Geological Society of America Centennial Field Guide—Cordilleran Section, 1987

Paleocene submarine-canyon fill, Point Lobos, CaliforniaH. Edward Clifton, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025Gary 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 PointLobos State Reserve. Geology modified from Nili-Esfahani (1965). Location names as shown in PointLobos State Reserve literature.

LOCATION SIGNIFICANCE

Point Lobos, a prominent headland at the southern side ofCarmel Bay on the central California coast (Fig. 1), is the site of apopular state reserve. Entrance to this reserve is from California1, 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 CarmelValley Road (County Road G16). Within the reserve, pavedroads and well-maintained foot trails provide excellent access tomany of the more prominent exposures (Fig. 1). Outcrops notserved by foot trails are off-limits to the public; however, thegeologically important exposures described herein are readilyaccessible.

Point Lobos State Reserve is beautifully maintained in apristine condition by its staff, and the rules are strictly enforced.Most important, from a geologic standpoint, are strictures againstcollecting or disturbing any natural object within the reserve, sogeological hammers are best left in vehicles. The rocks of thereserve are a striking esthetic resource and a mecca for amateurand professional photographers—they are not to be defaced.

The reserve opens in the morning (typically at 9:00) andcloses before sundown. A nominal entrance fee is charged tovisitors.

The rocks at Point Lobos provide a beautifully exposedexample of a filled part of a Paleocene submarine canyon carvedinto granodiorite of Cretaceus age. The fill, part of the PaleoceneCarmelo Formation of Bowen (1965), consists mostly of pebble-cobble conglomerate. Interbeds of sandstone are common withinthe 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 ofthe sedimentary processes that operated in such a setting, includ-ing the mechanisms of emplacement of the sand and gravel, thechange of facies owing to fluctuation of sediment supply, thelaterally shifting channel systems, and the trace fossils left by aprolific, exotic fauna.

SITE INFORMATION

The oldest rock exposed at Point Lobos is coarsely crystal-line granodiorite. Radiometric dates indicate that this plutonicrock crystallized slightly more than 100 my. ago during theCretaceus (Mattinson, 1978). Large, crudely aligned orthoclasecrystals attest to a slow crystallization, probably under directedstresses.

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240 H. E. Clifton and G. W. Hill

Figure 2. Paleocene conglomerate at Punta de los Lobos Marines, PointLobos State Reserve. Note abutting of Paleocene strata against the lightercolored granodiorite in left background at the northern wall of the postu-lated submarine canyon.

Depositionally overlying the granodiorite, in a roughly east-west belt across the central part of the reserve, are conglomeraticsedimentary rocks-part of the Carmelo Formation of Bowen(1965). The few fossils that have been found in these rocks indi-cate a Paleocene age (Nili-Esfahani, 1965). The northern contactwith the underlying plutonic rocks can be seen at close hand oneither side of Granite Point (Fig. 1) and from a distance in itsexposure east of Punta de los Lobos Marines (Fig. 1). The coastalexposure of the contact in the southern part of the reserve isobscured by overburden; the orientation of the contact relative tostratification in the conglomerate here suggests a fault. An iso-lated sliver of the Carmelo Formation is downfaulted into thegranodiorite at Gibson Beach near the southern boundary of thereserve (Fig. 1).

The depositional contact is steep, relative to stratification inthe Carmelo Formation (Fig. 2); it is broadly sinuous (Fig. 1),and appears to have a relief of at least tens (and possiblyhundreds) of meters. Pebble imbrication and ripple lamination inthe sedimentary rocks indicate paleocurrents parallel to the gran-odiorite walls. The Carmelo Formation thus appears to till a largevalley cut into the plutonic rocks. The cove east of Granite Point(Fig. 1) contains a horizontal section across the valley floor,which slopes to the southwest.

The Carmelo Formation appears to represent the fill of asubmarine canyon and probably accumulated in the middle toupper reaches of the canyon system. A gastropod found in mud-stone interbedded with the conglomerate in the cove east of Gran-ite Point suggests deposition in water depths greater than 330 to660 ft (100 to 200 m) (Clifton, 1981), and the abundant sedi-mentary structures in the sandstone of the Carmelo Formationare devoid of evidence of surface wave effects. A well-developedtrace-fossil assemblage suggests that deposition occurred at depthsof 660 to 5,000 ft (200 to 1,500 m) (Hill, 1981).

The sediment in the canyon fill consists largely of conglom-erate. Sandstone occurs as matrix to most of the conglomerate

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

and as interbeds within the conglomerate and, in the upper part ofthe succession at Point Lobos, within a few fining-upward se-quences that culminate in interbedded mudstone and thin beds ofsandstone. Mudstone is largely restricted to these finer intervals orto pebbly mudstone studded with dispersed pebbles and cobbles.Mudstone breccia occurs sporadically throughout the fill.

The clasts in the conglomerate are mostly well-roundedpebbles and cobbles of resistant composition, such as siliceousvolcanic rocks (Nili-Esfahani, 1965). These clasts probably attestto a long and complicated depositional history in which deposi-tion in a Paleocene submarine canyon is but the latest phase.Clasts composed of granodiorite are relatively uncommon; theytend to be anomalously large (as much as 10 ft-3 m—across)relative to other clasts and commonly are more angular. Thesand-sized sediment in the fill, in contrast, tends to be feldspathicand reflective of a granodioritic source (Nili-Esfanhani, 1965).

Two fundamentally different types of conglomerate occur atPoint Lobos. Disorganized conglomerate shows no internal strati-fication or alignment of clasts; the clasts in such conglomeratemay be supported by one another, or they maybe dispersed in asandy or muddy matrix (pebbly mudstone). Such conglomerate isreadily interpreted as the result of subaqueous debris flows wherethe dispersal of the clasts during transport is largely or totally dueto the strength of the matrix material. Organized conglomerate, incontrast, is stratified and/or shows an alignment of clasts (longaxes horizontal or imbricate); typically this conglomerate is clastsupported. Its origins require mechanisms of transport and depo-sition other than simple debris flow.

The arrangement of clasts in many of the organized con-glomerate beds (best seen where these beds are isolated insandstone, but also discernible within rock composed entirely ofconglomerate) suggests the nature of some of these mechanisms.A typical organized conglomeratic sedimentation unit forms alense traceable for meters to a few tens of meters parallel to thetransport direction and for a few meters transverse to the trans-port direction; the unit is some decimeters thick (Clifton, 1984).Well-defined textural variations exist in many of the organizedconglomerate units, particularly where they are encased in sand-stone (Clifton, 1984). Clasts in the conglomerate grade from tineto coarse upward within the bed (inverse grading) and in a down-transport direction toward the front of the deposit (Fig. 3). Theclasts in the conglomerate commonly are aligned with long axesparallel to flow direction and inclined (imbricated) in the up-transport direction. Inversely and laterally graded conglomeraticsedimentation units are particularly evident on Punta de los

Submarine-canyon fill, Point Lobos, California 241

Lobos Marines and on the point just south of Weston Beach(Fig. 1).

Conglomerate units that are encased in sandstone typicallyhave sharp, well-defined bases and poorly defined tops. The over-lying sandstone generally grades directly downward into thematrix of the conglomerate and thus appears to form the upperpart of a sandstone-conglomerate couplet. Most sandstone bedsshow an upward-fining textural trend (normal grading). Many ofthe sandstone interbeds contain parallel lamination, in contrast tothe conglomerate beds, which show no internal stratification. Thesandstone member of a couplet commonly can be traced somemeters in a down-transport direction beyond the terminus of theassociated subjacent conglomerate. Such sandstone typically con-tains large isolated pebbles or cobbles apparently derived fromthe front, coarse “nose” of the conglomerate. Many of these clastsare aligned with long axes normal to transport and are imbri-cated. A few sandstone beds contain units about 4 in (10 cm)thick of foresets that dip steeply in a down-transport direction (asindicated by other paleocurrent indicators).

The inverse grading and flow-parallel, long-axis alignmentwithin the conglomerate suggest transport just prior to depositionin a concentrated flow where intergranular collision among thepebbles and cobbles contributes to their dispersal as it moves.Pure “grain flow” (see Middleton and Hampton, 1976) of thistype is deemed an inefficient mechanism for transport over longdistances (Lowe, 1976), and accordingly the collisional sortinginto inversely graded beds probably reflects either the last phaseof transport from a high-density turbidity current or a “tractioncarpet” driven by an associated mass-flow of sand. The generalabsence of normal grading in the conglomerate suggests that tur-bulent sorting is uncommon and that the coarse clasts were rarelycarried by turbulence in high-density turbidity currents. Suchturbulence may have been a major factor, however, in themovement of the associated sand flows, the deposits of whichtypically are normally graded, and these flows may have carriedthe pebbles and cobbles in a colliding mass at the base of the flow.The sand flows apparently remained mobile after the gravel com-ponent “froze” into a deposit, rolling isolated coarse clasts a shortdistance from the down-transport nose of the conglomeraticdeposit.

The floor of the canyon during intervals of predominantlygravelly sedimentation probably had an internal relief of 3.3 to6.6 ft (1 to 2 m) produced by channeling and the small lobes leftby individual gravel-bearing flows. The presence of down-canyondirected high-angle cross-bedding in associated sand beds impliesthat the down-canyon slope of the floor was no more than a fewdegrees.

Slump structures and penecontemporaneous reformationalfeatures are common within the fill. Some of these, such as thelarge-scale reformational features exposed in the cove in thewestern side of Granite Point (Fig. 1), may reflect lateral slump-ing from the walls of the canyon. Other features may be due tosmaller-scale lateral slumping from the sides of small gullies orchannels on the floor of the canyon. Two excellent examples of

small rotational slumps of interbedded sandstone and mudstonelie within erosional recesses between steeply dipping resistantbeds of sandstone just northwest of the base of the rock stairwaythat descends southward from the point on the southeastern sideof Sand Hill Cove. These slumps, which locally cause nearly ameter of intercalated sandstone and mudstone strata to standvertically between the overlying and underlying beds, clearly oc-curred at the sea floor and not interstratally. The sand and mud atthe tops of the slumps were differentially eroded prior to (orduring) emplacement of the overlying beds.

The Paleocene rocks at Point Lobos present an assemblageof trace fossils that is particularly prominent in successions ofinterbedded sandstone and mudstone. The faunal traces are bestdisplayed in the exposure at Weston Beach, but they are presentin the finer deposits throughout the Carmelo Formation at PointLobos. A detailed description of the traces, which include amongothers Planolites, Ophiomorpha, Chrondrites, Thalassinoides,Arenicolites, and Scolicia, is presented by Hill (1981). One of themost striking traces is a complicated burrow that was previouslyidentified as the imprint of seaweed in the rocks (Herold, 1934;Nili-Esfahani, 1965). The nature of the burrow (Fig. 4) differsdepending on the arrangement of sandstone and mud interbedsrelative to its main tunnel, and the trace accordingly is manifestedin a surprising variety of ways depending on the location andorientation of exposure relative to the main tunnel (Hill, 1981).The nature of the burrowing organism is unknown; the trace hasnot been described elsewhere from the rock record.

Characteristics of the trace-fossil assemblage, such as taxo-nomic composition, diversity, abundance, behavioral and preser-vational types, and general bioturbation patterns, are useful insubdividing 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 oc-curred 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 theupper part of the Paleocene strata exposed on the western shore-line of the reserve. Where complete, these show an upward pro-gression from conglomerate through pebbly, then nonpebbly,sandstone into mudstone with thin sandstone interbeds. Some, asat “The Slot” midway between Sand Hill Cove and WestonBeach, are abruptly overlain by a thick succession ofconglomerate.

The best developed fining-upward sequence occurs at Wes-ton Beach; it appears to be the stratigraphically highest part of theCarmelo Formation in its seacoast exposure at Point Lobos. Thesection at Weston Beach (Figs. 5 and 6) is more than 100 ft(30 m) thick and is exposed on either flank of a faultedasymmetric syncline that plunges to the west. The rocks gradeprogressively 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 ofbroadly lenticular beds of sandstone generally decimeters thick.

242 H. E. Clifton and G. W. Hill

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

Submarine-canyon flll, Point Lobos, California 243

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

Figure 6. Sedimentary sequence as measured on the north side of Wes-ton Beach showing (a) lithologic succession, (b) thickness and distribu-tion of sandstone beds more than 5 cm thick, and (c) paleocurrentdirection.

Figure 7. Interpretive sketch of the origin of the fining-upward successionat Weston Beach. Note divergence of flow where turbidity current spillsover the levee and nature of vertical sequence (A-A) produced as chan-nel migrates laterally during sedimentation.

Most of these beds are visibly graded and show structureless tolaminated (Ta-b) or structureless to laminated to rippled (Ta-b-c)Bouma sequences. A few thin beds of mudstone (or intervals ofthinly interbedded mudstone and sandstone) drape over the sand-stone beds. These finer-grained intervals are remarkably consist-ent in thickness, internal stratification, and trace-fossil assemblageover the extent of their exposure. They appear to represent epi-sodes of relatively slow sedimentation between the flows thatdeposited the thicker beds of sandstone.

Paleocurrents in the thick-bedded sandstone are indicatedby ripple lamination, by ripples on bedding surfaces, and—nearthe base of the section—by pebble imbrication. Flow was con-sistently toward the southwest (Fig, 6), in marked contrast totransport directions to the northwest that prevail in the conglom-erate below the fining-upward sequence and in most of the otherexposures along the western shore of Point Lobos (Fig. 1).

The thick-bedded sandstone is overlain by several meters ofsandstone in which the beds are consistently in the range of 2 to6 in (5 to 15 cm) thick. Most of these beds are graded and showlaminated to rippled (Tb-c) sequences. The strata are relativelyundisturbed by bioturbation. Although the contact between thetwo is not erosional, the thin-bedded sandstones visibly dip moresteeply to the south than do the underlying strata (Fig. 5). Ripplesand ripple lamination indicate paleocurrents that deviate to thesouth by about 30° relative to those in the section below (Fig. 6).

The thin-bedded sandstone grades up into mudstone withnumerous thin beds of sandstone, nearly all of which are less than2 in (5 cm) thick (Fig. 6). The thin sandstone interbeds are gradedand/or ripple-laminated; isolated (“starved”) ripples are com-mon. Bioturbation is intense throughout this facies and in someparts of the section totally disrupts the stratification. Small, red-weathering concretions mark many horizons within the mud-stone. The direction of paleocurrents in this fine-grained rockparallels that in the thick-bedded sandstone in the lower part ofthe section.

The upward-fining succession resembles that which would

244 H. E. Clifton and G. W. Hill

be produced by a northerly shifting channel on the floor of thecanyon (Fig. 7). If so, the thick-bedded sandstone accumulatedon the north-facing channel margin. The lenticularity of the sand-stone beds suggests deposition from flows other than pure turbid-ity currents, perhaps from a form of fluidized flow (Middletonand Hampton, 1976). Deposition of the thin-bedded sand on asouth-facing levee margin (Fig. 7) would explain the divergenceof attitude and paleocurrent direction in these beds relative tothose below. The internal structures suggest a greater influence oftractive currents in these presumed levee deposits. The mudstonefacies that caps the sequence is interpreted as an interchanneldeposit. The thinness of the sandstone beds in this facies and thegeneral intensity of Bioturbation suggest that sand deposition wasdominantly from relatively infrequent flows that exceeded thecapacity of the channel. The direction of the paleocurrents rela-tive to those in the subjacent conglomerate implies that the postu-lated channel wandered sinuously across the floor of thesubmarine canyon.

A slump higher in the canyon may have diverted currentsenough to cut temporarily into the accretionary bank of thechannel at Weston Beach. Beds in the upper part of a thick-bedded sandstone on the northern side of the cove are truncatedby a steep erosional surface several meters high (Fig. 8). Amudstone-clast breccia forms talus at the base of the cut andintertongues with parallel-laminated and ripple-bedded sandaway from the cut. The sand, which shows virtually no bioturba-tion, probably accumulated rapidly from the erosion of thegraded beds at the margin of the cut. Sediment carried by theeroding currents seemingly bypassed this location. A graded andsomewhat bioturbated sandstone bed of irregular thickness thatcaps both the fill and the cut wall represents the first down-canyon flow to be deposited in this site after the episode oferosion into the bank.

It is unclear whether the fining-upward sequences representfacies that persisted through time along the margins of channelsthrough which gravel moved and was deposited or are facies thatoccupied the entire canyon floor during episodes of nondeposi-

REFERENCES

Bowen, O. E., 1965, Stratigraphy, structure, and oil possibilities in Monterey andSalinas Quadrangles, California, in Rennie, E. W., Jr., ed., Symposium ofpapers: Bakersfield, California, Pacific Section, American Association ofPetroleum Geologists, p. 48-69.

Clifton, H. E., 1981, Submarine canyon deposits, Point Lobos, California, inFrizzell, V., cd., Upper Cretaceus and Paleocene turbidites, central Califor-nia Coast: Pacific Section, Society of Economic Paleontologists and Miner-alogists, Guide Book to Field Trip No. 6, p. 79-92.

— , 1984, Sedimentation units in stratified drop-water conglomerate, Paleo-cene submarine canyon fill, Point Lobos, California, in Koster, E. H., andSteele, R. J., eds., Sedimentology of gravels and conglomerates CanadianSociety 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, PointLobos, California, in Frizzell, V., cd., Upper Cretaceus and Paleocene

Figure 8. Top Photograph of channel margin shown between 16 and26 ft (5 and 8 m) in the lithologic column of Figure 6. Bottom. Sketch oflithologic relations at this channel margin. Strata shown in original hori-zontal position.

tion of gravel. The absence of the sequences in the superb expo-sures of much of the section on Punts de los Lobos Marines andthe increasing prevalence of these sequences in the upper part ofthe section suggest that they formed episodically, perhaps duringtemporary high stands of the sea when gravel deposition wassuppressed.

In summary, Point Lobos is a site highly deserving of theattention of anyone interested in sedimentary geology. The re-serve presents a superbly exposed array of unusual rocks in agorgeously scenic setting. It is well worth a half or whole day ofstudy. Bring lots of film.

turbidites, central California Coast: Pacific Section, Society of EconomicPaleontologists and Mineralogists, Guide Book to Field Trip No. 6,p. 93-104.

Lowe, D. R., 1976, Grain flow and grain flow deposits Journal of SedimentaryPetrology, v. 36, p. 188-199.

Mattinson, J. M., 1978, Age, origin, and thermal histories of some plutonic rocksfrom the Salinian block of California Contributions to Mineralogy andPaleontology, v. 67, p. 233-245.

Middleton, G. V., and Hampton, M. A., 1976, Subaqueous sediment transportand deposition by sediment gravity flows, in Stanley, D. J., and Swift,D.J.P., eds., Marine sediment transport and environmental managementNew York, John Wiley and Sons, p. 197-218.

Nili-Esfahani, A., 1965, Investigation of Paleocene strata, Point Lobos, MontereyCounty, California [M.A. thesis]: Los Angeles, University of California,228 p.