MESOZOIC MOLLUSCAN FOSSILS FROM THE GOLDEN GATE NATIONAL RECREATION AREA AND THEIR SIGNIFICANCE TO FRANCISCAN COMPLEX TERRANE RECONSTRUCTIONS, SAN FRANCISCO BAY AREA, CALIFORNIA
William P. Elder
National Park Service, Fort Mason, Building 201, San Francisco, CA 94123
Blake et al. (1984) assigned Alcatraz Island to a terrane separate from the nearby Marin Headlands terrane on the basis of apparent age and petrographic differences between graywackes of the two terranes. However, the inoceramid specimen recently found on Alcatraz, as well as others described by Gabb (1869) as Inoceramus elliotii, resemble Cenomanian species, implying a similar age for graywackes of the two terranes and diminishing the need for a separate Alcatraz terrane. However, minimal stratigraphic separation between the Cenomanian and Valanginian fossils on Alcatraz suggests a problem with one of the age calls. A reasonable alternative solution, therefore, is assignment of the inoceramids to the Inoceramus neocomiensis group of Neocomian age, thus, indicating that the Alcatraz terrane is indeed separate from the Marin Headlands terrane.
The Franciscan Complex is comprised of a complexly deformed amalgamation of tectonostratigraphic terranes of differing depositional and deformational histories. The terranes are composed of oceanic blocks that may include both mafic basement and overlying sedimentary rocks. These blocks were shingled against the western margin of North America as the Pacific Plate was subducted under the North American Plate prior to formation of the San Andreas fault. In the San Francisco Bay area, the Franciscan Complex has been divided into numerous terranes contained within the Eastern and Central melange belts. The Eastern belt lies inboard and structurally higher than the Central belt and is of higher metamorphic grade. Blake et al. (1984) defined essentially eight terranes in the Central belt in the San Francisco Bay area (Figure 1) on the basis of differences in basement rock types and ages, in the ages and types of overlying sedimentary sequences, and in metamorphic grade. Of particular importance to this report are the Marin Headlands and the Alcatraz terranes, where the molluscan fossils discussed herein were found.
Figure 1Location map showing terranes of the Franciscan Complex identified in the San Francisco Bay area by Blake et al. (1984), and the three GOGA molluscan localities discussed herein: 1) Marin Headlands locality where Cenomanian Mantelliceras was found; 2) Baker Beach locality with Albian Douvilleiceras; 3) Alcatraz localities where Inoceramus, Buchia, and other bivalves have been found. Modified from Blake et al. (1984).
Figure 2Map of Alcatraz Island showing fossil localities (triangles) and geologic cross sections with Inoceramus and Buchia localities projected onto them, indicating approximate stratigraphic separation. Modified from Armstrong and Gallagher (1977).
Figure 31, Scanned image of plaster cast of syntype of Inoceramus elliotii Gabb, 1869, from which Gabb's (1869; pl. 31, fig. 90) was drawn. Specimen also was illustrated by Stewart (1930; pl. 2, fig. 2) and is an internal mold; Accession No. ANSP# 4411, Cat. No. 28869. 2) Scanned image of plaster cast of syntype of Inoceramus elliotii from Gabb's collection but never before illustrated. Specimen is an internal mold; Accession No. ANSP# 4411, Cat. No. 28870. 3-4) Inoceramid found by Ted Stout in 1992 and identified as Inoceramus ex gr. pictus in Elder and Miller (1993). Accession # GOGA-1651, Cat. No. GOGA-18502a&b. 3 - Scanned image of latex pull of external mold. 4 - Scanned image of plaster cast of internal mold. 5-6) Scanned images of Woods' (1911) figures of Inoceramus neocomiensis d'Orbigny, 1846. 5 - Woods (1911, pl. XLV, fig. 1). 6 - Woods (1911, pl. XLV, fig. 2).
Sedimentary rocks of the Franciscan Complex are predominantly composed of continental margin sediments, primarily argillite and graywacke sandstone, but also are represented to a lesser extent by open-ocean facies, such as chert or limestone primarily composed of radiolaria or foraminifera, respectively. Where preserved, these microfossils provide age control for times of open-ocean deposition on Franciscan terrane blocks. However, the time of accretion of these oceanic blocks onto the western margin of North America is typically poorly constrained, because the clastic facies deposited on the blocks when they neared the continental margin generally preserve no fossils. Therefore, the molluscan fossils found in the GOGA are highly significant because they provide critical control on the time when these oceanic blocks collided with North America. In addition, the fossils help in defining and assigning rocks to the different terrane blocks.
Molluscan fossils from the Franciscan Complex in lands that now lie in the GOGA have been known for over 130 years, with the earliest specimens being found in a barge containing sandstone blocks quarried from Alcatraz Island in the 1860s. These fossils were presented to paleontologist William M. Gabb by Major George H. Elliot. Gabb (1869, p. 193, pl. 31, fig. 90) subsequently described Inoceramus elliotii based on several internal and external molds of these specimens and additional material that he collected on the island in 1864 (Figures 2 and 3). No other specimens have been assigned to this species, however, due to the poor condition of the syntypes on which it is based, although Crame (1985, p. 483, text-fig. 4b) identified an Inoceramus aff. I. elliotii Gabb from the Lower Cretaceous of Antarctica.
Because of the significance of Alcatraz specimens on
providing an age for the Franciscan Complex, they have
been further discussed or illustrated by Stewart (1930, pl. 2, fig.
2), Anderson (1938, pl. 7, fig. 1), Matsumoto (1960), Bailey
et al. (1964, p. 115), and Elder and Miller (1993, p. 8).
Anderson (1938, p. 99) considered Inoceramus
elliotii to be related to I. ovatus Stanton and thought it to be of Neocomian age.
In contrast, Matsumoto (1960) and Elder and Miller (1993) speculated that I. elliotii may be equivalent to I. crippsi Mantell of Cenomanian age. Anderson (1938, p. 121) also described Lucina alcatrazis on the basis of three molds obtained from Gabb's Alcatraz material. Likewise, this species has not been identified elsewhere and is of little age significance.
In 1976, the next fossil find on Alcatraz (Figure 2) produced specimens identified as Buchia pacifica (Jeletzky) and Pleuromya sp. by David Jones in Armstrong and Gallagher (1977). Buchia pacifica is indicative of a latest Berriasian to early Valanginian age and Pleuromya is a genus typical of the Neocomian rocks of the west coast (Jones et al., 1969; Bralower et al., 1990; Anderson, 1938). The most recent fossil find on Alcatraz was in 1992, when Ranger Ted Stout found a juvenile inoceramid bivalve (Figures 2 and 3). Elder and Miller (1993) assigned this specimen to the Inoceramus pictus group of late Cenomanian age.
A Cenomanian age for the new inoceramid from Alcatraz Island is consistent with the age indicated by a specimen of the ammonite genus Mantelliceras, found at the north end of the Golden Gate Bridge and identified by Hertlein (1956) (Figure 1). The early Cenomanian age indicated by that ammonite is slightly younger than the early Albian age indicated by a specimen found just south of the Golden Gate (Figure 1), at the northend of Baker Beach (Schlocker et al., 1954). Both of those ammonites were found in rocks assigned to the Marin Headlands terrane of the Franciscan Complex.
Blake et al. (1984) defined the Alcatraz terrane as
separate from the Marin Headlands terrane on the basis of
the former being primarily composed of a relatively thick
sequence of turbidite sandstone with minor shale of Valanginian age,
in contrast to the latter terrane, which includes oceanic
volcanic basement overlain by pelagic chert that is as young as
late Albian to early Cenomanian. The chert of the Marin
Headlands terrane is overlain, however, by a thick
turbiditic graywacke sequence from which the
Cenomanian Mantelliceras and Albian
Douvilleiceras ammonites have been
found. In addition to the apparent age differences in the graywackes of these two terranes, they also differ petrologically, with the Alcatraz sandstones containing a significantly lower lithic component (Jayko and Blake, 1984, figs. 4a-b). However, if the rocks on Alcatraz are Cenomanian rather than Valanginian in age, as suggested by the inoceramids, then designation of a separate Alcatraz terrane is less compelling, thus, somewhat simplifying Bay Area geology.
The main obstacle to declaring a Cenomanian in age for all the rocks on Alcatraz is the Valanginian Buchia specimens identified in Armstrong and Gallagher (1977). Unfortunately, those specimens have been lost and, therefore, cannot be restudied. Several recent attempts to recollect the locality have failed to yield identifiable fossils. A Valanginian age assignment for these specimens presents two problems. First, the presence of Franciscan graywackes of that age requires establishment of a separate Alcatraz terrane; a requirement that is supported, however, by pertrographic differences between the rocks on Alcatraz and those of the Marin Headlands terrane. Second, the stratigraphy on Alcatraz Island indicates that the Valanginian fossils lay only about 60 m stratigraphically below the inoceramids of apparent Cenomanian age (Figure 2). In the rapid depositional setting of turbidite environments, this is an insufficient stratigraphic separation to account for a 40 Myr age difference, and there are no obvious intervening faults or other structural features that can account for this missing time. In re-evaluating the age implications of the fossils reported on by Armstrong and Gallagher (1977), Elder and Miller (1993) suggested that the Buchia pacifica identified may actually be deformed Inoceramus gradilis Pergament, which looks very similar to B. pacifica and is of middle Cenomanian age. There are two problems with this interpretation, however. First, Inoceramus gradilis has not been found on the Pacific coast south of Alaska (Elder and Box, 1992). Second, the co-occurring genus Pleuromya is largely restricted to and typical of Neocomian age rocks on the west coast.
One solution to the above-noted age problem is that the Cenomanian inoceramid calls are wrong, and that the rocks on Alcatraz are all of Neocomian age. Neocomian inoceramids on the Pacific coast of North America are not diverse and are poorly documented. In addition, the rocks and fossils of Alcatraz are somewhat deformed, making identifications tenuous. However, inspection of plaster casts made from three of Gabb's inoceramid specimens, as well as the specimen found in 1992, provides a better idea of the morphology of the inoceramid species than was previously possible (Figure 3). The morphology of these specimens is not like that of typical west coast Neocomian inoceramids, such as I. ovatoides Anderson, but is compatible with them belonging to the Inoceramus neocomiensis d'Orbigny group (Figure 3.5, 3.6; also see Woods, 1911, pl. XLV, figs. 1-2). Specimens of this species group, which resemble I. anglicus Woods but have coarser concentric rugae, occur in Hauterivian age rocks of Oregon in association with Pleuromya (Imlay, 1960, p. 177). The I. neocomiensis group is typical of the Valanginian to Berramian interval (Dhondt, 1992).
In conclusion, until more definitive paleontologic evidence is unearthed on Alcatraz, the age of the rocks on the island, and the validity of the Alcatraz terrane, will remain in question. However, the bulk of the paleontologic data, coupled with the stratigraphic constraints on Alcatraz, argue for a Neocomian age for the Franciscan graywacke on the island. This age is significantly older than the Albian to Cenomanian graywacke of the Marin Headlands terrane, supporting a distinct Alcatraz terrane with petrologically different sandstone.
The late Clyde Wahrhaftig initially brought the
Inoceramus collected in 1992 to me for identification.
Clark Blake, David Jones, and Clyde Wahrhaftig provided
helpful discussions regarding the fossils, geologic problems
ated with their age determinations, and terrane assignments of Alcatraz. LouElla Saul provided the casts of Gabb's type material which the Los Angeles County Museum of Natural History had obtained from the Academy of Natural Sciences of Philadelphia (ANSP). Two anonymous reviewers improved the manuscript.
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