Prodeltaic deposits record a distal, minimally reworked archive of dominant processes active at the fluvial-marine interface. The Fish Creek-Vallecito Basin (CA, US) preserves an ~3 km-thick, lower Pliocene, progradational deltaic succession formed when the ancestral Colorado River infiltrated the early Gulf of California. The interpreted prodeltaic unit in this succession (Mud Hills Member of the Deguynos Formation) contains an ~40 m-thick interval of rhythmic bedding with consistently alternating silt- to fine sand-dominated and clay-dominated beds forming couplets with an average thickness of 12 cm. Sedimentological analysis of the rhythmites reveals the couplets are laterally persistent with gradational to sharp, flat contacts, parallel, wavy, and ripple cross-lamination, and localized internal scours. Notably, grain size analysis performed with laser diffraction techniques shows a consistent pattern of inverse grading transitioning to normal grading. The cumulative sedimentological evidence indicates that deposition of the rhythmites was accomplished via hyperpycnal flows, each couplet likely representing one event in a setting characterized by high overall depositional rates. Time series analysis performed on bed thickness of 265 measured couplets obtained a prominent spectral peak above the 99% confidence level corresponding to a periodicity of ~18 couplets. Interpretations are tentative, but potential controls on cyclicity include seasonal climatic events or the lunar nodal cycle. Finally, a review of previous studies of the progression of the lower Colorado River to the Gulf of California, via the fill and overspill of a series of lakes, places deposition of the Mud Hills Member hyperpycnites immediately after the fourth, and last, lake drains, commencing unimpeded Colorado River flow and formation of a fluvial-dominated proximal prodelta. This study contributes to the recently established record of fine-grained hyperpycnites in the ancient rock record and to previous work substantiating the theory that ancient prodeltaic mudrocks may have been primarily deposited rapidly via hyperpycnal flow rather than slowly via suspension settling. Also, this study complements previous work on the initiation and progradation of the Pliocene Colorado River delta and adds to the growing evidence for the lake-spillover model of the advancement of the lower Colorado River to the Gulf of California.
School of Medicine
Master of Science (MS)
Year Degree Awarded
Date (Title Page)
Library of Congress/MESH Subject Headings
Sedimentology - Colorado River; Sediments - Geology - Colorado River; Geology - Colorado River - California; Fish Creek-Vallecito Basin - California;
Subject - Local
Prodeltaic deposits; Rhythmites; Pliocene Colorado River Delta; Hyperpycnites; Hyperpycnal flows;
Loma Linda University Libraries
This title appears here courtesy of the author, who has granted Loma Linda University a limited, non-exclusive right to make this publication available to the public. The author retains all other copyrights.
Waresak, Sandra R., "Cyclically-Forced Hyperpycnites of the Ancient Colorado River Proximal Prodelta" (2016). Loma Linda University Electronic Theses, Dissertations & Projects. 406.
Loma Linda University Electronic Theses and Dissertations
Loma Linda University. Del E. Webb Memorial Library. University Archives