Pancho Rico Formation Salinas Valley, California

Pancho Rico Formation Salinas Valley, California G E 0 L 0 G I CAL SURVEY P R 0 FE S S I 0 N A L PAPER 524-A Pancho Rico Formation Salinas Valley, California By DAVID L. DURHAM and WARREN 0. ADDICOTT SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 524-A A study of stratigraphy and paleontology of Pliocene marine strata in southern Salinas Vallry, California UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY PANCHO RICO FORMATION, SALINAS VALLEY, CALIFORNIA By DAVID L. DuRHAM and WARREN 0. AnniCOTT ABSTRACT northwestern part of the area are of a northern, cool-water type and may indicate interchange with Pliocene faunas to the north The Pancho Rico Formation of Pliocene age comprises sandy through a northwest-trending marine trough. marine strata and interbedded finer grainr.d rocks. It generally overlies the marine Monterey Shale of Miocene age and under INTRODUCTION lies the nonmarine Paso Robles Formation of Pliocene and fleistocene( ?) age in southern Monterey County, Calif. Lo Sandy marine strata succeed mudstone, porcelanite, ~ally it lies on the basement complex or on the Santa Mar and chert beds of the Monterey Shale at most places in galita Formation of upper Miocene age. It crops otrt from north the Salinas Valley, southern Monterey County, Calif. of King City southeastward for more than 50 miles to beyond the Nacimiento River and Vineyard Canyon. The Pancho Rico These sandy strata, together with interspersed finer contains sandstone, mudstone, porcelaneous mudstone, por grained rocks, constitute the Paneho Rico Formation celanite, diatomaceous mudstone, and conglomerate. Where the (Durham and Addicott, 1964). The following diseus- Pancho Rico overlies the Monterey Shale, its lower contact is at sion of the lithologic eharacter, stratigraphic relations, the base of the stratigraphically lowest sandstone unit above the and thickness of the Pancho Rico Formation is by Dur typically finer grained Monterey. ·where the Pancho Rico over lies the Santa Margarita Formation, its lower contact is at the ham, and is based on geologic 1napping and related top of the stratigraphically highest bed that contains Ostrea studies (Durham, 1963, 1964, 1965). Paleontologic. de titan, a giant oyster characteristic of the Santa Margarita. terminations, correlation of strata, and discussion of The upper contact of the Pancho Rico is at the top of the strati mollusean paleoecology and paleogeography are by Ad graphically highest lithologic unit that contains marine fossils. dicott. 'The Pancho Rico was referred to as an un Both the upper and lower contacts are conformable and grada tional. At some localities the unit probably intertongues with named formation of Pliocene age in three previous re the Paso Robles Formation, and at others, with the Monterey ports (Durham, 1963, 1964,1965). Shale. The thickness of the Pancho Rico ranges from less than 20 feet to more than1,000 feet. ACKNOWLEDGMENTS The molluscan fauna of the Pancho Rico Formation consists principally of species not previously reported from the Salinas E. J. Moore studied eolleetions of fossil mollusks from Valley area. Among the 141 larger invertebrate taxa recorded 18 localities (see list of fossil loes., M902-M1067) in in this paper are many species indicative of Pliocene age, as 1960 and 1961. These collections were rexamined and that epoch is recognized in the Pacific Coast megafossil additional fossils were colleeted during preparation of chronology. Faunal correlation is made with the type Jacalitos )!'ormation-exposed near Coalinga-a biostratigraphic unit this paper. Most of the echinoid identifications were that has been traditionally regarded as the standard for the made by J. Wyatt Durham. C. L. Rice made the initial lower Pliocene of the Pacific Coast. collections frmn localities between San Lucas and Pan The molluscan fauna of the Pancho Rico implies a shallow cho Rico Creek in 1960. J. G. Vedder and C. A. He water marine environment considerably warmer than exists penning eolleeted the fossils from locality M982, north today at comparable latitudes and similar to that off the west coast of Baja California, Mexico. The wide distribution of west of Bradle,y, in 1960. A. Myra l{een provided ac several warm-water indicators in the early Pliocene of the cess to the Stanford University collections and lent Salinas Valley area and the absence of these indicators from some specimens figured herein. J. H. Peck, Jr., ar contemporaneous strata to the north and northwest suggest ranged a loan of specimens from the Pliocene that a Pliocene seaway connected the Salinas Valley area to the collections in the University of California Museum Santa Maria basin to the south. Local moderate-depth as semblages in exposures of the Pancho Rico Formation in the of Paleontology. Al A2 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY DEFINITION OF THE PANCHO RICO FORMATION by recognition o:f lateral variation in lithologic charac Reed (1925, p. 606) "grouped together as the Poncho ter, thickness, and stratigraphic relations. Rico 1 formation" 1narine "diatomite * * * clastic Many geologists have assigned part or all of the shale, yellowish sandstone, and conglomerate beds" on Pancho Rico Formation, as here defined, to the Santa the east side of the Salinas Valley. He presumably Margarita Formation (fig. 2). However, use of the intended that the marine strata exposed along Pancho name Santa Margarita in the Salinas Valley is con Rico Creek should be considered typical of the forma sidered appropriate only at the type area, near the town tion, although he later (Reed, 1933, p. 231) admitted of Santa Margarita, and in adjacent areas where beds that the unit had "never yet been adequately defined." occur that are similar to typical Santa Margarita in To help overcome this need, we defined the Pancho Rico both lithologic character and stratigraphic position Formation as comprising the sandy marine strata and (Durham and Addicott, 1964). At the type area, the interbedded finer grained rocks that generally overlie Santa Margarita overlies the Monterey Shale and is the marine Monterey Shale and underlie the nonmarine light-colored massive coarse-grained arkosic sandstone Paso Robles Formation in the Salinas Vall~y area, that locally contains numerous large fossil oysters. The southern Monterey County (Durham and Addicott, concept that the Santa Margarita is progressively 1964). Locally the Pancho Rico overlies units other younger northward from the type area is commonly than the Monterey, as northeast of IGng City, where accepted, mainly on the basis of including in the Santa it lies nonconformably on the basement complex, and Margarita at least some beds that here are considered about 35 n1iles farther southeast near Vineyard Canyon, part of the Pancho Rico. The concept of a northward where it apparently lies conformably on the Santa Mar time transgression by the Santa Margarita is rejected garita Formation. Where the Pancho Rico overlies the in favor of the idea that in the southern part of the Monterey, the contact is conformable and is at the base Salinas Valley the Santa Margarita overlies the Mon of the stratigraphically lowest sandstone unit above the terey but that farther north it intertongues with the typically finer grained Monterey. Sandstone beds oc uppermost part of that formation (Durham and Addi curring locally in the Monterey Shale in the subsurface cott, 1964). Accordingly, the Santa Margarita in near the Salinas River (Gribi, 1963, p. 19) are distin southern Monterey County is considered to be a lentic guished from lithologically similar beds in the Pancho ular sandstone unit at or near the top of the Monterey, Rico by their lo,ver stratigraphic position and their whereas the Pancho Rico Formation overlies both. proximity to the sloping surface of the basement com plex. The contact of the Pancho Rico with the over AREAL DISTRIBUTION lying nonmarine Paso Robles Formation is conformable The Pancho Rico Formation crops out on the east and is placed at the top of the stratigraphically highest side of the Salinas Valley from north of King City lithologic unit containing marine fossils. The Pancho southeastward for about 50 miles to beyond Vineyard Rico Formation, as so defined, includes beds in the lower Canyon. It crops out at places along the west side of part that are lithologically identical with those in the the valley from north of the Arroyo Seco southeastward Monterey Shale, and beds in the upper part that are for about 50 miles to beyond the Nacimiento River. It lithologically similar to those in the Paso Robles Forma also crops out farther west, in the San Antonio River tion. In this sense, both upper and lower contacts of the Valley, north and northwest of Jolon and south of Pancho Rico are gradational and probably inter Lockwood (fig. 1). tonguing. Strata exposed along Pancho Rico Creek are con PREVIOUS NOMENCLATURE sidered typical of the Pancho Rico Formation for the Names previously applied to strata of late Miocene purpose of definition, but designation of a particular and Pliocene age in the Salinas Valley and the nomen sequence of beds as typical of the unit must be qualified clature used in this paper are shown in figure 2. Inter tonguing relations are omitted from the diagram, and 1 Authors' note: Maps available to Reed-for example, the 1915 edi correlation of unit boundaries from column to column tion of the Priest Valley 30-minute quadrangle-have the "Poncho Rico" Creek joining the Salinas River at San Ardo. Later maps, includ may be inexact. Figure 2 illustrates the divergence ing the 1927 reprint of the Pril'st Valley 30-minute quadrangle, have the of opinion among previous investigators as to classifi differently spelled name "Pancho Rico" for the same stream. Gudde (1949, p. 269) listed the name "Poncho Rico," but stated that it "prob cation of strata here assigned to the Pancho Rico ably commemorates Francisco ('Pancho') Rico, grantee of San Lorenzo Formation.

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