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The study of the Cretaceous and Paleocene in the Santa Lucia Range has been of value not only in connection with the sedimentation history of the state but also in deciphering the diastrophic history. Practically all the orogenies that have affected the Coast Ranges since the close of the Jurassic have been strongest in the west, decreasing in effect eastward. Thus, disturbances that resulted in slight disconformities along the west side of the San Joaquin Valley are represented by profound unconformities in the Santa Lucia Range. Relations that in some places are obscure on the east are clear and unmistakable on the west. This is particularly true of the Cretaceous.
Most of the recognized divisions of the Cretaceous are present in the Santa Lucia Range but they are somewhat thinner than in the better known sections in the east part of the Coast Ranges and on the west border of the Great Valley of California.
The Cretaceous of the Santa Lucia Range is divided into three formations separated by profound unconformities. The Lower Cretaceous, the Marmolejo formation, is of rather limited distribution, having been largely removed or covered with later sediments, and occurs only in the south part of the range where it consists of 4,000-5,000 feet of dark shales with minor amounts of sandstone and conglomerate. This rests either disconformably or unconformably on Franciscan-Knoxville rocks with a thin fossiliferous basal conglomerate, except on the east side of the range where there are local, thick, and very coarse breccias largely made up of angular blocks of chert, basalt, diabase, and sandstone derived from the Franciscan. These breccias occur only immediately west of the Las Tablas fault, t inning not far westward away from it, indicating movement, either of folding or faulting, along this zone in the very late Jurassic or early Cretaceous. Local movements of considerable magnitude took place at this time throughout the Coast Ranges. This diastrophism is called the Diablan orogeny by the writer. The meager fauna of the Marmolejo formation indicates that it belongs to the Paskenta stage of the Lower Cretaceous; the Horsetown stage does not appear to be represented.
The Marmolejo formation was strongly deformed and largely removed, as a result of the mid-Cretaceous disturbance, before the deposition of the Upper Cretaceous. Long continued erosion appears to have greatly reduced the region since the earliest Upper Cretaceous sea spread over an area of low relief, as shown by the character of the Jack Creek formation, which consists predominantly of fine-grained detritus, shale and silt, and which rests unconformably on the earlier Mesozoic rocks either without any basal conglomerate or with one only 2 or 3 feet thick. The Jack Creek formation, which is confined to the central and south part of the range, contains a very scanty fauna which has not yet been studied; from evidence in other areas it is believed to represent the Cenomanian and Turonian The maximum exposed thickness is 2,900 feet but it is ordinarily much thinner because of strong erosion prior to the deposition of the next succeeding Upper Cretaceous unit, the Asuncion. The Jack Creek everywhere rests on Franciscan-Knoxville and Lower Cretaceous rocks; nowhere does it transgress them onto the ancient crystalline complex.
The Asuncion formation is made up predominantly of coarse detritus, arkose sandstones, and boulder conglomerates and the sea in which it was deposited spread over an area of considerable relief. The Asuncion rests unconformably on the Jack Creek with an angular discordance ranging from a few degrees to 70°. Strong folding, faulting, and, in places, deep erosion took place after the deposition of the Jack Creek before the deposition of the Asuncion. This diastrophism is called the Santa Lucian orogeny by the writer. It uplifted wide areas and caused deep erosion, resulting in the removal of much of the earlier Mesozoic rocks and, in places, exposing the ancient crystalline basement. Although it affected a large area the sea was not completely withdrawn from all of the present Coas Ranges; it decreased in severity eastward and there appear to be areas along the west side of the San Joaquin Valley where deposition was continuous. However, even here there are lenses of coarse boulder conglomerates, containing reworked Cenomanian and Turonian fossils, that are believed to reflect the uplift, or uplifts, caused by the Santa Lucian orogeny.
The Asuncion becomes coarser toward the west and also contains an increasing amount of Franciscan debris in that direction. In the south part of the range it was chiefly derived from the west but in the central and north part of the range there is some evidence that it was derived both from the west and northeast.
The Asuncion, like the earlier Mesozoic rocks of the region, is strongly folded and faulted and commonly stands at high angles. Because of this the total thickness is not known as there are no sections in which both the base and top are exposed. The greatest single section is about 6,000 feet thick; the total original thickness may have been as much as 10,000 feet. Only the upper 1,500-2,500 feet contains determinable fossils. The fauna in the upper part is correlated with the Garzas fauna along the west side of the San Joaquin Valley and the Glycymeris veatchii fauna of the Santa Ana Mountains. The range of this fauna is thought to be from the upper part of the Senonian, through the Maestrichtian, into the Danian. The entire Asuncion is believed to be later than the Turonian.
The representative of the Martinez (restricted Martinez, lowermost Eocene) is called the Dip Creek formation. This occurs in only two small remnants about 50 miles apart. In the south part of the range it is 1,320 feet thick and rests with slight unconformity on the Asuncion. The Dip Creek formation is lithologically identical with the Asuncion and the faunas have much in common. The lower conglomerates of the Dip Creek contain abundant debris of the Asuncion and there is a slight angular discordance between them. The movements at the close of the Cretaceous were not as severe or as widespread as those between the Paleocene and lower Miocene.
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