Paleocene submarine-canyon fill, Point Lobos, California

Geological Society of America Centennial Field Guide—Cordilleran Section, 1987 Paleocene submarine-canyon fill, Point Lobos, California H. Edward Cli...
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Geological Society of America Centennial Field Guide—Cordilleran Section, 1987

Paleocene submarine-canyon fill, Point Lobos, California H. Edward Clifton, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Gary W. Hill, US. Geological Survey, 915 National Center, 12201 Sunrise Valley Drive, Reston, Virginia 22092

Figure 1. Distribution of the Carmelo Formation of Bowen ( 1965) and Cretaceus granodiorite at Point Lobos State Reserve. Geology modified from Nili-Esfahani (1965). Location names as shown in Point Lobos State Reserve literature.

LOCATION

SIGNIFICANCE

Point Lobos, a prominent headland at the southern side of

Carmel Bay on the central California coast (Fig. 1), is the site of a popular state reserve. Entrance to this reserve is from California 1, about 4 mi (6.4 km) south of the village of Carmel and 2.5 mi (4 km) southwest of the intersection of California 1 and Carmel Valley Road (County Road G16). Within the reserve, paved roads and well-maintained foot trails provide excellent access to many of the more prominent exposures (Fig. 1). Outcrops not served by foot trails are off-limits to the public; however, the geologically important exposures described herein are readily accessible. Point Lobos State Reserve is beautifully maintained in a pristine condition by its staff, and the rules are strictly enforced. Most important, from a geologic standpoint, are strictures against collecting or disturbing any natural object within the reserve, so geological hammers are best left in vehicles. The rocks of the reserve are a striking esthetic resource and a mecca for amateur and professional photographers—they are not to be defaced. The reserve opens in the morning (typically at 9:00) and closes before sundown. A nominal entrance fee is charged to visitors.

The rocks at Point Lobos provide a beautifully exposed example of a filled part of a Paleocene submarine canyon carved into granodiorite of Cretaceus age. The fill, part of the Paleocene Carmelo Formation of Bowen (1965), consists mostly of pebblecobble conglomerate. Interbeds of sandstone are common within the conglomerate, and a few intervals (up to 100 ft-30 m— thick) of pebble-free sandstone and mudstone exist within the fill. The rocks afford an excellent opportunity to examine evidence of the sedimentary processes that operated in such a setting, including the mechanisms of emplacement of the sand and gravel, the change of facies owing to fluctuation of sediment supply, the laterally shifting channel systems, and the trace fossils left by a prolific, exotic fauna. SITE INFORMATION

The oldest rock exposed at Point Lobos is coarsely crystalline granodiorite. Radiometric dates indicate that this plutonic rock crystallized slightly more than 100 my. ago during the Cretaceus (Mattinson, 1978). Large, crudely aligned orthoclase crystals attest to a slow crystallization, probably under directed stresses.

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Figure 3. Characteristic vertical and lateral textural variation within an organized conglomerate bed at Point Lobos.

and as interbeds within the conglomerate and, in the upper part of the succession at Point Lobos, within a few fining-upward sequences that culminate in interbedded mudstone and thin beds of sandstone. Mudstone is largely restricted to these finer intervals or Figure 2. Paleocene conglomerate at Punta de los Lobos Marines, Point to pebbly mudstone studded with dispersed pebbles and cobbles. Lobos State Reserve. Note abutting of Paleocene strata against the lighter Mudstone breccia occurs sporadically throughout the fill. colored granodiorite in left background at the northern wall of the postuThe clasts in the conglomerate are mostly well-rounded lated submarine canyon. pebbles and cobbles of resistant composition, such as siliceous volcanic rocks (Nili-Esfahani, 1965). These clasts probably attest to a long and complicated depositional history in which deposiDepositionally overlying the granodiorite, in a roughly east- tion in a Paleocene submarine canyon is but the latest phase. west belt across the central part of the reserve, are conglomeratic Clasts composed of granodiorite are relatively uncommon; they sedimentary rocks-part of the Carmelo Formation of Bowen tend to be anomalously large (as much as 10 ft-3 m—across) (1965). The few fossils that have been found in these rocks indi- relative to other clasts and commonly are more angular. The cate a Paleocene age (Nili-Esfahani, 1965). The northern contact sand-sized sediment in the fill, in contrast, tends to be feldspathic with the underlying plutonic rocks can be seen at close hand on and reflective of a granodioritic source (Nili-Esfanhani, 1965). Two fundamentally different types of conglomerate occur at either side of Granite Point (Fig. 1) and from a distance in its exposure east of Punta de los Lobos Marines (Fig. 1). The coastal Point Lobos. Disorganized conglomerate shows no internal stratiexposure of the contact in the southern part of the reserve is fication or alignment of clasts; the clasts in such conglomerate obscured by overburden; the orientation of the contact relative to may be supported by one another, or they maybe dispersed in a stratification in the conglomerate here suggests a fault. An iso- sandy or muddy matrix (pebbly mudstone). Such conglomerate is lated sliver of the Carmelo Formation is downfaulted into the readily interpreted as the result of subaqueous debris flows where granodiorite at Gibson Beach near the southern boundary of the the dispersal of the clasts during transport is largely or totally due to the strength of the matrix material. Organized conglomerate, in reserve (Fig. 1). The depositional contact is steep, relative to stratification in contrast, is stratified and/or shows an alignment of clasts (long the Carmelo Formation (Fig. 2); it is broadly sinuous (Fig. 1), axes horizontal or imbricate); typically this conglomerate is clast and appears to have a relief of at least tens (and possibly supported. Its origins require mechanisms of transport and depohundreds) of meters. Pebble imbrication and ripple lamination in sition other than simple debris flow. The arrangement of clasts in many of the organized conthe sedimentary rocks indicate paleocurrents parallel to the granglomerate beds (best seen where these beds are isolated in odiorite walls. The Carmelo Formation thus appears to till a large sandstone, but also discernible within rock composed entirely of valley cut into the plutonic rocks. The cove east of Granite Point (Fig. 1) contains a horizontal section across the valley floor, conglomerate) suggests the nature of some of these mechanisms. A typical organized conglomeratic sedimentation unit forms a which slopes to the southwest. The Carmelo Formation appears to represent the fill of a lense traceable for meters to a few tens of meters parallel to the submarine canyon and probably accumulated in the middle to transport direction and for a few meters transverse to the transupper reaches of the canyon system. A gastropod found in mud- port direction; the unit is some decimeters thick (Clifton, 1984). stone interbedded with the conglomerate in the cove east of Gran- Well-defined textural variations exist in many of the organized ite Point suggests deposition in water depths greater than 330 to conglomerate units, particularly where they are encased in sand660 ft (100 to 200 m) (Clifton, 1981), and the abundant sedi- stone (Clifton, 1984). Clasts in the conglomerate grade from tine mentary structures in the sandstone of the Carmelo Formation to coarse upward within the bed (inverse grading) and in a downare devoid of evidence of surface wave effects. A well-developed transport direction toward the front of the deposit (Fig. 3). The trace-fossil assemblage suggests that deposition occurred at depths clasts in the conglomerate commonly are aligned with long axes of 660 to 5,000 ft (200 to 1,500 m) (Hill, 1981). parallel to flow direction and inclined (imbricated) in the upThe sediment in the canyon fill consists largely of conglom- transport direction. Inversely and laterally graded conglomeratic erate. Sandstone occurs as matrix to most of the conglomerate sedimentation units are particularly evident on Punta de los

Submarine-canyon fill, Point Lobos, California Lobos Marines and on the point just south of Weston Beach (Fig. 1). Conglomerate units that are encased in sandstone typically have sharp, well-defined bases and poorly defined tops. The overlying sandstone generally grades directly downward into the matrix of the conglomerate and thus appears to form the upper part of a sandstone-conglomerate couplet. Most sandstone beds show an upward-fining textural trend (normal grading). Many of the sandstone interbeds contain parallel lamination, in contrast to the conglomerate beds, which show no internal stratification. The sandstone member of a couplet commonly can be traced some meters in a down-transport direction beyond the terminus of the associated subjacent conglomerate. Such sandstone typically contains large isolated pebbles or cobbles apparently derived from the front, coarse “nose” of the conglomerate. Many of these clasts are aligned with long axes normal to transport and are imbricated. A few sandstone beds contain units about 4 in (10 cm) thick of foresets that dip steeply in a down-transport direction (as indicated by other paleocurrent indicators). The inverse grading and flow-parallel, long-axis alignment within the conglomerate suggest transport just prior to deposition in a concentrated flow where intergranular collision among the pebbles and cobbles contributes to their dispersal as it moves. Pure “grain flow” (see Middleton and Hampton, 1976) of this type is deemed an inefficient mechanism for transport over long distances (Lowe, 1976), and accordingly the collisional sorting into inversely graded beds probably reflects either the last phase of transport from a high-density turbidity current or a “traction carpet” driven by an associated mass-flow of sand. The general absence of normal grading in the conglomerate suggests that turbulent sorting is uncommon and that the coarse clasts were rarely carried by turbulence in high-density turbidity currents. Such turbulence may have been a major factor, however, in the movement of the associated sand flows, the deposits of which typically are normally graded, and these flows may have carried the pebbles and cobbles in a colliding mass at the base of the flow. The sand flows apparently remained mobile after the gravel component “froze” into a deposit, rolling isolated coarse clasts a short distance from the down-transport nose of the conglomeratic deposit. The floor of the canyon during intervals of predominantly gravelly sedimentation probably had an internal relief of 3.3 to 6.6 ft (1 to 2 m) produced by channeling and the small lobes left by individual gravel-bearing flows. The presence of down-canyon directed high-angle cross-bedding in associated sand beds implies that the down-canyon slope of the floor was no more than a few degrees. Slump structures and penecontemporaneous reformational features are common within the fill. Some of these, such as the large-scale reformational features exposed in the cove in the western side of Granite Point (Fig. 1), may reflect lateral slumping from the walls of the canyon. Other features may be due to smaller-scale lateral slumping from the sides of small gullies or channels on the floor of the canyon. Two excellent examples of

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small rotational slumps of interbedded sandstone and mudstone lie within erosional recesses between steeply dipping resistant beds of sandstone just northwest of the base of the rock stairway that descends southward from the point on the southeastern side of Sand Hill Cove. These slumps, which locally cause nearly a meter of intercalated sandstone and mudstone strata to stand vertically between the overlying and underlying beds, clearly occurred at the sea floor and not interstratally. The sand and mud at the tops of the slumps were differentially eroded prior to (or during) emplacement of the overlying beds. The Paleocene rocks at Point Lobos present an assemblage of trace fossils that is particularly prominent in successions of interbedded sandstone and mudstone. The faunal traces are best displayed in the exposure at Weston Beach, but they are present in the finer deposits throughout the Carmelo Formation at Point Lobos. A detailed description of the traces, which include among others Planolites, Ophiomorpha, Chrondrites, Thalassinoides, Arenicolites, and Scolicia, is presented by Hill (1981). One of the most striking traces is a complicated burrow that was previously identified as the imprint of seaweed in the rocks (Herold, 1934; Nili-Esfahani, 1965). The nature of the burrow (Fig. 4) differs depending on the arrangement of sandstone and mud interbeds relative to its main tunnel, and the trace accordingly is manifested in a surprising variety of ways depending on the location and orientation of exposure relative to the main tunnel (Hill, 1981). The nature of the burrowing organism is unknown; the trace has not been described elsewhere from the rock record. Characteristics of the trace-fossil assemblage, such as taxonomic composition, diversity, abundance, behavioral and preservational types, and general bioturbation patterns, are useful in subdividing the rocks into specific depositional facies. In addition, the ichnoassemblage represents a mixing of “shallow-water” and “deep-water” types, leading to speculation that deposition occurred in water depths no deeper than upper to mid-bathyal (600-5,000 ft; 200-1,500 m). Distinctive fining-upward successions are present in the upper part of the Paleocene strata exposed on the western shoreline of the reserve. Where complete, these show an upward progression from conglomerate through pebbly, then nonpebbly, sandstone into mudstone with thin sandstone interbeds. Some, as at “The Slot” midway between Sand Hill Cove and Weston Beach, are abruptly overlain by a thick succession of conglomerate. The best developed fining-upward sequence occurs at Weston Beach; it appears to be the stratigraphically highest part of the Carmelo Formation in its seacoast exposure at Point Lobos. The section at Weston Beach (Figs. 5 and 6) is more than 100 ft (30 m) thick and is exposed on either flank of a faulted asymmetric syncline that plunges to the west. The rocks grade progressively upward through conglomerate, pebbly sandstone, thick-bedded nonpebbly sandstone, and thin-bedded sandstone, to mudstone with thin sandstone interbeds (Fig. 6). The lower part of the sandstone section consists mostly of broadly lenticular beds of sandstone generally decimeters thick.

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Figure 4. Trace fossils from the Carmelo Formation of Bowen (1965) at Point Lobos. Left: Schematic diagram showing various manifestations of the complex unnamed trace that is particularly abundant in the sandstone-mudstone succession at Weston Beach. Right: Examples of the different manifestations of this unnamed trace.

Submarine-canyon flll, Point Lobos, California

Figure 5. Fining-upward succession at Weston Beach. Note upwardthinning of sandstone beds.

Figure 6. Sedimentary sequence as measured on the north side of Weston Beach showing (a) lithologic succession, (b) thickness and distribution of sandstone beds more than 5 cm thick, and (c) paleocurrent direction.

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Figure 7. Interpretive sketch of the origin of the fining-upward succession at Weston Beach. Note divergence of flow where turbidity current spills over the levee and nature of vertical sequence (A-A) produced as channel migrates laterally during sedimentation.

Most of these beds are visibly graded and show structureless to laminated (Ta-b) or structureless to laminated to rippled (Ta-b-c ) Bouma sequences. A few thin beds of mudstone (or intervals of thinly interbedded mudstone and sandstone) drape over the sandstone beds. These finer-grained intervals are remarkably consistent in thickness, internal stratification, and trace-fossil assemblage over the extent of their exposure. They appear to represent episodes of relatively slow sedimentation between the flows that deposited the thicker beds of sandstone. Paleocurrents in the thick-bedded sandstone are indicated by ripple lamination, by ripples on bedding surfaces, and—near the base of the section—by pebble imbrication. Flow was consistently toward the southwest (Fig, 6), in marked contrast to transport directions to the northwest that prevail in the conglomerate below the fining-upward sequence and in most of the other exposures along the western shore of Point Lobos (Fig. 1). The thick-bedded sandstone is overlain by several meters of sandstone in which the beds are consistently in the range of 2 to 6 in (5 to 15 cm) thick. Most of these beds are graded and show laminated to rippled (Tb-c) sequences. The strata are relatively undisturbed by bioturbation. Although the contact between the two is not erosional, the thin-bedded sandstones visibly dip more steeply to the south than do the underlying strata (Fig. 5). Ripples and ripple lamination indicate paleocurrents that deviate to the south by about 30° relative to those in the section below (Fig. 6). The thin-bedded sandstone grades up into mudstone with numerous thin beds of sandstone, nearly all of which are less than 2 in (5 cm) thick (Fig. 6). The thin sandstone interbeds are graded and/or ripple-laminated; isolated (“starved”) ripples are common. Bioturbation is intense throughout this facies and in some parts of the section totally disrupts the stratification. Small, redweathering concretions mark many horizons within the mudstone. The direction of paleocurrents in this fine-grained rock parallels that in the thick-bedded sandstone in the lower part of the section. The upward-fining succession resembles that which would

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be produced by a northerly shifting channel on the floor of the canyon (Fig. 7). If so, the thick-bedded sandstone accumulated on the north-facing channel margin. The lenticularity of the sandstone beds suggests deposition from flows other than pure turbidity currents, perhaps from a form of fluidized flow (Middleton and Hampton, 1976). Deposition of the thin-bedded sand on a south-facing levee margin (Fig. 7) would explain the divergence of attitude and paleocurrent direction in these beds relative to those below. The internal structures suggest a greater influence of tractive currents in these presumed levee deposits. The mudstone facies that caps the sequence is interpreted as an interchannel deposit. The thinness of the sandstone beds in this facies and the general intensity of Bioturbation suggest that sand deposition was dominantly from relatively infrequent flows that exceeded the capacity of the channel. The direction of the paleocurrents relative to those in the subjacent conglomerate implies that the postulated channel wandered sinuously across the floor of the submarine canyon. A slump higher in the canyon may have diverted currents enough to cut temporarily into the accretionary bank of the channel at Weston Beach. Beds in the upper part of a thickbedded sandstone on the northern side of the cove are truncated by a steep erosional surface several meters high (Fig. 8). A mudstone-clast breccia forms talus at the base of the cut and intertongues with parallel-laminated and ripple-bedded sand away from the cut. The sand, which shows virtually no bioturbation, probably accumulated rapidly from the erosion of the graded beds at the margin of the cut. Sediment carried by the eroding currents seemingly bypassed this location. A graded and somewhat bioturbated sandstone bed of irregular thickness that caps both the fill and the cut wall represents the first downcanyon flow to be deposited in this site after the episode of erosion into the bank. It is unclear whether the fining-upward sequences represent facies that persisted through time along the margins of channels through which gravel moved and was deposited or are facies that occupied the entire canyon floor during episodes of nondeposi-

Figure 8. Top Photograph of channel margin shown between 16 and 26 ft (5 and 8 m) in the lithologic column of Figure 6. Bottom. Sketch of lithologic relations at this channel margin. Strata shown in original horizontal position.

tion of gravel. The absence of the sequences in the superb exposures of much of the section on Punts de los Lobos Marines and the increasing prevalence of these sequences in the upper part of the section suggest that they formed episodically, perhaps during temporary high stands of the sea when gravel deposition was suppressed. In summary, Point Lobos is a site highly deserving of the attention of anyone interested in sedimentary geology. The reserve presents a superbly exposed array of unusual rocks in a gorgeously scenic setting. It is well worth a half or whole day of study. Bring lots of film.

REFERENCES Bowen, O. E., 1965, Stratigraphy, structure, and oil possibilities in Monterey and Salinas Quadrangles, California, in Rennie, E. W., Jr., ed., Symposium of papers: Bakersfield, California, Pacific Section, American Association of Petroleum Geologists, p. 48-69. Clifton, H. E., 1981, Submarine canyon deposits, Point Lobos, California, in Frizzell, V., cd., Upper Cretaceus and Paleocene turbidites, central California Coast: Pacific Section, Society of Economic Paleontologists and Mineralogists, Guide Book to Field Trip No. 6, p. 79-92. — , 1984, Sedimentation units in stratified drop-water conglomerate, Paleocene submarine canyon fill, Point Lobos, California, in Koster, E. H., and Steele, R. J., eds., Sedimentology of gravels and conglomerates Canadian Society of Petroleum Geologists Memoir 10, p. 429-441. Herold, C. L., 1934, Fossil markings in the Carmelo Series (Upper Cretaceous[?]), Point Lobos, California Journal of Geology, v. 42, p. 630-640. Hill, G. W., 1981, Ichnocoenoses of a Paleocene submarine-canyon floor, Point Lobos, California, in Frizzell, V., cd., Upper Cretaceus and Paleocene

turbidites, central California Coast: Pacific Section, Society of Economic Paleontologists and Mineralogists, Guide Book to Field Trip No. 6, p. 93-104. Lowe, D. R., 1976, Grain flow and grain flow deposits Journal of Sedimentary Petrology, v. 36, p. 188-199. Mattinson, J. M., 1978, Age, origin, and thermal histories of some plutonic rocks from the Salinian block of California Contributions to Mineralogy and Paleontology, v. 67, p. 233-245. Middleton, G. V., and Hampton, M. A., 1976, Subaqueous sediment transport and deposition by sediment gravity flows, in Stanley, D. J., and Swift, D.J.P., eds., Marine sediment transport and environmental management New York, John Wiley and Sons, p. 197-218. Nili-Esfahani, A., 1965, Investigation of Paleocene strata, Point Lobos, Monterey County, California [M.A. thesis]: Los Angeles, University of California, 228 p.