OF WESTERN AND SOUTHERN NORTH AMERICA OF WESTERN AND SOUTHERN NORTH PALEONTOLOGY GEOLOGY AND VERTEBRATE Geology and Vertebrate Paleontology of Western and Southern North America Edited By Xiaoming Wang and Lawrence G. Barnes Contributions in Honor of David P. Whistler WANG | BARNES 900 Exposition Boulevard Los Angeles, California 90007 Natural History Museum of Los Angeles County Science Series 41 May 28, 2008 Magnetic stratigraphy of the Eocene to Miocene Sespe and Vaqueros formations, Los Angeles and Orange counties, California Gino Calvano,1 Donald R. Prothero,2,3 Joshua Ludtke,4 and E. Bruce Lander1,3 ABSTRACT. The middle Eocene to lower Miocene age Sespe and Vaqueros formations in northern Orange County and northwestern Los Angeles County, California, USA, yield many important vertebrate and invertebrate fossils. These include Uintan (late middle Eocene) and Arikareean to Hemingfordian (late Oligocene to early Miocene) land mammals in the Sespe Formation, and important marine mammals (including the oldest sirenians and desmostylians in California and many early whales) and mollusks of the Turritella inezana Zone in the Vaqueros Formation. Paleomagnetic analyses of five important fossil-bearing sections in northern Orange County and one in Los Angeles County were undertaken to determine the age of these fossils and to supplement previous paleomagnetic studies of the Sespe and Vaqueros sequence. More than 225 samples were collected and subjected to both thermal and alternating field demagnetization. Although most samples had a slight chemical remanence overprint resulting from goethite, their characteristic remanence was held mostly in magnetite with little hematite (despite the red color of many rocks). Both normal and reversed polarities were documented and passed reversal tests, demonstrating that the overprints had been removed and a primary or characteristic remanence revealed. There was no evidence of statistically significant tectonic rotation or translation in the Orange County rocks, but the Los Angeles County rocks showed a clockwise tectonic rotation of about 47 6 13u (typical of other results from the western Transverse Ranges). Two sections (Jamboree Road and Santiago Canyon Road) yield late Uintan mammals in rocks of reversed polarity, which correlate with Chron C18r (40.0–41.1 Ma) on the basis of comparisons with Sespe faunas and magnetic stratigraphy of Simi Valley, Ventura County, California. The upper part of the Santiago Canyon Road and the Irvine Lake Sespe Formation sections probably correlates with Chrons C10n–C10r (28.3– 29.5 Ma) on the basis of their early Arikareean mammals. The lower Piuma Road section in the Santa Monica Mountains correlates with Chrons C8n–C9r (26– 28.2 Ma) on the basis of early Arikareean mammals and a date of 27.2 6 0.2 Ma for a tuff just above the fossil-bearing interval. The upper Piuma Road section probably is assignable to Chrons C5Er–C6Ar (18.6–21.9 Ma) on the basis of early Hem- ingfordian land mammals at the top and a date of 21.1 6 0.2 Ma near the base. The Laguna Canyon Vaqueros Formation sections, which yield whales, desmostylians, and sirenians, were mostly normal in polarity (with a short reversed zone at the base), but their correlation is less certain. They probably correlate with Chrons C5Dn–C5Er (17.5–19.0 Ma; early Miocene) on the basis of the occurrence of early Heming- fordian mammals in the underlying Sespe redbeds. INTRODUCTION 1 Paleo Environmental Associates, Inc., 2248 Win- rock Avenue, Altadena, California 91001. Email: The Sespe and Vaqueros formations are distinc- [email protected] tive and widespread middle Tertiary rock units 2 Department of Geology, Occidental College, Los found throughout central and southern Califor- Angeles, California 90041 nia. First defined by Watts (1897) and based on 3 Department of Vertebrate Paleontology, Natural History Museum of Los Angeles County, 900 Exposi- exposures in lower Sespe Creek near Fillmore, tion Boulevard, Los Angeles, California 90007 Ventura County, California, the distinctive red 4 Department of Biology, San Diego State University, sandstones and mudstones of the Sespe Forma- San Diego, California 92182 tion crop out in many places from northern Santa Science Series, Number 41, pp. 43–61 Natural History Museum of Los Angeles County, 2008 44 & Science Series 41 Calvano et al.: Paleomagnetism of Sespe and Vaqueros formations Barbara County to southern Orange County, as depositional sequence in North America where well as on Santa Rosa Island in the Santa Barbara land mammal faunas assignable to these land Channel. This formation also occurs in the mammal ages occur in stratigraphic succession, subsurface. In some places, the Sespe Formation but faunas from the intervening Chadronian (late exceeds 1,700 m in thickness, although most Eocene) and Orellan–Whitneyan (early Oligo- exposures are discontinuous and highly faulted. cene) land mammal ages are missing. Originally, In most places, the Sespe Formation is con- it was thought that the Sespe Formation spanned formably overlain by the widespread gray marine the entire late Uintan through early Arikareean sandstone that is known as the Vaqueros Forma- interval. Subsequent analysis has shown that tion. Originally based on exposures in the Santa upper Eocene and lower Oligocene rocks are Lucia Range near King City (Hamlin, 1904), the missing from the Sespe Formation wherever it Vaqueros Formation crops out widely from the occurs (Prothero et al., 1996), with an unconfor- type area all the way to southern Orange County mity spanning at least 7 million years occurring (Loel and Corey, 1932). The Vaqueros Formation between the middle Eocene and late Oligocene has long been known for its abundant shallow- portions of the section. The middle Eocene water marine molluscan fauna that includes such megasequence of the Sespe Formation was taxa as Turritella inezana Conrad, 1857, and deposited in an alluvial coastal plain setting Rapana vaquerosensis Arnold, 1907, which were during a period of major marine regression the basis for the Vaquerosian molluscan stage (Howard, 1988), a regression that culminated in (Loel and Corey, 1932; Addicott, 1972). Al- the largest eustatic drop of sea level in the entire though the color differences between these two Phanerozoic time interval. The lowered base level formations are easily mapped in most places, in led to a period of erosion that removed a great some places, the contact is gradational, and in deal of the Sespe Formation. The second mega- others redbeds interfinger with gray shallow sequence of the Sespe Formation represents marine sandstones. backfilling of the basin that coincided with a Most studies of the Sespe and Vaqueros major transgression (Howard, 1988). Lander sequence have focused on the outcrops and fossils (1994a, 1994b, 1997, 2002b) and Lander et al. from the Transverse Ranges and farther north (2001b), however, consider the regression and (Fig. 1A). For example, outcrops of the Sespe and transgression to reflect collisional and extensional Vaqueros sequence are widespread in the Santa tectonic events along the continental margin. Monica Mountains (Yerkes and Campbell, 1979, In Orange County, paleontological studies of 1980; Howard, 1988; Dibblee, 1993). The Piuma Sespe Formation vertebrate assemblages began in Road section (Fig. 1B) in this study is one of the 1947. However, these assemblages from Orange thickest and best dated Sespe and Vaqueros County were less extensive than those that had sequences in the Santa Monica Mountains. In been documented in the Sespe Formation to the addition, the Santa Ana Mountains and Laguna north until recent excavations for new housing Hills in Orange County (Fig. 1C) also produce a and freeway projects produced many large thick sequence of land mammal–bearing Sespe collections of fossils (Whistler and Lander, Formation redbeds that are overlain by marine 2003). These collections reveal that the Sespe mollusk–bearing Vaqueros Formation strata. Formation of Orange County yields fossils of late These beds were mapped and described by a Uintan, Duchesnean (middle Eocene), Arikareean number of geologists (Vedder et al., 1957; (late Oligocene), and even early Hemingfordian Schoellhamer et al., 1981; Belyea, 1984; Howard, (early Miocene) age (summarized in Prothero and 1988; Belyea and Minch, 1989). Here, the Sespe Donohoo, 2001; Whistler and Lander, 2003). and Vaqueros sequence ranges in thickness from Thus, outcrops of the Sespe Formation in Orange 300 to 900 m, and it looks very similar to the County span a greater age range than any other Sespe and Vaqueros outcrops that are 160 km exposures of the unit (from about 41.0 Ma to as away in Ventura County. Because the contact is young as 17.5 Ma). in most places gradational, Schoellhamer et al. The Vaqueros Formation exposures in Orange (1981) mapped them as ‘‘Sespe and Vaqueros County received little attention prior to the study undifferentiated,’’ as have all later workers (e.g., by Schoellhamer et al. (1981). Recent excavations Lander, 1994a, 1994b; Lucas et al., 1997). related to construction, however, have produced Stock (1920, 1930, 1948) and Kew (1934) abundant Vaquerosian age molluscan faunas, as were the first to report fossil land mammals from well as an important assemblage of marine the Sespe Formation, and these faunas soon mammals, many of which are still in preparation became critical to our understanding of mamma- and some not yet reported. Noteworthy are 11 lian evolution on the Pacific Coast of North