a potential hydrocarbon resource, the unit is considered to be equivalent to the Sacate, and is grouped with the lower-lying Cozy Dell Shale and Matilija Sandstone as the Gaviota-Sacate-Matilija Sandstone Play. The likely source rocks for any petroleum accumulation in the Coldwater-Sacate are the lower-lying organic-rich shale units, such as the Cozy Dell and the Juncal-Anita Formations. Since this deep-lying play is incompletely explored, the estimates for hydrocarbon resources are speculative, with the United States Bureau of Ocean Energy Management, Regulation and Enforcement giving a range of 117 to 127 million barrels of oil recoverable using current technology for the entire Santa Barbara-Ventura Basin Province. Some onshore oil fields produce from the Coldwater, including the Ojai Oil Field north of Santa Paula and Sespe oil field north of Fillmore, not far from the type locality for the formation. Deposition environment and tectonic history The unit is of middle to upper Eocene age, being deposited in a shallow marine environment, during a time in which the sea was receding and becoming more shallow. Because the process was not uniform, with periods of shallow-water deposition alternating with deeper water conditions, the predominantly sandstone unit is interbedded with siltstones and shales, with the siltstone and shale making up around 20% of the total composition. Late in the period of deposition, large oyster beds appear in the stratigraphic record, indicating a brackish-water nearshore environment. The source rock for the sediment making up the Coldwater was mostly granitic, and pebbles or cobbles of granite and quartzite appear in some of the nearshore sandstones near the top of the unit. At its top the unit grades into the redbeds and conglomerates of the Sespe Formation, which was deposited in a floodplain, riverine, and estuarine environment onshore. A 1996 study by Donald R. Prothero and Robert J. Emry determined the age of the unit using magnetostratigraphy, and gave its period of deposition as from 39.5 Ma (million years ago) to 42.5 Ma. Additionally, studies of the correlation of the underlying Cozy Dell and Matilija Sandstone with similar units in the California Central Valley, along with paleomagnetic data, show that the tectonic block on which the Coldwater and its surrounding formations were deposited – including the western part of the Transverse Ranges – has rotated clockwise almost 90 degrees since the time of deposition, also moving north-northwest from its original location. This finding is consistent with earlier studies