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
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^ * 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).
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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
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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). Irwin (1959), lies a zone of mostly incoherent Studies of heavy minerals or conglomerate melanges. This zone, named the Central Belt clasts, or both, from Paleocene strata revealed of Berkland and others (1972), consists of a no Franciscan debris at Lower Lake (Brr.ce, generally chaotic mixture of resistant Francis- 1953, p. 27-30), at Middle Mountain (Berk- can rock types—including glaucophane am- land, 1969, 1972b), or at Round Valley (Swe phibolites and eclogile—enclosed within a and Dickinson, 1970, p. 184). However, a flood sheared matrix of pumpellyitic shales and gray- of Franciscan detritus appears beneath the wackes. A few miles north of Rice Valley, Meganos Stage fossils at Rice Valley (Berkland, these rocks have been informally known as the 1971). The timing and distribution of the first "Skunk Rock mélange" by geologists of the appearance of Franciscan debris has important California Department of Water Resources and plate-tectonic implications (Page, 1970, 1972; other workers. The Ric e Valley outlier, as well Berkland, 1972a, 1972b). as the other known outliers in the northern Coast Ranges, are surrounded by mélanged rocks similar to those described by Brown STRATIGRAPHY (1964) as a "friction carpet" beneath a thrust Mesozoic System plate in the Stonyford quadrangle, about 15 mi east of Rice Valley. Franciscan Complex. The Franciscan Com- plex, formally renamed by Berkland and others Both the Central Belt and the Eastern Belt (1972), is a diverse assemblage of highly de- contain scarce but widely distributed fossils formed rocks having a classical eugeosyncl.nal (especially Buchia) of latest Jurassic to Early character (Bailey and others, 1964). It consists Cretaceous age. Both belts also contain at least predominantly of variably metamorphosed one locality with fossil; of Late Cretaceous age graywacke and shale, together with metamor- (Berkland, 1972b; D. L. Jones, 1969, oral phosed mafic volcanic rocks (greenstones), commun.). The younger fossils appear to date radiolarian chert, and serpentinite. Most of large slabs of rock (broken formations?) not the rocks from the longitude of Clear Lake typical of the rocks of the surrounding belts. eastward to the base of the Great Valley se- The age of the Franciscan Complex has been quence are foliated metaclastic rocks (In,vin, controversial for more than 100 yrs, but most 1960) which have been assigned to the Eastern workers have assigned it to either the Jurassic Belt of the Franciscan Complex (Berkland and or Cretaceous period, or to both. Irwin (1957) others, 1972). These rocks typically contain and Bailey and others (1964) established that metamorphic phases such as pumpellyite, law- fossils from widely separated Franciscan ter- sonite, phengitic mica, and, locally, jadeitic ranes ranged from Late Jurassic (Tithonian) to pyroxene and glaucophane. In the Rice Valley at least early Late Cretaceous (Turonian) in area, clastic rocks are of textural zones I and II age. Berkland (1964) showed that the Coastal of Blake and others (1967). The rocks have a Belt (more than 25 mi west of Rice Valley) metamorphic fabric which parallels the bed- contained fossils of late Late Cretaceous ding and retains primary sedimentary features (Senonian) age. Berkland (1969, 1972a, 1972b) such as graded bedding. The Eastern Belt con- later described a Maestrichtian unit within sists of a series of relatively coherent masses, the Central Belt of the Franciscan Complex. hundreds to thousands of feet in thickness, This unit was the youngest known phase of the separated by northwest-trending mélanges. The Franciscan until recently when a number of deformational style of the mélanges may be Tertiary fossil localities were established within
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the Coastal Belt with microfossils as young as adjacent sedimentary rocks suggests that the middle Eocene (O'Day and Kramer, 1972; ultramafites were emplaced as cold intrusives Kramer and others, 1973). Thus the Franciscan along fault zones. However, it is possible that Complex, as a whole, spans a time interval an original depositional contact has been dis- ranging from at least Late Jurassic to early rupted by differential tectonic movements be- Tertiary—approximately 100 m.y. tween adjacent rocks of varying competence. Ultramafic Rocks. Ultramafic rocks were Such movements could account for the com- traditionally included within the Franciscan mon slickensiding and shearing near the basal Complex in the California Coast Ranges, but contact of the sedimentary sequence at Rice mounting evidence shows that many of the Valley. However, if this sequence were de- exposed ultramafic masses are part of an posited upon an eroded segment of oceanic ophiolite sequence which represents pre-late lithosphere (the ultramafic rocks), later fault- Tithonian oceanic lithosphere upon which ing has destroyed the evidence. The present the Great Valley sequence was deposited contact relations show that the western band (Moiseyev, 1966; Bailey and others, 1964; of ultramafic rocks truncates the Rice Valley Bezore, 1969; Raymond, 1970; Page, 1970, synclinal axis (Fig. 2) and cuts out more than 1972). The Franciscan Complex also may have 1,500 ft of strata. been deposited upon mafic-ultramafic base- Clasts of serpentine and serpentinized perido- ment rocks, but no unfaulted contacts with tite are not found in the overlying Cretaceous such rocks have been observed beneath un- strata, but are common higher in the section as questioned Franciscan strata. constituents of a Paleocene conglomerate that Significantly, no ultramafic rocks are known is comprised mostly of Franciscan detritus. to intrude either the outliers or the main mass Presumably most of the clasts were derived by of the Great Valley sequence, except near the local erosion of uplifted basement rocks follow- base of the Upper Jurassic Knoxville Formation ing Cretaceous deposition. (McKee, 1965). However, olistostrome units Rice Valley Sequence: Cretaceous System. composed of serpentinous debris are found The lower and more widely distributed part within the Jurassic and Lower Cretaceous parts of the Rice Valley sequence is comprised of the Great Valley sequence (Taliaferro, 1943; of unnamed Cretaceous (Hauterivian to Moiseyev, 1970), but merely form sedimentary Cenomanian) marine strata. These correlate tongues within that sequence. The provenance with a portion of the Great Valley sequence, of the serpentine has not been determined, but which forms a great homocline about 20 mi to fossils of a distinctive Brachiopod associated the east of Rice Valley. In the outlier, only with the sedimentary serpentines were dis- about 3,500 ft of the Cretaceous system is covered in situ at the west edge of Rice Valley, represented, compared with about 40,000 ft where abundant brachiopodal limestone beds at the western edge of the Sacramento Valley. in a shale sequence structurally overlie serpen- However, fossils were obtained at only two tinized peridotite. horizons within the Cretaceous part of the The ultramafic belts near Rice Valley sepa- Rice Valley sequence and the time represented rate metamorphosed and intensely deformed by the 3,500 ft of strata there is incompletely lower plate rocks of the Franciscan Complex known. At the base of the section, Lower Cre- from mildly deformed Cretaceous and Paleo- taceous (Hauterivian) rocks are faulted against gene strata of the upper plate. The ultramafic ultramafic rocks; at the top of the section, rocks are typically massive, strongly jointed, Upper Cretaceous strata are unconformably and reddish weathering, with abundant bastite overlain by Paleocene strata. pseudomorphs after pyroxene. Most of the The Cretaceous rocks are mainly arkosic primary rock was harzburgite with minor sandstone and greenish-gray to dark-gray silt- dunite, all of which has been 75 to 95 percent stone containing variably sized lenses of gray serpentinized. Along the margins of the ultra- limestone. Minor pebble conglomerate consists mafic bodies, shearing has produced slickentite of subrounded clasts about x/i in. in diameter, serpentine; within the bodies there are wide- mainly of black chert and shale. The arkosic spread but minor chromite segregations and sandstone contains 15 to 20 percent potassium chrysotile veinlets. feldspar and abundant biotite flakes. Spheroidal The linear outcrop pattern of the ultramafic weathering is characteristic of the thicker beds rocks and the lack of thermal effects in the which attain maximum thicknesses exceeding
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Figure 2. Geologic map of the Rice Valley area in minute series, California). the northwest part of the Potato Hill quadrangle 3 ft. The Cretaceous sandstones do not provide basal fault contact with peridotite, where the bold outcrops of the overlying massive crumpling, slickensiding, and rare quartz veins Tertiary sandstones, partly because the latter are present. are richer in calcareous cement. The Cretaceous The lowermost beds of the Rice Valley se- rocks are steeply dipping but not highly de- quence are found near the south end of Rice formed, except within a few tens of feet of the Valley, where shales with limestone lenses up
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EXPLANATION
Qal Quaternary alluvium Sand, silt, clay, and gravel. Mostly less than (100 ft? max) 10 ft thick except in Rice Valley and lower McLeod Cr.
I [QT] Quaternary terrace Flat-lying terrace deposits dissected by McLeod (15 ft+) Cr. Mostly silty sands and silty gravel. Caliche
0 unconf. Rice Valley sequence N o Te? Glauconitic sandstone Friable green sandstone with onion-skin weathered z (500 ft) claystone (Capay?). w r •e Tepi Concretionary sandstone Limy sandstone with 4-ft-thick coquinoid zone u u (150 ft) with Meganos Stage fossils. H Tepa Quartz grit Quartz arenite with coarse, angular grains. Minor I (150 ft) coal seams to 1 in. * Partly contemporaneous with Cretaceous section of Rice V. sequence. to 15 ft in diam are locally rich in fossils from along the west side of Rice Valley. The lime- the middle part of the Early Cretaceous (Hau- stones produce a strongly foetid odor when terivian). The Early Cretaceous strata of the struck with a hammer and have yielded the Rice Valley sequence appear to be restricted following Hauterivian Stage megafossils: to the drainage basin of Rice Valley, itself. No Peregrinella whitneyii, Turbo wilburensis, part of the sequence extends across the Bear Belemnites sp., and a modiolid pelecypod. Creek Road, about 1 mi south of Rice Valley, Toward the north the western band of ultra- or to the Stonyford-Upper Lake Road, about mafic rock truncates progressively younger 1 mi north of Rice Valley. beds in the Rice Valley sequence. At the base The most striking fossiliferous rocks at Rice of the sequence on the south side of McLeod Valley are coquinoid pale-gray limestone lenses Creek, the strata are no older than Albian, the from shales within 100 ft of the ultramafic belt uppermost stage of the Early Cretaceous. The 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 rocks are dated by a distinctive nodular lime- abundant along some hori;x>ns in the sandstone. stone horizon about 15 ft thick, which lies They occur as ovoid lenses reaching maximum within 50 ft of the ultramafic belt. The nodules dimensions of 3 to 4 ft and consist of maroon- are typically pale gray, but are reddish weath- weathering, bluish-gray limestone. Small pieces ering close to the fault contact because ol of shell obtained from these lenses have thus readily oxidized pyrite. The nodules weather far been undiagnostic of age. out of the enclosing shales and resemble loose The probability that these beds represent potatoes on the hillslope; however, individual the Martinez Stage is enhanced by the observa- nodules are spheroidal and reach diameters up tions of Merriam and Turner (1937, p. 92) to 16 in. The limestone horizon contains rare that: ". . . we find the true Meganos and its megafossils but extremely abundant and well- correlatives preceded in stratigraphie order by preserved dinoflagellates diagnostic of middle the Martinez formation at all known occur- Cretaceous (Albian-Cenomanian) age, accord- rences." At Rice Valley, Meganos Stage fossils ing to W. R. Evitt (1970, written commun.). occur abundantly about 400 ft stratigraphically At least one sample also contained angiosperm above the supposed Martinez formation. pollen of Late Cretaceous age, thus permitting the assignment of the limestone nodule horizon Polished Pebble Conglomerate to the Cenomanian, rather than Albian-Ceno- The cross-bedded Martinez Stage(?) sand- manian. The dinoflagellates identified by Evitt stone contains sporadic pebbles within 10 ft of included a large number of undescribed species, the top of the unit. In the bed of McCleod but the following diagnostic forms were Creek the sandstone can be seen to become present: Litosphaeridium siphoniphorum, Wal- increasingly pebbly upward to a gradational lodinium sp., Deflandrea sp., and (long ranging) contact with a polished-pebble conglomerate, Odontochitina operculata. about 150 ft thick. The clasts are mostly sub- The only megafossils found at this horizon rounded Franciscan litholDgic types, from ]/i were a small bivalve resembling Cucullaea ? and to 3 in. in diameter, with a maximum size of a 5-in. long, slender and tapering cone of 6 in. A pebble count of 152 clasts (Table 1) calcite, possibly a recrystallized Baculites. shows that more than three-fourths are attrib- As mentioned previously, the upper age utable to a Franciscan provenance. Especially limit of the Cretaceous beds at Rice Valley is diagnostic of the Franciscan Complex are unknown, but it is probable that most of the quartz-veined metagraywacke pebbles, two of which were sectioned and found to contain post-Cenomanian section is absent. The uncon- lawsonite and pumpellyite. The metagray- formity which separates the Cretaceous strata wacke pebbles are indistinguishable in texture from overlying Paleocene beds, along with the and mineralogy from the bedrock of the East- presence of Cretaceous detritus in Paleocene ern Belt of the Franciscan Complex, which conglomerate, suggests considerable erosion of completely surrounds Rice Valley. Other con- the Cretaceous rocks during Late Cretaceous glomerate clasts probably referable to the or earliest Tertiary time. Franciscan are red and green radiolarian chert Tertiary System and riebeckitic metachert. Less diagnostic, but also possible indicators of the Franciscan Com- Martinez Stage(?). Massive, cross-bedded plex are unmetamorphosed graywacke, green- sandstone, about 200 ft thick, overlies the Cre- stone, keratophyre, serpentine, jasper, and taceous rocks with slight angular discordance. vein quartz. Minor coal seams and plant detritus occur in several horizons, as do worm tubes and frag- I am familiar with late Cenozoic con- mental shells of invertebrates. No diagnostic glomerates at a number of localities in fossils were found, but the yellowish-buff color, western California where a Franciscan prove- cross-bedding, and distinctive alveolar weath- nance is undoubted (for example, Cache for- ering of the calcareous sandstone are character- mation, Red Bluff gravels, Tulare gravels, istic of known Paleocene Martinez Stage rocks Livermore gravels, Calpella gravels). The early at Middle Mountain, 10 mi to the west; at Cenozoic conglomerate at Rice Valley is strik- Lower Lake, 30 mi to the southeast; and near ingly similar in composition to these younger Round Valley, 30 mi to the northwest. Future units, and all contrast strongly with late search for fossils in the Martinez Stage(?) Mesozoic conglomerates iri the Great Valley strata at Rice Valley should be concentrated in sequence (Bailey and others, 1964; Page, 1970). large limestone concretions, which are fairly At Rice Valley, the Paleocene conglomerate 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 contains about 1 percent Cretaceous detritus proximately 70 percent of the grains are derived from local equivalents of the Great quartz. The grit contains thin seams of glossy Valley sequence, all of which are easily erodible black coal about 2 in. in thickness but measur- clastic rocks and are thus not abundantly ing only a few feet long. No fossils have been represented. About 18 percent of the pebbles obtained from any part of the quartz grit, but are mafic porphyries having a non-Franciscan the attitudes of adjacent units suggest con- character. These may have been derived from formity; however, the unusual character of the area of the northern Sierra Nevada, where the grit and its rapid wedging-out toward the early Tertiary volcanic activity occurred east indicate deposition during a time of (Slemmons, 1966). rapidly fluctuating conditions. I first correlated the Rice Valley conglomer- Abundant resistant quartz grains, as well as ate with a lithologically similar unit near their angularity, suggest minimal transport Round Valley, about 30 mi to the north. How- and deep chemical decay in a subtropical ever, the more northerly conglomerate is of environment, a milieu common to many middle Miocene (Temblor) age, whereas the Paleogene formations of interior California Rice Valley conglomerate underlies richly (Allen, 1929, 1941). The quartz grit is here fossiliferous Meganos Stage strata and can be considered to belong to the Meganos Stage of no younger than late Paleocene. The Tertiary the late Paleocene. The unit attains a maximum part of the section at Rice Valley appears to of 150 ft in thickness at Rice Valley, but 30 mi record continuous deposition from early Paleo- to the southeast, a quartz-rich grit of identical cene to possibly early Eocene time, and the appearance is about 1,000 ft thick (Brice, 1953, polished-pebble conglomerate is considered to p. 12). This much thicker section is at Lower be an upper phase of the early Paleocene Lake, where the quartz-rich sandstone may Martinez Stage. The appearance of abundant represent the entire Meganos Stage. At Round Franciscan detritus is the earliest known within Valley, near Covelo, about 30 mi northwest of the continental shelf-type sediments of the Rice Valley, Clark (1940) mapped 300 ft of upper plate. Most other references to Francis- Meganos Stage strata resting upon 130 ft of can debris in pre-Miocene strata rely upon the Martinez Formation. Both of these units nondiagnostic clasts such as greenstone, gray- at Round Valley are tawny brown-weathering, wacke, or chert of various colors, or upon heavy carbonaceous sandstones separable only on minerals, such as blue amphibole and chromite. faunal grounds. All of these materials could have had a prove- The Rice Valley Meganos Stage rocks lie nance in the Sierra Nevada or Klamath Moun- almost exactly halfway between the two previ- tains (Bailey and others, 1964, p. 41; C. Dur- ously known occurrences in the northern Cali- rell, 1971, oral commun.). Comprehensive fornia Coast Ranges, and may help to define a studies of the lower Eocene lone Formation formerly continuous seaway, perhaps an inun- and its correlatives (Allen, 1929, 1941; Morris, dated trough resulting from block faulting 1962) showed some detrital glaucophane along associated with late stages of isostatic rebound the west side of the Great Valley, but none of subducted Franciscan materials (Ernst, along the east side, adjacent to the Sierra 1970). Nevada. This distribution argues against a Sierran source, but does not eliminate the Concretionary Sandstone Klamath Mountains as a provenance for blue amphibole detritus in Mesozoic or younger Conformably or disconformably overlying formations. However, coarse clasts of meta- the quartz grit is a highly fossiliferous, calcare- graywacke, chert and metachert, as well as ous sandstone that maintains a uniform thick- nonresistant serpentinite, strongly indicate a ness of about 150 ft. A resistant arenaceous local derivation of Franciscan materials. The limestone bed, about 5 ft thick, lies within the significance of this detritus is discussed in a upper third of the unit. This horizon has pro- later section of this article. duced most of the Tertiary fossils discovered at Rice Valley and is equivalent to the Meganos "D" horizon of Clark (1918, 1921). The Quartz Grit calcareous sandstone contains arenaceous lime- Overlying the polished-pebble conglomerate stone concretions which are maroon weathering on the western limb of the Rice Valley syncline and form a dark-colored, resistant, narrow is a tongue of distinctive quartz grit, containing ledge, which projects 1 to 3 ft above the hill- angular particles about 1 in. in diameter. Ap- slopes. Thus it forms a distinctive marker 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 horizon except along the extremely brushy ar.d TABLE 1. PEBBLE-COUNT OF PALEOCENE serpentinite-mantled upper slopes bordering CONGLOMERATE (UNIT Tep 1) McLeod Creek. In such areas only the massive sandstone outcrops of the Martinez Stage( 5) (%) are easily seen. Franciscan types Graywack: 52 Several fossil collections were identified by Metagraywacke (foliated)* 26 W. O. Addicott (June 2 and November 6, Chert (rec, radiolarian) 16 1970, written commun.). Of more than 40 Chert (green, radiolarian)* 4 different invertebrate species present, 30 were Jasper 3 Greenstone _2 identified to generic or specific levels (Table 103 (68) 2). The most diagnostic forms are: Turritelia Possibly Franciscan types meganosensis, T. merriami brevitabulata, Chert (gray to black) 10 Whitneyella meganosensis, Spisula meganosensis, Bull quartz 8 Diplodonta cretacea, and Nuculana (Leda) Serpentine _2 gabbi. Regarding the fauna, Addicott (Novem- 20 (13) Great Valley Sequence types ber 6, 1970) stated: Arkosic sandstone 1 The assemblages are referable to the Paleocene Foraminiferal limestone 1 "Meganos Stage." The only other occurrences of 2 (1) the "Meganos" in northern California are at the Unknown affinities type locality near Mount Diablo, about 100 mi t3 Basalt 1 the southeast, and at Round Valley, about 30 mi Andesite porphyry 12 Dacite 4 to the northwest. Rhyolite 3 Addicott was not aware that the "Tejon" Keratophyre 4 strata at Lower Lake had been reassigned to Granite-microgranite 3 Unidentified _1_ the Meganos Stage by Clark and Vokes (1936), 28 (18) thus making Rice Valley the fourth occurrence Total clasts 153 (100) of Meganos Stage fossils in the northern Coast Ranges. * With pumpellyite or lawsonite, or both. According to Addicott, the fossil assem- t One bluish chert with needles of blue amphibole. blages tend to confirm the field evidence from Rice Valley (that is, cross-bedded sandstones, well preserved as those from the underlying coal seams, fragmental shells, glauconitic sand- Meganos "D" horizon. The glauconitic strata stones, rapid intertonguing of units) and indi- are rather arbitrarily assigned to the Meganos cate a shallow-marine origin for the Tertiary "E" horizon of Clark (1918, 1921), which is section. Some variation in conditions is sug- equivalent to the later recognized Capay for- gested by Addicott's comment that "The three mation of Crook and Kirby (1935). Fossils stratigraphically highest localities may repre- correlative with those from the Capay forma- sent somewhat deeper water conditions (but) tion were later assigned to the "Capay Stage" all probably represent sublittoral depths of the early Eocene by Merriam and Turner ( =neritic zone)." (1937). Tney defined the Capay Stage as: "... a well-characterized faunal and deposi- Glauconitic Sandstone tional stage between the Meganos and the The youngest marine strata at Rice Valley Domengine (including) . . . certain deposits are glauconitic sandstone and greenish silt- which have previously been regarded as cor- stone, which together have a maximum thick- relatives of the Meganos." ness of about 500 ft. They lie along the syn- Fossils from the Rice Valley glauconitic clinal axis of Rice Valley and the best exposures sandstone and siltstone were not distinctive are along the banks—and rarely the bed—of from those of the underlying Meganos "D" McLeod Creek. The siltstone is hackly frac- zone, except for the presence of fragments of tured and exhibits onion-skin weathering. I'; crab carapace in the younger beds. However, contains minor calcareous siltstone lenses, Addicott considered the scanty fauna to repre- which become punky and pinkish colored sent a horizon "... possibly as young as when weathered. Rare invertebrate fossils have Eocene" based on the presence of Scaphander been obtained from various horizons in this costatus, Nuculana sp., Tellina?, Turritelia sp., unit, but they tend to be smaller and not so and crab carapace fragments. Thus, the Pa- 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 TABLE 2. RICE VALLEY MEGAFOSSILS, MEGANOS STAGE (LATE PALEOCENE) Bathybembix-like Trochid Brachysphingus mammilatus (Clark and Woodford) Rm, Rc, Lm, Mm Calyptraea cf. C. diegoana (Conrad) Lm, Cc, Mm Cryptoconus injucundus Hanna? Lm, Mm Ficopsis sp. Rm, Rc, Mm Molopophorusl Mm Neverita cf. N. globosa (Gabb) Lm, Cc, Mm Scaphander costatus (Gabb) Rm, Lm, Cc, Mm Turbonilla sp. Cc, Mm Turritella meganosensis Clark and Woodford Rm, Mm Turritella cf. T. merriami brevitabulata Merriam and Turner Rm, Mm Turritella sp. Rm, Rc, Cc, Mm Whiineyella meganosensis Turner Mm Naticid, turrid-Crassispira-\iV.e Mm About 10 undetermined taxa (minute species or internal molds) Acila decisa (Conrad) Rm, Rc, Cc, Mm Diplodonta cretacea (Gabb) Mm Mytilus sp. (minute form) Nuculana gabbi (Gabb) Rm, Rc, Lm, Cc, Mm Nuculana (Saccella) sp. Rm, Rc, Lm, Cc, Mm Ostrea sp. Rc, Lm, Cc, Mm Pitar sp. Rm, Mm Schedocardia hartleyensis (Clark and Woodford) Rc, Cc Solen cf. S. stantoni Weaver Lm, Mm Sphenia meganosensis Clark and Woodford? Mm Spisula maganosensis Clark and Woodford Lm, Mm Tellina longa Gabb Lm, Mm Venericardia sp. Rm, Rc, Lm, Cc, Mm Arcid Mm Cardiid Mm Crab carapace and appendage fragments Mm Identifications by W. O. Addicott, U.S. Geological Survey. Letter symbols refer to other localities where these fossils are known. (Rm) Round Valley Meganos; (Rc) Round Valley Capay; (Lm) Lower Lake Meganos; (Cc) Type Capay at Capay Valley; (Mm) "Type Meganos" at Mt. Diablo. leogene glauconitic sandstone unit at Rice This assumption has been made realistic by the Valley is questionably assigned to the early reassignment of the supposed upper Eocene Eocene Capay Stage. "Tejon" of that area to the late Paleocene The Meganos-Capay contact at Round Val- Meganos Stage (Clark and Vokes, 1936). ley was reported by Clark (1940, p. 131) to be conformable and the absence of a suitable Quaternary System lithologic break caused him to place the contact Quaternary sediments of the Rice Valley arbitrarily within a 210-ft-thick, unfossiliferous area consist of terrace alluvium, valley-fill al- shale unit. Similarly, the contact at Rice luvium, and landslide deposits. Most of the Valley is considered conformable due to con- sedimentary cover is rather thin (less than 10 sistent attitudes in the known Meganos and ft) and bedrock is exposed in many places in the Capay (?) Stage horizons. However, the the bottom of McLeod and Rice Creeks. How- presence of the Capay Stage at Rice Valley ever, the most heavily alluviated portions of is yet to be proven. Also at Lower Lake the Rice Valley may be 100 ft in thickness. Paleogene section is not known to include the Upper McLeod Creek has cut a channel Capay Stage. However, since early in this about 20 ft deep into fiat-lying, fine-grained century, workers in the Lower Lake area have terrace materials, apparently as a result of largely ignored the lower Tertiary rocks. Gen- rejuvenation. The middle course of the creek erally, these strata have been lumped into the flows largely along bedrock and a nick point Martinez and "Tejon" formations, but it is is migrating upstream into extensive terrace possible that the Capay Stage may overlie the alluvium, which conceals the northern bound- "Tejon" of Dickerson (1916) at Lower Lake. ary of the Rice Valley sequence. 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 Landslide or slopewash, or both, are mainly movements may account for the observed surficial deposits, especially common along the relations. ridges bordering McLeod Creek. The mobile Similar to other outliers of Great Valley materials were derived chiefly from the elon- sequence rocks in the Franciscan Complex, the gated ultramafic belts near the ridge tops, but Rice Valley sequence is bordered by serpen- there is considerable admixture with various tinite slivers and chaotic terrane. Such terrane rocks of the Rice Valley sequence. has been considered by some workers as great shear zones or tectonic zones (Bailey and Structure others, 1964). Other workers have adopted the The Cretaceous and Paleogene beds of the mélange concept of Hsu (1971) to explain the Rice Valley sequence (and possibly the under- intensely sheared, crushed, and mixed nature lying ultramafic rocks as well) are folded into of such terrane. I (Berkland and others, an open syncline which plunges about 30° tc 1972) consider these rocks in the northern N. 50° W. The western limb is oversteepened to Coast Ranges as the characteristic phase the vertical, whereas the eastern limb dips no of the Central Belt of the Franciscan Com- more steeply than about 65°. A primary depo- plex. The chaotic rocks of the Central Belt sitional contact of sediments upon ultramafic may constitute a "friction carpet" (Brown, rock is unlikely, but cannot be totally dis- 1964) between upper and lower thrust plates. counted; however, later movements of the In terms of recent plate tectonics, the ultramafic rocks along marginal faults have chaotic zone may mark a tectonic mixture of progressively cut out younger sedimentary adjacent plates during subc.uction of Franciscan beds toward the northwest. Even the synclina.. rocks beneath the Gres.t Valley sequence axis is truncated by the serpentinite, leaving (Page, 1970; Berkland, 1972b). The latter no doubt that the marginal faults postdate suggestion is supported by the presence of a folding of the Rice Valley sequence. slab of unmetamorphosed gray wacke and minor A reverse fault, trending east-west, has been chert pebble conglomerate along the east side inferred beneath the terrace alluvium at the of Rice Valley. This slab contains up to 3 per- head of McLeod Creek. Evidence for this fault cent K-feldspar and is similar to the Lower includes the unusual east-west drainage of up- Cretaceous part of the Great Valley sequence; per McLeod Creek and the lineament of it may have been torn from the upper plate serpentinite slivers and landslide zones east of during subduction. McLeod Creek. The fault explains the absence of Tertiary rocks ]4, mi north of McLeod Significance of Franciscan Detritus Creek along the Stonyford-Upper Lake Road, The first appearance of Franciscan debris in which has been cut into serpentine and meta- other units (or reworked within itself) has morphosed Franciscan rocks in this area. considerable significance fo:r interpretations of Less than 1 mi south of the mapped area, paleogeography and paleo tectonics of Cali- an east-west road to Bear Creek Ranger Station fornia. Currently popular plate-tectonic models also exposes only Franciscan Complex rocks. (for example, Ernst, 1970) suggest that follow- It appears likely that the ultramafic belts ing an interval of sea-floor spreading, a sub- bordering McLeod Creek intersect south of ducted oceanic plate will undergo isostatic Rice Valley and cut off the Rice Valley se- rebound, owing to the rise of the temporarily quence in that direction. displaced denser mantle material. When this A fault coincident with the axis of the Rice concept is applied to the Franciscan debris, it Valley syncline produces a left-lateral separa- explains the preservation of blueschist facies tion of the polished-pebble conglomerate and minerals which form under conditions of great other members of the sequence. The horizontal pressure, but relatively low temperature. Thus, separation measures about 400 ft, but because they must have been uplifted rapidly before of the north plunge of the syncline, the separa- normal geothermal gradients could be estab- tion might be explained by dip-slip movement lished. along a steeply west-dipping normal fault, Quantitative data regarding the rates of with the eastern block relatively uplifted. This uplift of Franciscan materials have been lack- explanation is preferred but cannot yet be ing, except in the most general terms. Now proven as strike-slip, or a combination of the evidence from Rice Valley Paleocene con- 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 glomerate appears to provide the most critical Brown and Ritter (1969) reported "phenome- data thus far available. The rate of buoyant nal" denudation rates for the Eel River basin uplift, as well as the denudation rate, deter- averaging as much as 204 cm per 1,000 yrs mined how rapidly the subducted rocks of the during a 10-yr study. This figure is close to lower plate might have become available to that suggested for Paleocene time by the erosion. Evidence of uplift provided by sub- geologic evidence presented herein. marine landslides (including detrital serpen- Some degree of uniformity through time in tines) does not have the significance of sub- the erosion rate is to be expected as the Eel aerial or shallow-marine deposits because the River flows mainly through the incompetent, latter indicate a much greater degree of uplift. landslide-prone mélange of the Central Belt of The Rice Valley Paleocene strata are typical of the Franciscan Complex. The ancestral Eel shallow-marine deposits, so Franciscan rocks River system probably had similar lithologic must have been raised above sea level in order and structural controls. Its lower reaches were to have contributed in quantity to those de- the site of a thick accumulation of middle to posits. The presence of lawsonitic metagray- late Cenozoic marine and nonmarine sedimen- wacke clasts in the Paleocene conglomerate tary deposits (Ogle, 1953), showing the longev- suggests that Franciscan rocks were uplifted ity of the area as a negative crustal feature, from at least a depth of 12 km (4 kb) before following the great isostatic uplift and denuda- being eroded about 60 m.y. ago. The time tion of early Cenozoic time. The available data available for such uplift can be determined indicate that an erosion rate of more than 200 from the age of the conglomerate (60 m.y.) cm per 1,000 yrs is realistic for the Rice Valley and the time of cessation of the late Mesozoic area during Paleocene time, as modern rates interval of subduction. The evidence from are similar for the same area and the same kind Middle Mountain, about 13 km west of Rice of rocks. It is likely, but not required by this Valley, suggests that late Maestrichtian rocks hypothesis, that the mountain peaks surround- (65 m.y. old) were subducted near the end of ing the Eel River basin during Paleocene time an interval of subduction (Berkland, 1969, exceeded the present elevations of 5,000 to 1972a, 1972b). Thus, in this area, the time 8,000 ft, because the buoyant uplift of the available for uplift and erosion of rocks was crustal rocks was at a maximum then. approximately 5 m.y., and the suggested rate is 12 km per 5 m.y., or 0.24 cm per yr. This The products of the rapid denudation and seems conservative in terms of sea-floor spread- uplift in the Rice Valley region were carried ing and consumption rates in the Pacific basin, westward during Paleogene time, and accumu- which have been commonly calculated at 6 to lated as a thick clastic wedge along the Pacific 10 cm per yr (Atwater, 1970). border. It is appropriate that the Coastal Belt of the Franciscan Complex is characterized by However, the rate of 0.24 cm per yr appears abundant K-feldspar (reworked from the Great surprisingly high in terms of denudation rates, Valley sequence?) and by blueschist detritus where it would be expressed as 240 cm per in the form of conglomerate clasts and heavy 1,000 yrs and compared with calculated rates minerals (reworked from older parts of the for the Himalayas, which approximate 100 cm Franciscan Complex?; compare Berkland, per 1,000 yrs (Menard, 1961; Schumm, 1963). 1964; Bailey and others, 1964; Boyle, 1967). Higher values for uplift rates were reported Thus, the Coastal Belt had a mixed provenance by Bloom (1969, p. 87), who quoted figures of of probable Great Valley sequence and pre- 120 to 1,050 cm per 1,000 yrs for New Zealand. Coastal Belt Franciscan terranes. The newly Also, Schumm (1963, p. 7) cited 450 cm per recognized Paleocene-Eocene age of much of 1,000 yrs as an average modern value for the Coastal Belt (O'Day and Kramer, 1972; Japan and 500 cm per 1,000 yrs for the Lake Kramer and others, 1973) coincides with the Superior region. The latter rate is for post- postulated time for rapid erosion of isostati- glacial rebound of cratonic rocks and may not cally uplifting rocks to the east. The volume of be strictly analogous to the situation in Japan eroded materials may be largely accounted for 3 or to the hypothetical one in northern Cali- by the estimated 20,000 mi of mainly clastic fornia during Paleocene time. More pertinent detritus comprising the Coastal Belt. The is the known modern erosion rate of the Eel evidence is ambiguous as to whether this River basin, of which Rice Valley is a part. clastic material has been subducted during a 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 ¿lev. '5300 ft 2000 A A' Figure 3. Structural cross-section A-A' across McLeod Creek, Rice Valley area. post-Eocene interval of sea-floor spreading, within Franciscan terrane. Presumably the but no blueschist metamorphism has been rocks constituting the Rice Valley sequence demonstrated for the Coastal Belt; thus, if it are autochthonous and were underthrust by were subducted it must have been limited i?. an oceanic plate of Franciscan materials. Simi- depth. The alternative is that deeply sub- lar isolated remnants of the "upper plate" ducted materials have not been exposed, or, occur less than 35 mi to the north, west, and perhaps, not recognized as part of the Coastal south of Rice Valley at Round Valley, Middle Belt. Mountain, and Lower Like, respectively. The Rice Valley secuence is unique in SUMMARY having very abundant Franciscan debris in a A new section of Cretaceous and early conglomeratic unit of Paleocene age, the Tertiary strata lies 5 mi southeast of Lake earliest documented appearance of such ma- Pillsbury, California, within metamorphosed terial in an "upper plate" environment. The terrane of the Eastern Belt of the Franciscan conglomerate is significant because it provides Complex. The section totals about 4,650 ft in a means of determining uplift and erosion rates thickness and includes fossiliferous strata rang- for deeply subducted Franciscan rocks, some ing in age from Lower Cretaceous to late of which were metamorphosed to the blue- Paleocene. schist facies. Earlier published evidence had The Rice Valley sequence consists of: (1) merely permitted broad, nonquantitative state- arkosic sandstones with shales containing fos- ments that Franciscan blueschists had been siliferous limestone concretions of Hauterivian subjected to deep and rapid burial followed to Cenomanian age; (2) cross-bedded, cavern- by rapid uplift, before a normal geothermal ous-weathering, massive sandstone of probable gradient could be established. Evidence in the early Paleocene age (Martinez? Stage); (3) Rice Valley-Middle Mountain area suggests polished-pebble conglomerate, rich in Fran- that Franciscan rocks, structurally buried to ciscan detritus; (4) quartz-rich gritty sandstone a depth of at least 12 km, were uplifted and similar to the Lower Lake beds of the late eroded during the time from 65 to 60 m.y. Paleocene (Meganos Stage); (5) fossiliferous, ago. The time restrictions thus would imply a calcareous sandstone containing more than 30 rapid uplift and denudation rate of 240 cm per genera of shallow marine invertebrates, in- 1,000 yrs, comparable with some of the most cluding Turritella meganosensis, diagnostic of rapid rates known, but only slightly greater the Meganos Stage; and (6) glauconitic sand- than measured historic denudation rates in stone and siltstone with scattered fossus, the same area. possibly as young as Eocene. The section is tightly folded, with nearly ACKNOWLEDGMENTS vertical limbs, and has been down-faulted I am indebted to Foyle Mason, of Upper against serpentinized peridotite. The synclinal Lake, California, whose observation led folding, together with the graben structure, directly to the recognit ion of the Rice Valley has preserved this 1-sq-mi segment of the outlier. Thanks are also due the Wilson B. Great Valley sequence and younger rocks Goddard family for their hospitality at Rice 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 Valley ranch. My thanks go also to W. R. Soc. America, Abs. with Programs for 1971, Evitt, Stanford University, for identification v. 3, no. 2, p. 81-82. of palynomorphs, and to W. O. Addicott and • 1972a, Franciscan seamount records final D. L. Jones, U.S. Geological Survey, for phase of Kula Plate subduction in northern identifying Paleogene and Cretaceous mega- California: Geol. Soc. America, Abs. with Programs for 1972 (Cordilleran Sec.), v. 4, fossils. no. 3, p. 127. I thank M. C. Blake, Jr., U.S. Geological 1972b, Paleogene "frozen" subduction zone Survey, W. R. Dickinson and B. M. Page, in the Coast Ranges of northern California: Stanford University, and L. A. Raymond, Internat. Geol. Cong., 24th, Tectonics, Appalachian State University, for their careful Montreal, Sec. 3, p. 99-105. reviews of earlier drafts of this paper. To the Berkland, J. O., Raymond, L. A., Kramer, J. C., latter goes my special appreciation for many Moores, E. M., and O'Day, Michael, 1972, stimulating discussions regarding the Rice What is Franciscan?: Am. Assoc. Petroleum Valley outlier and related subjects. Geologists Bull., v. 56, no. 12, p. 2295-2302. Bezore, S. 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