Stratigraphy, paleontology, and geologic history.-A basement' consisting of igneous rocks of the Jurassic(?) Franciscan formation and sediments of the Upper Jurassic Knoxville formation, and formations of Tertiary and Quaternary age are exposed in the Santa Maria district. The outcrop section, exclusive of the Franciscan, has a maximum thickness of about 10,000 feet, the subsurface section about 27,000 feet. At no locality, however, is either outcrop or subsurface section as thick as the total maxima for the formations.

The outcrop Franciscan is made up of altered basalt, gabbro (generally- greatly altered), and minor areas of peridotite and serpentine.

The marine Knoxville formation, consisting of shale, thin-bedded calcareous sandstone, and conglomerate, was deposited on the igneous rocks of the Franciscan. It has an outcrop thickness of about 500 feet and a known subsurface thickness of at least 1,250 feet. Aucella cf. A. piochii, which occurs in both outcrop and subsurface sections, suggests late Jurassic age.

The Franciscan and Knoxville were probably uplifted during Taliaferro's Diablan orogeny at the close of the Jurassic and formed a land area that presumably endured during early Cretaceous and perhaps during much of late Cretaceous time. If sediments were laid down while 10,000 feet of Upper Cretaceous were deposited in the adjoining San Rafael Mountains, they were eroded following uplift at the close of the Cretaceous, when Reed and Hollister's San Rafael uplift was formed. The district is inferred to have been part of a Franciscan and Knoxville land area at the south border of the San Rafael uplift during all of early Tertiary time.

The known history of the district as part of a Tertiary basin began in the early Miocene(?), possibly a little earlier or possibly 3: little later, when the nonmarine sediments of the early Miocene(?) Lospe formation were deposited. The Lospe formation has a maximum thickness of 2, 700 feet, and is made up of coarse-grained reddish sandstone and conglomerate, and greenish sandstone, gypsiferous siltstone, and mudstone. White tuff is a minor but conspicuous constituent. The Lospe formation overlies the Knoxville, or· overlaps it and rests on the Franciscan.

The first known Tertiary invasion of the sea took place immediately thereafter in early middle Miocene time, and from then on until approximately the end of the Pliocene the sea occupied continuously at least most of the region. The extent of the Miocene basin is not certainly known, but it was part of an extensive· basin that has been designated the Santa Barbara embayment.

The early middle Miocene Point Sal formation is the earliest marine Tertiary formation. It has an outcrop thickness of as much as 1,500 feet and a maximum subsurface thickness of 3,600 feet, and consists of siltstone, mudstone, and thin beds of sandstone. The Point Sal formation overlies the Lospe formation without marked discontinuity, or overlaps it and rests on Knoxville or Franciscan. It contains a large foraminiferal fauna representing the Siphogenerina hughesi zone, or the lower part of Kleinpell's Relizian stage.

The Monterey shale overlies the Point Sal formation without noticeable discontinuity, or overlaps all the older sedimentary formations and rests on the Franciscan. It has a maximum outcrop thickness of 2,100 feet, but is as much as 5,000 feet thick in some subsurface sections. The Monterey is divided into three mapped members. The lower member is characterized by phosphatic shale and somewhat porcelaneous shale; the middle member by chert and cherty shale; and the upper member by porcelaneous shale, or by both porcelaneous shale and diatomaceous strata. The lower member contains Foraminifera representing the upper part of Kleinpell's Relizian stage and all of his Luisian stage, the middle member a few species indicating the lower part of his Mohnian stage, and the upper member a fauna representing the upper part of the Bolivina hughesi zone, at the top of the Mohnian, and in part a younger unnamed faunal division; that is, the Monterey is of late middle and late Miocene age.

The diverse stratigraphic relations of the Lospe and Point Sal formations and Monterey shale along the borders of the Franciscan rocks forming Point Sal Ridge are inferred to be the result of repeated movements during Miocene time in the area of basement rocks west of the district, presumably extending westward beyond the present coast.

Toward the close of the Miocene, low ridges appeared on the floor of the sea, ridges that grew during Pliocene time and were destined to become anticlines. At the same time deformation took place in the northeastern part of the district, and elsewhere on some anticlines, or on other structural highs bounded by faults. In those areas the Sisquoc formation overlies the Monterey shale with marked discordance; elsewhere there is no discordance.

Two facies of the Sisquoc formation are mapped: a marginal sandstone facies, designated the Tinaquaic sandstone member, and a basin facies. The Tinaquaic sandstone member is 1,400 feet thick and contains megafossils of middle Pliocene age (also early Pliocene just east of the mapped area). It is unconformable on the Monterey. The basin facies, at least 3,000 feet thick in outcrop sections and 5,000 in some subsurface sections, consists of diatomaceous mudstone, other types of diatomaceous ·strata, somewhat porcelaneous mudstone, and porcelaneous shale-deposits that are ordinarily characteristic of the Monterey shale. Even in areas where the two formations are lithologically indistinguishable and conformable, a field basis for differentiating them has been established. The basin facies of the Sisquoc conformably overlies the Monter.ey in outcrop sections, but in some subsurface sections the formations are unconformable, and on the north limb of the Santa Maria Valley syncline the basin facies of the Sisquoc overlaps the Monterey onto the basement, thus forming the overlap trap for the oil in the Monterey in the Santa Maria Valley field. The lower and middle parts of the basin facies contain Foraminifera of the Bolivina obliqua zone. Kleinpell assigned that zone to the lower part of his late upper Miocene Delmontian stage. Assignment to the upper Delmontian, however, appears to be preferable. The upper few hundred feet of the basin facies contain Foraminifera similar to those in the overlying Foxen mudstone and megafossils of middle Pliocene affinities. The basin facies is therefore considered late upper Miocene to middle Pliocene.

The submarine ridges were growing during Pilocene time. The Foxen mudstone is missing on them and in the northeastern part of the district. On the north limb of the Santa Maria Valley syncline, the Foxen overlaps the Sisquoc formation and rests on the basement. In the basins between the submarine ridges, the mudstone, siltstone, and fine-grained sandstone of the Foxen (800 feet thick in outcrop sections and as much as 2, 750 in subsurface sections) overlie conformably the Sisquoc formation. In some of the areas where the Foxen is missing as a lithologic unit, it appears to be represented by condensed deposits of phosphatic pellets, mapped with the underlying or overlying Formation, depending on the matrix, or by a condensed section of fine-grained sand, mapped with lithologically indistinguishable sand in the basal part of the overlying formation. Though foraminifera are abundant in the Foxen mudstone, relatively few species are represented. Those from the lower part of the formation may be of middle Pliocene age. Megafossils from the upper part of the Foxen are considered late Pliocene. The formation is therefore assigned to the middle(?) and upper Pliocene.

Movements during Pliocene time in the basement area west of Point Sal Ridge are indicated by the occurrence in the western Casmalia Hills of coarse detritus from the Monterey in the upper part of the Sisquoc formation and in the Foxen mudstone.

During late Pliocene time, when the Careaga sandstone was deposited, the Pliocene sea had its greatest extent. The region then flooded may be referred to as the Santa Maria basin. Throughout most of the district two mapped members of the Careaga sandstone are differentiated: the Cebada fine-grained member, which has a maximum outcrop thickness of 1,000 feet, and the Graciosa coarse-grained member, 50 to 425 feet thick. Mild deformation continued during the late Pliocene. The Cebada fine-grained member is missing on the embryonic anticlines, where it is overlapped by the Graciosa coarse-grained member. Both members of the Careaga sandstone contain a large megafauna.

The nonmarine Paso Robles formation conformably overlies the Careaga sandstone. The Paso Robles consists chiefly of sand and gravel, but clay, marl, and limestone are the most characteristic constituents. The maximum outcrop thickness is 2,000 feet, the estimated maximum subsurface thickness 4,500 feet. The Paso Robles fresh-water fauna is meager, consisting of a few species similar to living forms. The formation is currently assigned to the interval including late Pliocene and early Pleistocene(?). The age assignment late Pliocene(?) and early Pleistocene, however, may be preferable for the Santa Maria district.

Then followed the only period of strong general deformation in the known Tertiary and Pleistocene history of the district. The present structural features of the district were formed at that time, and the submarine ridges appeared as fully formed anticlines. Dating of the deformation is uncertain, because of uncertainty concerning the age of the Paso Robles formation. It is, however, without much doubt of the same age as the welldated strong middle Pleistocene deformation in the Ventura basin.

Terrace deposits, laid down on both wave-cut and stream-cut platforms, are rather arbitrarily assigned to the late Pleistocene. The oldest and most extensive terrace deposits, not more than 100 feet thick, are designated the Orcutt sand. The Orcutt sand itself is tilted as much as 12° on the limbs of anticlines and is faulted on the north limb of the Graciosa anticline, indicating renewed growth of the anticlines, presumably in late Pleistocene time. Terrace deposits apparently younger than the Orcutt sand are arched in a low anticline west of lower Foxen Canyon.

Structure.-Santa Maria Valley is the boundary between two structural provinces. To the north, valleys and hills are either synclines or anticlines. To the south, on the contrary, major valleys coincide generally with major synclines, and the hills are anticlinal. Santa Maria Valley itself is a syncline. Unlike valleys farther south in the district, however, it lies athwart an older uplift. Also unlike most of the valleys farther south, the axis of the syncline is not in the middle of the valley, but is far to the south near the bordering hills. Indeed, in the western Casmalia Hills, the anticline in the bordering hills is overturned and overrides the axis of the syncline.

The major structural features of the district have a general west-northwestward trend parallel to the trend of the basin. Minor westward-trending and northward-trending folds and faults, however, extend across the trend of the major features. The district includes areas of wide, open folds and also areas of narrow, closely spaced, and steeply tilted folds, as well as some major overturned anticlines, most of the latter overturned northward. The closely spaced folds coincide almost invariably with outcrops of the Monterey shale and Point Sal formation.

Physiography.-The surface on which the terrace deposits, designated the Orcutt sand, were deposited is extensive but is locally deformed, and only remnants are preserved. Toward the coast it changes from a stream-cut surface to a wave-cut surface.

In the coastal area three main marine terraces are recognized: the high terrace (altitude about 800 feet), the intermediate terrace (altitude about 600 feet), and the low terrace (altitude 50 to 125 feet).

An indurated layer is present at or near the surface at many localities scattered throughout the district. It is suggested that the indurated layer is an ancient hardpan, the incomplete skeleton of a former soil profile developed on a former surface of less relief than the present surface.

Sand dunes extend inland from the coast at the north and south borders of the mapped area. They are classified under three age groups: old, intermediate, and modern. The old dunes, which have a protective cover of natural vegetation and are now inactive, are far more extensive than those of the other two groups. They cover many square miles on a terrace bordering Santa Maria Valley, and extend 20 miles inland. It has not been determined whether their inactivity is due to a cutting off of the supply of sand, or to a climatic change.

Occurrence of oil.--Oil has been produced in the Santa Maria district since 1901, the total production to the end of 1947 being 269,657,000 barrels. Throughout the district most of the oil is heavy. Seven producing fields are located in the mapped area. In order of discovery from oldest to youngest the fields are as follows: Orcutt, Lompoc, West Cat Canyon, East Cat Canyon, Casmalia, Gato Ridge, and Santa Maria Valley. The Santa Maria Valley field is the largest in both area and productive capacity. It is the largest overlap field in coastal California, and one of the last major fields found in the State up to 1947. The Las Flores (Monterey) pool is transforming the West Cat Canyon field into one of the major fields of the district.

The Monterey shale is the chief oil-bearing formation, and the principal reservoir in the Monterey consists of fractured chert and cherty shale. Sand in the Sisquoc formation is the sole reservoir in the minor East Cat Canyon field and in the Pliocene pool of the West Cat Canyon field.

The Point Sal formation yields some oil in the southeastern part of the Santa Maria Valley field (the only part of that field where the formation is present) and a recent well in the Casmalia field is producing a small amount of relatively light oil from the Point Sal. The Lospe formation is also productive in a recent well in the Casmalia field. The Knoxville formation is productive in three areas in the northern part of the Santa Maria Valley field. The Point Sal formation offers the greatest promise for deeper-zone production. The Lospe and Knoxville formations can no longer be ignored in areas where younger marine formations overlap against them.

Oil possibilities in undeveloped areas.--Two matters weigh heavily in prospecting in the Santa Maria district: the degree of fracturing of chert and cherty shale in the Monterey shale, and the gravity of the oil. Other things being equal, the productivity of the Monterey varies directly with the amount of fracturing. Very heavy oil, too heavy to produce commercially under present conditions, has befln found ii the Monterey in the northeastern part of the district, where five discoveries (one east of the mapped area) have been made in recent years.

Among areas of possible interest, three appear to be favorable for prospecting on the basis of surface geology: an area east of Foxen Canyon, where oil may be trapped in the basal part of the Tinaquaic sandstone member of the Sisquoc formation by westward overlap of successively higher Tinaquaic strata onto the Monterey shale; an area so1tth of the I ... ions Head fault, where oil may be trapped by the fault; and the offshore extension of the north border of Point Sal Ridge, where oil may possibly be trapped in the Monterey by overlap of the Sisquoc formation.