protected from contamination by terri- genous rna terial. As these enclosed basins filled with deposits, the reducing en- vironment gradually ameliorated, and.
Franciscan Limestones and Their Environments of Deposition ABSTRACT Detailed study of the distribution, petrography, and microfossils of the Calera and Laytonville Franciscan limestones allows us to describe their most probable areas of deposition. The Calera Limestone Member was deposited in shallow submerged basins situated o n a ridge. Coccolith ooze and rarer radicr larian limestone were deposited on basin floors which were generally below the lysodint:. Planktonic foraminifera deposited above the lysocline on basin walls were sporadically swept down into the basins as turbidity currents and, because of this rapid deposition , were preserved. The ridge along which the basins were located may have been located behind an island arc of Mesozoic age. The Laytonville limestone was deposited in deep water, possibly in the arc-trench gap, or as abyssal plain deposits that were subsequently subducted or accreted to the continent.
Daniel Wachs Earth Sciences Board, University of California Santa Cruz, California 95064
James R. Hein U.S. Geological Survey, Office of Marine Geology Menlo Park , California 94025
N • LAYTONVlLLE
l
INTRODUCTION This paper considers the possibilities of depositional environments and mechanisms for Franciscan limestone. The limestone can be divided into two 'major groups, the Calera Limestone Member (Lawson, 1902, 19 14) and the Laytonville limestone (Bailey and others, 1964). Limestone in the Franciscan assemblage occurs as isolated bodies associated with graywacke, shale, and greenstone (Fig. 1). Planktonic foraminifera and radiolarians are common in both limestone groups as are discontinuous interbeds and lenses of chert. Foraminiferal species in the Calera Member indicate an early Cenomanian age (Wachs, 1973), and the Laytonville limestone may have formed during Cenomanian to Turonian time (Bailey and others, 196 4). The fine-grained nature of the limestone matrix and the association of limestone, chert, and volcanic rocks led Darrow (1963) and Bailey and others (1964) to suggest that the limestone was chemically precipitated. Later, Garrison and Bailey ( 1967) discovered that the limestone matrix is composed partly of nannofossil fragments and little if any clastic detritus. They interpreted the limestone as a pelagic deposit and, having noted the close association with volcanic rock, suggested that the limes tone was GEO LOGY
0
10miles
El FRANCISCAN FORMATION
e •
PELAGIC
SHALLOW WATER
} CALERA LIMESTONE
Figure 1. Map showing location of Calera Limestone Member along San Andreas fault in central California. Inset map (uppet: right corner) shows three major locations of Laytonville limestone. Inset (right-middle) is simplified columnar section of Calera Member as exposed in Permanente Quarry_ Hollister (mentioned in text) is just off lower right-hand corner of map.
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Figure 2. Photograph of approximately 90 m of Calera Member as exposed in Permanente Quarry. Broken line marks transition between lower black limestone and upper white limestone; dark zone near top of section is diabase sill (see Fig. l inset). Thin dark horizon below sill is volcanic ash.
30
JANUARY 1975
deposited on the tops of seamounts above the area of clastic sedimentation. We have suggested (Wachs and Hein, 1974) that the micrite (limestone matrix) was originally comp osed of at least 50 percent calcareous nannofossils and that recrystallization altered or destroyed much of this matrix material. The important parameters that aid in distinguishing the environments and mechanisms of Franciscan limestone deposition of the Franciscan assemblage (discussed in detail below) include the shape and distribution of the limestone bodies, the nature of the bedding, the degree of contamination by terrigenous or volcanic detritus, the associations with shallow-water limestone and pillow basalts, and the microfossil distributions. Significantly, these limestones are not affected by regional metamorphism (Wachs and Hein, 1974 ). CALERA LIMESTONE MEMBER The Calera Member is exposed as a discontinuous belt from Rockaway Beach southward to near Hollister (see Fig. I). The thin, discontinuous belt lies immediately east of the San Andreas fault system, which, in thi s area, forms the western boundary of Franciscan rocks. The most complete stratigraphic sec tion ( ISO m thick and several hundred meters wide) is exposed at Permanente Quarry (Fig. I ). Ceopetal texture and graded bedding clearly demonstrate (for the first time) that the section is not overturned (Wachs and Hein, 1974). The limestone bodies, including the largest body at Permanente Quarry, are usually tabular in nature, whe ther in the range of meters or tens of meters thick. Walker (I 950) believed that the limestone masses are fau lt slivers. In contrast, Darrow (I 963) interpreted the discontinuous outcrops as an expression of a primary deposition pattern on a topographically varying sea floor. In all areas limes tone bedding is not disrupted, regardless of whet her the outcrop is 1 or I 00 m in thickness. It is unlikely that the Calera Member has been involved in chaotic transportation from an original upslope site of deposition, because bedding is well preserved in all outcrops (especially in Permanente Quarry, Fig. 2) regardless of size (thickness and areal extent), and because the sections are not overturned. Chaotic transportation would be most easily recognized by definition of the con tact relations between masses GEOLOGY
of limestone of the Calera Member and the surrounding Franciscan rocks. However, most contacts are highly weathered, faulted, or not exposed, and further work is required for a better understanding. Raymond (1974) pointed out that pelagic deposits in the Mount Oso area of the Franciscan assemblage display primary depositional relations. The oldest part of the Calera Limestone Member is bituminous (the lower gjack Calera Member of Wachs and Hein, nn4) and grades upward into the upper white Calera limestone. The upward decrease in organic matter probably indica tes a change from a highly reducing environment to a slightly reducing one. All of the Calera Member examined is pure limestone devoid of even minor amounts of terrigenous and volcanic detritus. Examination of the Calera Member (see Wachs and Hein, 1974) reveals that, in addition to planktonic foraminifera and radiolarian molds, benthic foraminifera are present. Benthic microfossils are found only in the micritic limestone, which typically lacks planktonic foraminifera. Furthermore, planktonic foraminifera form limestone with grainsupported textures that, in places, is grainstone composed solely of planktonic foraminiferal and echin oderm fragme nts. The grainstone beds that are found in sharp contact with the micritic limestone and in places are graded are interpreted as bioturbidites. Radiolarians are found only in limest one that has a micritesupported texture and are never mixed with planktonic foraminifera. Again, sharp upper and lower contacts mark the change from micritic limestone to radiolarian micritic limestone. Shallow-water limestone containing oolites, pellets, and fragments of coral, echinoderms, mollusks, and coralline algae occurs as part of the Calera Member. Megafossils, however, are rare. Also associated with the Calera Member is vesicular pillow basalt, an eruptive rock that was emplaced in water only several hundred meters deep (Matthews and Wachs, 1973). LAYTONVILLE LIMESTONE The red Laytonville li mestone is found north of San Francisco (Fig. I) and occurs as thin, discontinuous blocks. The most northerly exposures are in the Scotia quadrangle (Bailey and others, 1964 ). Distribution of irregular Layton-
ville limestone masses are more scattered, smaller, and less abundan t than those of the Calera Member to the south. The Laytonville limestone is associated with shale and greenstone masses and appears as tec tonic inclusions in shale. Similar "knockers" of blueschist facies rocks occur within several meters of the limestone inclusions. The red limestone occurs, less commonly, as matrix between pillows of nonvesicular basalt. Like the Calera Member, the Laytonville limestone is free of volcanic de tritus. In contrast to the Calera, no benthic foraminifera, shallow-water limestone, or nea rby vesicular pillow basalts are found in association with most Laytonville limestone outcrops. Planktonic foraminifera typically make up I 0 to 30 percent of the rock (much higher values than commonly found in the Calera Member); however, the texture is always micrite supported. Radiolarian molds are fou nd only in chert lenses. DISCUSSION Depositional Environment of Calera. We infer that the lower black limestone was deposited in isolated, enclosed basins with limited water circulation. The basins were probably perched basins on a ridge protected from contamination by terrigenous rna terial. As these enclosed basins filled with deposits, the reducing environment gradually ameliorated, and white rather than black fetid limestone accumulated. Because the basin floors were below the lysocline, only the more solution-resistant nannofossils accumulated here. However, planktonic foraminifera were preserved in turbidite shed from the upper walls of the basin. These upper walls and surrounding areas must have been above the lysocline, a circumstance that allowed preservation of pelagically deposited foraminiferal as well as skeletal debris of shallow-water organisms. The dissolution of planktonic foraminifera deposited pelagically on the basin floors allowed the less abundant benthic foraminifera (which are less susceptible to solution) to be selectively preserved in the micritic limestone. Radiolarian "blooms" appeared from time to time in the Late Cretaceous seas, and their resistant silica tests are preserved in radiolarian micritic limestones. Radiolarian mudstone of Cenozoic age occurs interbedded with nannoplankton ooze in the North Atlantic (Peterson and others, 1970) and the South Atlan tic 31
(Maxwell and others, 1970) and similarly may be attributed to radiolarian blooms stimulated by enrichment of the sea water with silica possibly supplied by submarine volcanism. Because all the planktonic foraminifera are selectively dissolved, even a small radiolarian bloom would be sufficient to form a pelagic sediment rich with radiolarians. Nannoplankton are more stable than planktonic foraminifera (Tracey and others, 1971 ; Berger, 1972) and are preserved despite their slow rate of pelagic deposition. Berger (1970) demonstrated that the depth of the lysocline can vary as much as several kilometers, but it typically lies at a depth of 3 to 4 km. The greatest variation occurs near continents. Matthews and Wachs (1973) showed that the Calera Limestone Member was deposited in several hundred meters of water. Evidence for this contention is its association with pillow basalt that has a vesicularity indicative of shallowwater emplacement. It is quite possible that the lysocline was much shallower during parts of the Late Cretaceous Period. This condition may have occurred on a local or regional scale. Benson and others ( 1970) documented vertical fluctuations of the lysocline in the Atlantic Ocean that occurred during Cretaceous and early Tertiary time. They suggested that the lysocline climbed well into the photic zone near the end of the Cretaceous Period. Scholl and Creager (1973) identified a high level for the calcite compensation depth in the North Pacific during late Maestrichtian and Paleocene times. In any case, the limestone was deposited in irregular basins on the flanks of a ridge, in shallow water, and apparently beneath a local or regional lysocline. Although shallow-water fossils are absent in the micrite, they occur in calcareous turbidite interbedded with the micritic limestone and in nearby shallowwater limestone (see Wachs and Hein, 1974) outcrops. These associations with shallow-water fossils coupled with the presence of vesicular pillow lava rocks suggest a shallow-water depositional environment for the Calera Member as a whole. Location of the Depositional Basins. The Calera Limestone Member was deposited in relatively shallow stagnant basins. It is possible that these basins were atop seamounts (for example, as calderas) located on an abyssal plain (for example, see Garrison and Bailey, 1967). However, oxygen-poor bottom-
32
water conditions are most apt to occur on island arcs or on oceanic ridges, because these features have a greater probability for topographic enclosures. Moreover, to explain the linear belt of Franciscan limestone as part of a volcanic seamount chain, each volcano would necessarily be topped with a caldera capable of sustaining stagnant bottomwater conditions. We believe that in light of the available data, such a locale for limestone deposition is not the most likely. A second possible depositional site for the Calera Limestone Member is an active ocean ridge. No high-pressure metamorphism is recognized in the Calera Member (Wachs and Hein, 1974), indica ting that it was never subducted (that is, thrust deeply under the continental margin). However, the limestone bodies could conceivably have been at an elevated position on a ridge and thereby would have been "saved" from subduction and subsequent deep burial in an active trench. A modern analog for deposition of limestone on a ridge is given by van Andel and Komar ( 1969). Numerous valleys or ponds on the flanks of the Mid-Atlantic Ridge are partly filled with pelagic sediment, and two of these basins contain carbonate turbidite that was " ... derived from calcareous pelagic deposits which mantle the surrounding hills. Adjacent valleys are separated by divides so each valley contains an independent sedimentation unit" (p. 1163). The thickness of the sediment in the basins ranges from 100 to 500 m; the Calera Member is approximately 200m thick (Wachs, 1973). There is no independent evidence that supports the existence of a volcanic ridge (whether a spreading center or not) just seaward of a Franciscan trench of Mesozoic age or that such a ridge directly interacted with the North American plate during or near Cenomanian time. One would certainly expect large amounts of volcanic debris admixed with limestone formed on a volcanically active ridge. The Calera Limestone Member is pure, and we found it difficult to locate even trace amounts of detrit al quartz grains (Wachs and Hein, 1974). In contrast, study of limestone on Guam (southern end of the Mariana island arc) revealed a large variation in th~: mixture of volcanic and carbonate material. Compositions range from slightly calcareous tuff to rather pure reefal limestone, but still with some volcanic debris (Schlanger, 1964; Garrison and others, in prep.).
All the characteristics of the Calera Limestone Member discussed in this paper and in Wachs and Hein (1974), such as the lack of detritus in the limestone bodies, seem to fit most closely an environment similar to Karig's (1971) third arc (West Mariana Ridge), a submerged remnant arc broken by active faults but lacking volcanism. The West Mariana Ridge is mantled by foraminiferal ooze, coralline-algal colonies, and coral, echinoderm, and pelecepod fragments. A similar ridge has been hypothesized to explain Franciscan stratigraphy in the northern Coast Ranges, although unequivocal outcrops of the ridge are not known (Blake and Jones, 1974; Ross and others, 1973). Karig ( 1971, p. 336) showed that the West Mariana Ridge has subsided more than 1 km since cessation of volcanism in Pliocene time. A combination of erosion, subsidence, and depositional burial could easily account for the disappearance of the "Franciscan Ridge." A subsiding ridge allows growth of reefs or limestone deposition on a limited scale uncontaminated by volcanic debris before submergence of the ridge and consequent termination of reef development (this could occur on any initially hot and subsequently cooling ridge). The small amount of shallow-water limestone in the Franciscan assemblage may be explained in a similar fashion. Of course, coccolith-foraminifera deposition would continue during subsidence. An interesting aspect of the Calera Limestone Member is that it forms an elongate belt parallel to and east of the San Andreas fault system. It is quite possible, therefore, that the Calera Member was deposited in basins on the flanks of a ridge located behind the island arc of Mesozoic age and that the ridge may have been associated with an ancestral San Andreas fault of Cretaceous age (Garfunkel, 1973; Silver and others, 197 1). Similar ridges and associated strikeslip faults in back-arc areas have been discussed by many workers (for example, Karig, 1971; Karig and others, 1973; van Andel and others, 1973; and Crowell, 1972). Such a fault ridge need not have been associated with interarc basin spreading or ex tension. Depositional Environment of Laytonville. The depositional environment of the Laytonville limestone is difficult to infer because masses of the limestone are only preserved as tectonic blocks. The micrite- (nannofossil-) supported texture, the slow rate of sedimentation JANUARY 1975
(Wachs, 1973), the absence of graded bedding or any other sedimentary structures, and the mixing of two sizes of planktonic foraminifera (Wachs and Hein, 1974) suggest that the Laytonville limestone is a biogenic pelagic deposit. The depth of water in which this red limestone formed is not as clearly readable as is that for the Calera Limestone Member. However, the absence of shallowwater benthic foraminifera and vesicular pillow basalt and the pelagic nature of the limestone indicate deposition at some distance from shallow water. The lack of detrital material and bioturbidites in the limestone places the area of deposition beyond the influence of volcanic activity and turbidity currents. Foraminiferal productivity in the surface waters, a factor that in itself could have greatly lowered the local or regional lysocline and promoted the preservation of planktonic foraminifera, may have been high, as indicated by the large percentage of these organisms in the rock. Alternatively, the lysocline probably developed at a greater water depth than in the case of the Calera Member, a circumstance that would preserve the foraminifera deposited in deep water. CONCLUSIONS
It is now clear that the two Franciscan limestones are of a very different nature. The nannofossil matrix is the only textural aspect they have in common. The Calera Limestone Member was probably deposited in shallow water on a linear ridge in a back arc region , whereas the Laytonville limestone was deposited in relatively deep water in the arc-trench gap or on the abyssal sea floor. Presumably the Laytonville limestone masses are tectonic blocks (phacoids) in a melange, the formation of which has so overshadowed recent thought concerning Franciscan-like deposits. However, the Calera Member is not as easily accounted for by a subduction zone-trench model. Scholl and Marlow (I 974) believed that many if not most of the circum-Pacific Franciscan-like deposits, commonly explained as trench deposits, are probably not formed in the trench environment at all, but rather they originated and evolved behind the active arc or in the arc-trench gap. The distribution of rock types in the Franciscan assemblage leads one to conclude that there are major distinct units within this assemblage (Berkland and others, I 972). As demonstrated by RayGEOLOGY
mond (1974), Blake and Jones (I 974), and Wachs and Hein (I 974), all of the assemblage is not a melange. Careful study may reveal additional preserved and continuous units that co uld not have been thrust to any great depth under the continent and that accumulated in areas other than trenches and abyssal plains. REFERENCES CITED Bailey, E. H., Irwin, W. P., and Jones, D . L. , 1964, Franciscan and related rocks, and their significance in the geo logy of western California: California Div. Mines and Geology Bull. 183, 177 p. Be nson , W. E., Gerard, R. D., an d Hay, W. W., 1970, Summary and conclusio ns, Leg 4, Deep Sea Drilling Project, in Bador, R. G., and others, Initial reports of the DSDP, Vo l. 4: Washington, D.C., U.S . Govt. Printing Office, p. 659-673. Berger, W. H., 1970, Plan ktonic fo raminifera: Selective solution an d the lysocline: Marine Geology, v. 8, p. 111-138. - - 1972, Deep sea carbonates: Dissolution facies and age-depth constancy: Nature, v. 236, p. 392-395. Berkland, J. 0 ., Ra y mo nd, L. A., Kramer, J. C., Moores, E. M., and O'Day, M., 1972, What is Franciscan?: Am. Assoc. Petroleum Geologists Bull., v. 56, p. 2295-2302 . Blake, M. C ., Jr. , and Jones, D. L., 1974 , Origin of Franciscan mela nges in northern Ca lifornia : Soc. Eco n. Paleontologists and Mineralogists Spec. Pub. 19, p. 345. Oowell, J . C., 1972, Sedimentation along continental transform fault s: Geo l. Soc. America, Abs. with Programs (Cordilleran Sec.), v. 4, no. 3, p . 141. Darrow, R. L., 1963, Age and structural rela· tio ns hips of the Franciscan Formation in the Montara Mountain quadrangle, San Mateo County, California: California Div. Mines and Geology Spec. R ep t. 7 8, 2 3 p. Garfunkel, Z., 1973, History of the San Andreas fault as a plate bou ndary : Geol. Soc. America Bull., v. 84 , p . 2035-2042 . Garrison, R. E., and Bailey, ·E. H. , 1967, Electron microscopy of limestones in t he Franciscan Formation o f California : U.S. Geol. Survey Prof. Paper 575-B, p . B94-BIOO. Karig, D. E., 197 1, Structur al history of the Mariana Is land Arc system : Geo l. Soc. America Bull., v. 82, p. 323-344. Karig, D. E., Ingle, J. C. , Jr ., Bo uma , A. H., E llis, H., Haile, N., Koizumi , 1., MacGregor, I. D., Moore, J. C., Ujiie, H ., Watanabe, T ., White , S.M., Yasui, M., and Ling, H. Yi., 1973, Origin of the west Philippine basin : Nature, v. 246, p . 458-461 . Lawson, A. ·c., 1902, A geo logical sectio n o f the middle Coast Ranges of Ca lifo rnia: Science, v. I 5, p. 415-41 6. 19 14, Description of the San Francisco district, Tamalpais, San Francisco, Concord, San Mateo, and Ha yward quadrangles: U.S . Geo l. Survey Geol. Atlas, fo lio 193. Matthews, V., and Wachs, D. , 19 73, Mi xed depositional environments in the Franciscan geosynclinal asse mblage : Jour. Sed. Petro logy,v. 43, p. 516-517. Maxwell, A. E., and others, 1970, Initial report s o f the Deep Sea Drilling Project, Vol. 3: Washington, D.C., U.S . Govt. Printing Office, 806 p. PA INTED I N U.S A
Peterson, M.N.A., and others, 1970, Initial r eports of the Deep Sea Drilling Project, Vo l. 2: Washington, D.C., U.S. Govt. Printing Office, 491 p. Raymond, L.A., 1974, Possible modern analogs f or rocks of the Franciscan Complex, Mount Oso area, California: Geology, v. 2 , p. 143- 146. Ross, D. C., Wentworth, C. M., and McKee, E. H., 1973, Cretaceous mafic conglomerate near Gualala offset 350 miles by San Andreas fault from oceanic crus tal so urce near Eagle Rest Pea k, California: U.S. Geol. Survey Jour. Research, v. I, p. 45-52. Sc hlanger, S. 0., 1964, Petrology of the limestones of Guam: U.S. Geol. Survey Prof. Paper 403-D, 52 p. Scholl, D. W., and Creager, J. S., 1973, Geo logic synthesis of Leg 19 (DSDP) resu lts: Far north Pacific, and Aleutian Ridge, and Bering Sea, in Creager, J. S., Scholl, D. W. , and others, Initial reports of the Deep Sea Drilling Project, Vol. 19 : Washington, D.C., U.S. Govt. Printing Office, p. 897-9 1 3. Scholl, D . W., and Marlow, M. S., 1974, Sedime ntary sequences in modern Pacific tre nches and the deformed Pacific eugeosynclines: Soc. E con . Paleontologists and Mineralogists Spec. Pub . 19, p . 193. Silver, E. A., Curra y, J. R., and Coo per, A. K., 1971 , Tectonic developme nt of the continental margin off central Califo rnia, in Lipps, J. H ., and Moores, E. M., eds., Geo t. Soc. Sacramento Guidebook, Ann. Field Trip, Geologic guide to the northern Coast Ranges, Point Reyes region , California. Tracey, J. 1., and others, 1971, Initial r eports of the Deep Sea Drilling Project, Vol. 8: Washington, D.C., U.S. Govt. Printing Office, 1037 p. van Andel, Tj . H., and Komar , P. D., 1969, Po nded sediments of the Mid-Atlantic Ridge between 22° and 23° north la titude: Geo l. Soc. America Bull., v. 80, p. 116311 90. van A nde l, Tj. H., Rea, D. K., von Heezen, R. P., and Hoskins, H., 1973, Ascension fr acture zo ne , Ascension Island, and the Mid-Atlantic Ridge: Geo l. Soc. America Bull., v . 84, p. 1527-1546. Wachs, D., 1973, Petrology and d epositiona l history of limestones in the Franciscan Formation of California I Ph. D. thesis): Santa Cru z, University of California, Santa Cruz, 99 p. Wachs, D., and Hein , J. R. , 1974, Petrography and diagenesis of Franciscan limesto nes: Jour. Sed. Petrology (in press). Walker , G. W., 1950, The Calera lim estone in San Mateo and Santa C lara Counties , Califo rnia : Ca lifornia Div. Mines Spec. Rept. 1-B, 7 p.
ACKNOWLEDGMENTS Re viewed by D. W. Scholl and M. C. Blake, Jr. We greatly appreciate valuable discussions with D. W. Scholl, M. C. Blake, Jr. , and R. E. Garrison. D. W. Scholl also reviewed an earlier version , and L. A. Raymond provided useful suggestions. We thank D. W. Scholl, M. C. Blake, Jr., and R. E. Garrison for providing us with preprints of their respective articles. MANUSCRIPT RECEIVED JULY 3 1, 1974 MANUSCRIPT ACCEPTED NOV. 4, 1974
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