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Pigeon Point Formation, California

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Pigeon Point Formation, California PALEOGEOGRAPHIC IMPLICATIONS OF MOLLUSCAN ASSEMBLAGES IN THE UPPER CRETACEOUS (CAMPANIAN) PIGEON POINT FORMATION, CALIFORNIA William P. Elder LouElla R. Saul U. S. Geological Survey, Natural History Museum Mail Stop 915, of Los Angeles County, 345 Middlefield Road, 900 Exposition Boulevard, Menlo Park, CA 94025 Los Angeles, CA 90007 ABSTRACT formation's place of deposition relative to cratonic North America. The Pigeon Point Formation crops out along the San Mateo County coastline in a This paleogeographic controversy was northern and southern sequence of folded and generated by paleomagnetic data reported by faulted strata. Correlation of the two se- Champion and others (1981, 1984), which indi- quences remains somewhat equivocal, although cate that the Pigeon Point Formation was de- on the basis of biostratigraphy and a re- posited as much as 2500 km south of its pre- versed magnetic interval both appear to have sent latitude, or at the approximate latitude been deposited during the early to middle of southern Mexico in the Late Cretaceous. Campanian. Sedimentary structures suggest The formation has been considered by many to that the northern sequence was deposited by overlie basement rocks of the Salinian ter- turbidity currents in a continental rise set- rane (e.g. Howell and Vedder, 1978; Champion ting, whereas the southern sequence primarily and others, 1984), causing speculation that reflects deposition in shelf and slope envi- that terrane is far traveled. However, other ronments . geologic and paleomagnetic evidence suggests that Salinia has had more limited tectonic Right-lateral offset on the San Andreas transport (James and Mattinson, 1988; Lund and subsidiary faults to the east of the Pi- and others, 1991; Whidden et al., 1991). In geon Point Formation can account for 100's of addition, sandstone petrology and conglomer- km of northward transport since its deposi- ate clast composition of the Pigeon Point tion. However, Champion and others (1984) Formation is not characteristic of Cretaceous suggested 2500 km of northward transport from strata underlain by Salinian basement else- a tropical setting of about 21'N. Molluscan where (Lee-Wong and Howell, 1977; Grove, assemblages in the formation argue strongly 1989) . Instead, conglomerate clast composi- for a less tropical site of deposition. tion is suggestive of Late Cretaceous rocks Relative abundances of warm and temperate of the Sur-Obispo terrane (Grove, 1989), taxa and the presence or absence of key spe- which coincidentally does contain paleomag- cies are similar to those of the Santa Ana netic evidence of substantial northward Mountains Cretaceous section, and are indica- translation (Lund and others, 1991). tive of a warm-temperate to subtropical site of deposition. This paper will compare molluscan assem- blage data drawn from material collected in INTRODUCTION the Pigeon Point Format .ten, with molluscan distributions found in rocks of similar age Upper Cretaceous sedimentary rocks near along the Pacific margin of North America in Pigeon Point, California, were first recog- order to assess paleobiogeographic con- nized by Arnold (1908), who identified a num- straints on the tectonic transport history of ber of Late Cretaceous species and named sev- the formation. However, before addressing eral new species from the unit. These strata the paleogeographic questions, we will review were formally named the Pigeon Point Forma- the geologic and lithostratigraphic settings, tion by Hall and others (1959), who provided the age control and the paleoenvironmental molluscan evidence for a late Campanian or implications of the assemblage distributions, Maastrichtian age. Following these reports, since these factors play an important role in the Pigeon Point Formation generally has been developing paleobiogeographic comparisons. considered sparsely fossiliferous, although Saul and Popenoe (Saul and Popenoe, 1962, GEOLOGIC SETTING 1992; Saul, 1978, 1983, 1989) discussed and named a number of species from it. Continued The Pigeon Point Formation lies west of research of existing collections and analysis the San Andreas fault on a block that is of new collections by the authors indicates bounded to the northeast by the San Gregorio that approximately 100 megafossil species can fault (Fig. 1) . Cretaceous strata have not be identified from the formation, making the been identified between the San Gregorio and fauna one of the most diverse known for a San Andreas fault zones in the Santa Cruz unit apparently confined to middle Campanian Mountains, where Cenozoic strata apparently age on the west coast of North America. This lie directly on granitic basement rocks of paper provides a preliminary report of those the Salinian terrane (Clark and Brabb, 1978; taxa having paleobiogeographic implications McLaughlin and others, 1988). The offshore regarding the controversy surrounding the extent of the Pigeon Point Formation is not known, but some have suggested a hypothetical In Dunn, G., and McDougall, K., eds.,1993, Mcsozoic Palcogcography of the Western United Statcs-II Pacific Section SEPM, Book 71, p. 171-186. / f'-r extension of the Sur-Nacimiento fault zone through exposure or drill hole, identifica- not far offshore (e.g. Howell and others, tion of the amount of offset on the San Gre- 1977; Lowe, 1979), which may truncate the gorio fault is critical in determining the formation to the southwest (Fig. 1). basement rock and terrane to which the Pigeon Point Formation belongs. This is because the Because of the paleomagnetically implied Sur-Nacimiento fault, which separates grani- extensive transport of the Pigeon Point For- tic basement of the Salinian terrane to its mation since deposition (Champion and others, northeast from Franciscan basement of the Sur 1981, 1984), determination of the underlying Nacimiento block to its southwest, is appar- basement rock has been paramount to analysis ently offset by the San Gregorio fault off- of terrane movements. Barring the possibil- shore from Point Sur (Fig. 1) . Franciscan ity of direct investigation of the basement basement rocks dredged from the headward part of Ascension Canyon just south of Pigeon Point (Howell and Joyce, 1981) imply at least 90 km of dextral offset of the Sur-Nacimiento fault by the San Gregorio fault system. How- Point Arena ever, if offset on the San Gregorio fault is slightly greater, on the order of 110-115 km, then the Pigeon Point Formation would lie on the southwest side of the Sur-Nacimiento fault and be underlain by Franciscan basement of the Sur-Obispo terrane (Fig. 1) . Most es- timates of offset on the San Gregorio-Hosgri fault system fall into the range of 105-150 km (see discussion in Griscom and Jachens, 1989). Thus, a narrow adjustment of these constraints in either direction would allow the Pigeon Point Formation to be underlain by either Salinian or Sur-Obispo basement rocks. Cretaceous rocks nearest to the Pigeon Point Formation lie to the east of the San Andreas fault on Franciscan basement in Palo Alto and near Loma Prieta (Fig. 1). In Pal Alto, a small claystone outcrop on the St a- ford University campus contains foraminxit. and macrofossils of late Campanian ag*_- (Graham and Church, 1963), but exposure is too limited to make proper comparison with the Pigeon Point Formation. Upper Cretaceous rocks near Loma Prieta are turbidite deposits similar to those of the Pigeon Point Forma- tion and are of approximately the same age, as indicated by a number of bivalves in com- mon, including middle Campanian Meekia bella. However, the molluscan fauna near Loma Prieta contains turbidite transported nearshore com- ponents originating in littoral rocky coast- line habitats (Elder, 1991) that are not rep- resented by faunal components in the Pigeon Point Formation. LITHOLOGIC AND STRATIGRAPHIC SETTING The Pigeon Point Formation is well ex- posed along 16 km of coastline near Pigeon EXPLANATION Point in San Mateo County. Neither the de- Sur-Obispo positional base nor top of the formation are Salinian terrane exposed due to burial or fault contact and terrane erosional truncation, respectively (Howell and Joyce, 1981). Small to large scale fold- ing, faulting, and rotation of blocks within Figure 1. Map of west-central California the formation are extensive (Joyce, 1981) and showing the position of the Pigeon Point For- inhibit precise reconstruction of the strati- mation relative to nearby localities of simi- graphic section. Correlation is particularly lar age (LP = Loma Prieta; PA = Palo Alto, hampered by an unnamed fault 1.5 km south of Stanford University Campus) and major faults Bean Hollow Beach (Fig. 2), which effectively and terrane boundaries of significance to its divides the formation into northern and tectonic transport history. Fault abbrevia- southern sequences of uncertain relationship. tions are as follows: HF = Hosgri fault, PF = In spite of these structural difficulties, Pilarcitos fault, SAFZ = San Andreas fault reasonably accurate large-scale stratigraphic zone, SGF = San Gregorio fault, SNF = Sur sections through the approximately 2500-3300 Nacimiento fault. Position of offset Sur m of exposed formation have been constructed Nacimiento fault and Salinian/Sur Obispo ter- by Howell and others (1977), Lowe (1979), and rane boundary relative to the Pigeon Point Howell and Joyce (1981) . Formation is not known. / f'-r Northern Sequence Figure 2. Map showing fossil localities, major structures, and generalized dip directions and lithofacies units of the Pigeon Point Formation.
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