbonate suggesting active lake-groundwater interactions. The tufa mounds accumulated when fresh calcium-rich spring waters mixed with bicarbonate-rich lake waters. The original internal structure consists of porous and crystalline (thinolite) fabric often associated with aquatic vegetation. Gastropod, ostracod and plant fragment coquinas occur in the same bedding plane with the tufa mounds. Fringe cements consisting of several layers of finely laminated micrite often cover the plant material and many of the gastropods and ostracods often serve as the nucleus of peloids. Stable isotope data (δ18O range from -10.15 to -10.96‰ PDB) from the tufa mounds indicate the lake was fed by relatively fresh groundwater. The spring water allowed a more abundant and diverse fauna/flora population to thrive. Lateral measurements of stratigraphic sections demonstrate the tufa mounds were formed around the margin of the ancient Copper Canyon Lake.
The CCF represents a ~5 and 3Ma fanglomerate and fluvial-lacustrine basin fill deposit. The CCF is significant because it preserves 1800 meters of lacustrine deposits that contain shoreline playa features including highly abundant and diverse mammal and bird tracks. Track distribution and abundance is tied into the appearance of the tufa mounds and associated bioclastic carbonates. Lower in the lacustrine section tracks and bioclastic carbonates rich in gastropods, ostracods and plant fragments are scarce; however higher in the section, where tufa mound deposits appear, tracks and bioclastic carbonates are very abundant. Increase in mammal and bird tracks along with bioclastic carbonates indicates a groundwater influx into a closed basin and freshening of the ancient Copper Canyon Lake waters. The lacustrine tufa mounds of the Miocene-Pliocene Copper Canyon Lake and the stratigraphic and spatial association of fossil vertebrate tracks and invertebrate fossils is an excellent example of the interplay between lake water chemistry and organism distribution.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009