Magnetic Stratigraphy of the Late Middle Eocene Coldwater Formation, Central Ventura County California: Implications for Sequence Stratigraphy

Edward Vance and Donald R. Prothero

Until recently, the middle Eocene Coldwater Sandstone could not be dated more precisely than its "Tejon Stage" molluscs (which range from 33-44 Ma). Fossil mammals and magnetic stratigraphy provide much higher-resolution chronostratigraphy. Most samples yielded a stable primary remanence that passed a reversal and fold test, and shows a clockwise rotation of about 100° ± 17° (consistent with other pre-Miocene units in the western Transverse Ranges). The chronostratigraphy of the late Uintan and Duchesnean mammals found within the redbeds in the Coldwater shows that the upper Cozy Dell Coldwater-Sespe sequence in central Ventura County spans Chrons C19r-C18n (approximately 39.5-42.5 Ma).

These new data allow a test of two recent sequence-stratigraphic models proposed for the Cozy Dell-Coldwater-Sespe formations. Campion et al. (1994) placed three sequence boundaries in this interval, which they correlated with the Ta3.6-Ta4.4 parts of the Haq et al. (1987) curve. The new chronostratigraphy shows that none of these three sequence boundaries are eustatically controlled, and they are miscorrelated by 4-5 million years (they actually occur within Ta3.4 or Ta3.5). Clark (1994) recognized three different sequence boundaries in the same sections; none of his sequence boundaries match the Haq et al. (1987) curve, either. Such mismatches further reinforce the caveat that sequence-stratigraphic correlations are only as good as the biostratigraphic data on which they are based.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California