Pliocene Bouse Formation of the Lower Colorado River Valley – Marine Incursion or Lacustrine Deposits?
The initial development of the Lower Colorado River in the early Pliocene coincides with deposition of sediments comprising the Bouse Formation. The origin of the southern Bouse Formation is open to interpretation and there currently are two general schools of thought:
- An origin by one or two marine incursions associated with the developing Gulf of California, supported by the paleontological and sedimentological interpretations.
- Deposition of in a very large saline lake fed by the developing Colorado River in a region that formerly consisted of closed basins; supported by geochemical, paleontological, and geomorphic interpretations.
Today, the Arizona Geological Survey (AZGS) released a contributed report by Jon Spencer (formerly of the AZGS), Kurt Constenius and Jordon Bright of the University of Arizona Dept. of Geosciences arguing for a lacustrine origin for Bouse Formation sediments in the Milpitas Wash, Blythe Basin, California.
From their abstract, “The Pliocene Bouse Formation in the lower Colorado River trough locally contains laminated marl and claystone interpreted by O’Connell et al. (2017) to represent the spring-neap tide cycle during sediment deposition in an estuary. Tidal cycles, if present, should be detectable by Fourier spectral analysis of lamination thicknesses in continuous sequences of laminated sediments. To evaluate the tidal interpretation, we attempted to photograph several laminated sequences in the southern Bouse Formation (south of Blythe, California) so that thicknesses could be measured from the photographs. Only one sequence, in lower Milpitas Wash (California), was identified where thicknesses could be determined with adequate precision from field photography. Fourier analysis of that sequence failed to identify evidence of tides. Furthermore, electron-microprobe analysis determined that laminations consist of alternating claystone and marl, which is consistent with annual changes in lake chemistry and sediment sources rather than physical changes in sediment sorting and transport during tidal cycles. Fourier analysis of data presented by O’Connell et al. (2017) of two nearby laminated Bouse sequences interpreted as tidal rhythmites also failed to identify statistically significant evidence of tides.”
To review or download the entire paper, follow the link below.
Spencer, J.E., Constenius, K. and Bright, J., 2018, Evidence of a lacustrine origin for laminated marl of the Pliocene Bouse Formation, Milpitas Wash, Blythe Basin, lower Colorado River valley. Arizona Geological Survey Contributed Report, CR-18-K, 33 p.
Posted 12/19/2018
