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Rice Valley Outlier—New Sequence of Cretaceous-Paleocene Strata in Northern Coast Ranges, California

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Rice Valley Outlier—New Sequence of Cretaceous-Paleocene Strata in Northern Coast Ranges, California JAMES O. BERKLAND Department of Geography and Geology, Appalachian State University, Boone, North Carolina 28607 Rice Valley Outlier—New Sequence of Cretaceous-Paleocene Strata in Northern Coast Ranges, California ABSTRACT Valley sequence) environment. Time restric- tions on underthrusting and isostatic rebound Lower Cretaceous through Paleocene strata of deeply subducted Franciscan rocks imply are exposed in a small outlier within the Eastern rapid rates of uplift and denudation during Belt of the Franciscan Complex (Berkland and early Paleocene time. The suggested rate of others, 1972) near Rice Valley in the northern 240 cm per 1,000 yrs is comparable with the Coast Range of California. This outlier, and highest known rates in the world today. three others known in the region, are syncli- nally folded, downfaulted remnants of an INTRODUCTION upper thrust plate of Great Valley sequence Recognition in 1970 of a new Cretaceous- rocks. Paleocene outlier (Berkland, 1971) within the The Rice Valley synclinal remnant is sepa- Franciscan Complex of northern California rated by faults from surrounding rocks of the resulted when Foyle Mason showed me a Franciscan Complex. Two belts of serpentin- quartz-rich sandstone sample collected near ized dunite and harzburgite, converging toward Rice Valley. Field investigation revealed the the south, border the Rice Valley sequence on fourth known outlier of Great Valley sequence the east, west, and south. The northern border rocks in the northern Coast Range (Fig. 1) is a reverse fault that brings blueschist facies and it constitutes the first recognized in this metaclastic rocks of the Eastern Belt over century. This newest section, herein called the unmetamorphosed, mildly deformed strata of Rice Valley outlier, covers about 1 sq mi and the Rice Valley sequence. is located about 5 mi southeast of Lake Pills- Basal beds of the sequence are Lower Creta- bury in the central part of the northern Coast ceous (Hauterivian) sandstone, shale, and lime- Ranges. Exposures of the outlier occur partly stone that are steeply tilted and faulted against within Rice Valley and partly within the the ultramafic rocks. About 3,500 ft of Cre- valley of McLeod Creek, a tiny northern taceous marine strata are unconformably over- tributary of Rice Creek. lain by more than 1,150 ft of shallow-marine The Rice Valley outlier consists of more than lower Tertiary beds. Fossils are generally 4,650 ft of clastic marine strata ranging in age scarce throughout the sequence, but are abun- from Early Cretaceous (Hauterivian) through dant in limestone-rich horizons of Hauterivian Paleocene. The sequence is folded into a and Cenomanian age as well as in a calcareous northwest-plunging syncline and is separated sandstone bed with more than 30 genera of from surrounding metamorphosed rocks of the late Paleocene (Meganos Stage) invertebrates. Franciscan Complex by diapirically(P) in- Special significance is attached to a 150-ft- faulted bands of ultramafic rock. The Rice thick bed of polished-pebble conglomerate Valley outlier comprises an exposed section of: which underlies the Paleocene fossils. The (1) Cretaceous shales with limestones contain- conglomerate is composed chiefly of Franciscan ing Hauterivian megafossils, (2) Cretaceous detritus, with pebbles of red radiolarian chert, arkosic sandstones with shales and nodular lawsonitic metagraywacke, and serpentinite. limestones containing Albian-Cenomanian pal- This is the earliest known appearance of coarse ynomorphs, (3) Paleocene massive, cross- Franciscan debris in an upper plate (Great bedded sandstone resting unconformably upon Geological Society of America Bulletin, v. 84, p. 2389-2406, 3 figs., July 1973 2389 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2389/3433395/i0016-7606-84-7-2389.pdf by guest on 25 September 2021 r* o'» ¿/« »»»*>. \ \ >//.' ^ * Ì V. * • « * " x 123'» Figure 1. Generalized geologic map of the Middle plex (Coastal Belt, Central Belt, and Eastern Belt) are Mountain-Rice Valley region, northern Coast Ranges, shown, along with the known out:liers of Great Valley California. Three subdivisions of the Franciscan Com- sequence rocks (Kgv). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2389/3433395/i0016-7606-84-7-2389.pdf by guest on 25 September 2021 RICE VALLEY OUTLIER, CALIFORNIA 2391 the Cretaceous rocks, (4) coarse conglomerate lated the rocks with their newly defined consisting chiefly of Franciscan detritus in- "coastal belt" because of the abundance of cluding lawsonitic metagraywacke, (5) quartz- detrital potassium feldspar and the presence of rich grit lithologically identical to that found laumontite in both units. Later, Bailey and 30 mi to the southeast at Lower Lake and as- others (1964) showed the Middle Mountain signed to the Meganos Stage of the Paleocene outlier to be correlative with the Upper Creta- by Clark and Vokes (1936), (6) fossiliferous ceous part of the Great Valley sequence. More calcareous sandstone containing abundant shal- recent studies by the writer (Berkland, 1969, low-marine invertebrates of late Paleocene age 1972b, 1973, in prep.) have established more (Berkland, 1971), and (7) glauconitic sand- than 60 fossil localities within the Middle stone with mostly fragmental fossils of late Mountain sequence. The ages range from Paleocene to possibly early Eocene age (W. O. lowermost Cretaceous (Berriasian) to middle Addicott, 1970, written commun.). Paleocene (Ynezian), over a stratigraphic in- terval of more than 8,000 ft. PREVIOUS WORK About 30 mi northwest of Rice Valley, near The Rice Valley area has not been mapped Covelo, a section of marine strata ranges from geologically except where included in broad late Late Cretaceous through middle Miocene reconnaissance studies. All such maps (Irwin, (Clark, 1940). These rocks were first investi- 1960; Bailey and Irwin, 1959; Jennings and gated during the Whitney geologic survey Strand, 1960; Bailey and others, 1964) show (Gabb, 1866), but the structure of the outlier Rice Valley to be underlain by rocks of the is still poorly understood. The middle Miocene Franciscan Complex. Many workers have rec- strata (Temblor Stage) appear to be unique in ognized that the Franciscan can be subdivided this part of California. The section is domi- into northwest-trending belts having distinc- nantly conglomeratic, with abundant polished, tive lithologic, mineralogic, and structural well-rounded pebbles and cobbles of mostly characteristics. Three such belts (Fig. 1) were Franciscan lithologies. The Paleocene con- described by Berkland (1972b) and Berkland glomerate at Rice Valley is strikingly similar and others (1972). Also depicted on Figure 1 except for its greater age. are the four known outliers of Great Valley Approximately 30 mi southeast of Rice sequence and lower Tertiary rocks in the area. Valley, a composite outlier of imbricate' thrust The smallest such outlier is at Rice Valley and slices of Great Valley and lower Tertiary rocks the largest, about 8 mi west, is Middle Moun- was mapped by Swe and Dickinson (1970). tain (Fairbanks, 1893; Banks and others, 1958; The geology of this area near Lower Lake was Berkland, 1969, 1972a, 1972b). The Middle first described by Gabb (1866) and later by Mountain sequence was recognized as being Stanton (1896) and Dickerson (1916). Little separable from the Franciscan Complex during new information regarding the Tertiary marine 1890 field work by Fairbanks. He did not study rocks here has been published, but Anderson Middle Mountain closely but, on the basis of (1936) and Brice (1953) summarized the earlier lithology, he considered the mountain to be work. However, several workers have empha- comprised of the "Chico-Tejon series," a term sized the significant lack of Franciscan detritus formerly used to include many of the Creta- within the Tertiary marine outliers near Lower ceous-Paleogene rocks on the Pacific Coast. Lake (for example, Reed, 1933; Brice, 1953; Geologic mapping of Middle Mountain was Page, 1966). The lack of evidence for a Fran- not attempted until the late 1950s, when ciscan provenance was considered anomalous geologists of the California Department of as the outliers are surrounded by Franciscan Water Resources delimited the outlier (Banks terrain. Some workers have explained the and others, 1957). During that work, Early anomaly by post-Eocene thrusting of the Cretaceous microfossils were obtained from Paleogene rocks onto the Franciscan. two localities, providing the basis for desig- Dickerson (1916) studied the fauna of the nating the Middle Mountain outlier as Lower youngest marine beds at Lower Lake and as- Cretaceous on the California State Geologic signed them to the upper Eocene (Tejon Map (Jennings and Strand, 1960). Stage). The basis for this correlation was later Bailey and Irwin (1959) collected samples of shown to be in error by Clark and Vokes (1936, sandstone from Middle Mountain and corre- p. 856-858), who considered the beds to belong Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/7/2389/3433395/i0016-7606-84-7-2389.pdf by guest on 25 September 2021 2398 J. O. LiERKLAND to the late Paleocene (Meganos Stage). This attributable to repeated episodes of regional age revision of the Lower Lake "Tejon" beds shearing, to sedimenta:.-y processes, such as has apparently been overlooked by other work- submarine sliding, or to both mechanisms. ers in the area (Anderson, 1936; Brice, 1953; Either type of deformation would be accom- Koenig, 1963; Swe and Dickinson, 1970; Garri- modated by the environment generally con- son, 1972), who have perpetuated the "Tejon" sidered for much of the Franciscan—that of a designation in the absence of new fossil collec- subducted oceanic crustal plate and trench tions. The Meganos Stage correlation by Clark (Lowe, 1969; Ernst, 1970; Page, 1970; Hsu, and Vokes (1936) is enhanced by a more abun- 1971). dant Meganos Stage fauna to the north at Between the Eastern Belt of the Franciscan Round Valley (Clark, 1940) and by the latest Complex and the "coastal belt" of Bailey and discovery at Rice Valley (Berkland, 1971).
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