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Erratic continental rocks on volcanic seamounts off the US west coast
Marine Geology 246 (2007) 1 – 8 www.elsevier.com/locate/margeo
Erratic continental rocks on volcanic seamounts off the US west coast Jennifer B. Paduan ⁎, David A. Clague, Alicé S. Davis Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States Received 18 May 2007; received in revised form 13 July 2007; accepted 18 July 2007
Abstract Sampling of volcanic seamounts with dredges and the remotely operated vehicle Tiburon recovered erratic rocks in surprising abundance as far as 500km offshore of the US West coast. The erratics usually have continental lithologies and appear to have been weathered in nearshore environments. They are probably transported by kelp holdfasts, drift logs, and pinnipeds to the seamounts, where they accumulate over time. The erratics are concentrated as lag deposits and kept from becoming buried in sediment by currents that sweep the seamounts. The erratics often have thinner manganese-oxide crusts than rocks of the seamounts because they were delivered to the seafloor more recently and manganese-oxide crusts precipitate over time. The thinner crusts make erratics easier to collect. While most of the erratics clearly did not originate by the volcanic processes that formed the seamounts, careful evaluation of some is necessary to distinguish them as erratics. Failure to recognize the presence of erratics may result in unrealistically complex interpretations of regional geology. © 2007 Elsevier B.V. All rights reserved. Keywords: seamount; erratics; pinniped; kelp holdfast; driftwood; remotely operated vehicle
1. Introduction The volcanic seamounts offshore of the western continental US (Fig. 1) offer an ideal setting for the study of erratics, rocks transported from elsewhere. Exploration of these seamounts, first by dredging and then with a remotely operated vehicle (ROV), established their volcanic origin and characteristic basaltic lithologies (Davis and Clague, 2000; Davis et al., 2002). However, sampling also recovered a great number of rocks with a wide range of continental lithologies that could not have
⁎ Corresponding author. Tel.: +1 831 775 1729; fax: +1 831 775 1620. E-mail addresses: [email protected] (J.B. Paduan), [email protected] (D.A. Clague), [email protected] (A.S. Davis). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.07.007
originated through the volcanic processes that formed the seamounts. These rocks are recognized to be erratics. Continental material is transported offshore primarily by turbidity flows. The debris is deposited as distinctive sequences on continental slopes, submarine fans, and abyssal plains, with coarse materials carried shorter distances than fine particles (Menard, 1964). Erratic rocks have long been associated with rafting by icebergs in high latitudes, and as discarded ballast rock along fishing or shipping routes (i.e., dropstones). Emery (1941, 1955, 1963) and Emery and Tschudy (1941) proposed that rocks found sporadically in the Monterey Formation (fine-grained sedimentary sequences of the mid-latitude California continental margin) were transported attached to kelp holdfasts, tangled in roots of drift logs, or in the guts of pinnipeds. Among the accumulated
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sediments of the continental margin, observations of erratics are relatively rare, yet dredged materials have been used indiscriminately in support of complicated geological interpretations (e.g., Doyle and Gorsline, 1977). High current velocities on seamounts (Eriksen, 1991) and other sites lead to low sediment accumulation rates and more frequent erratics. Lonsdale (1991) mentioned that rounded granitic and metamorphic rocks found at seamounts off the US west coast were likely to be kelp-rafted erratics. We document here the widespread occurrence of erratics with a broad range of lithologies, sizes, and roundness at seamounts as far as 500km offshore. Some resemble in-situ lavas. With the increasing use of submersibles for sampling at seamounts and continental margins, the opportunity to be misled by erratics also increases. 2. Sample locations and methods Erratics were among rocks collected with dredges and the ROV Tiburon. Dredges collect rocks that are larger than the mesh size and that can be plucked from the seafloor; they cannot be selective. The hauls were not exhaustively or quantitatively searched for erratics. The ROV-collected rocks were picked up with the manipulator arm (Fig. 2A), and were selected based on what was large enough for analysis, small enough to fit in the sample drawer, and able to be broken loose from the manganese-encrusted outcrops. The rocks in-situ to the seamounts were the intended targets for collection on these dives, and obviously rounded rocks were deliberately avoided. Despite this discrimination, many erratics were collected on ROV dives between 1998 and 2007 on the seamounts and Patton Escarpment (Table 1). All samples were sawed open and examined as hand specimens. Roundness was estimated visually. Thin sections were made of some of the fine-grained erratics to confirm the lithologies. Basaltic erratics were analyzed by XRF and ICP-MS for major and trace elements. Erratics were collected with the ROV on the Vance, President Jackson, and Taney near-ridge seamounts off northern California and Oregon, on nine seamounts off the central to southern California margin, and on Patton Escarpment off southern California (Fig. 1). Dredge hauls from Davidson Seamount (L2-79-NC-D2), Pioneer Seamount (S5-79-NC-D13), the President Jackson Seamounts (L5-85-NC-31D), and Patton Escarpment (AG69-D3,-D4,-D9) contained erratics (Table 1). The near-ridge seamounts are chains of volcanic cones with mid-ocean ridge basalt (MORB) compositions that erupted sequentially near a mid-ocean ridge
Fig. 1. Locations of erratics collected with the ROV Tiburon (black dots) and selected from dredges (red squares). SJ = San Juan Seamount; SM = San Marcos Seamount; NEB = Northeast Bank; PE = Patton Escarpment, which is the edge of the continental shelf.
axis onto recently-formed oceanic crust (Clague et al., 2000; Davis and Clague, 2000). The California margin seamounts Gumdrop, Guide, Pioneer, Davidson, Rodriguez, San Juan, San Marcos, and Little Joe are NE–SW trending volcanic edifices composed of basaltic lavas ranging from tholeiite to trachyte erupted over millions of years onto much older oceanic crust (Davis et al., 2002); Northeast Bank is of similar age and origin, but erupted onto the continental shelf. These sites are 45.5 to 31.9°N latitude and from 50 to 500km from the nearest shore (Table 1). Patton Escarpment, the only non-seamount site in this study, is the NNW–SSE continental shelf edge off southern California. It is the relict accretionary prism of a Cretaceous subduction zone. Lithologies range from sedimentary to low-grade metamorphic rocks, with some MORB to andesitic volcanics (Marsaglia et al., 2006). 3. Results ROV Tiburon-collected samples include 108 rocks identifiable as erratics, which is 9.4% of the 1149 rocks
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Fig. 2. A) Granodiorite cobble T146-R17 from Davidson Seamount being collected with the manipulator arm of the ROV. B) Surface of conglomerate T666-R37 from Northeast Bank. C) Cross-section of T666-R37; rounded pebbles of sandstone, andesite, and rhyolite and an angular piece of limestone are visible, as well as basalt from the seamount. D) Cross-section of T670-R34, a rounded, bored marble from Rodriguez Seamount. E) Cross-section of T662-R27, a bored limestone from San Juan Seamount. F) T628-R7, a round cobble of andesite with an Mn-oxide patina from Rodriguez Seamount. G) Cross-section of T945-R2, a round cobble of sandstone with a thick Mn-oxide crust, from Davidson Seamount. Scale bars are 5 cm.
collected. Ninety-seven were from the California seamounts; 3 from Patton Escarpment, and 8 from the near-ridge seamounts (Table 1), representing 10.7%, 7.3%, and 2.9% of the total rocks collected on the dives, respectively. Erratics occurred in a random distribution along the dive tracks and with no relationship to depth. From the dredges an additional 17 rocks that had similar characteristics were found, for a total of 125 erratic rocks.
The erratics include an assortment of lithologies (Table 1, Figs. 2 and 3): sandstone, limestone, metamorphic rocks, silicic volcanics, granitic rocks, mudstone, chert, conglomerate, gabbro, and basalt. The basalt (T667-R19) was collected on Patton Escarpment, and the major and trace element composition indicates it is chemically similar to lavas from Rodriguez Seamount. A conglomerate from Northeast Bank (T666R37) contained pebbles of most of the above lithologies
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Table 1 Erratic collection data and lithologies Seamount
Latitude (°N)
Distance from land (km)
Summit depth (m) a
Age of seamount (Ma) b
Tiburon-collected erratics (%) c
Vance Pres. Jackson Taney Gumdrop Pioneer Guide Davidson Rodriguez
45.5 42.5 36.7 37.5 37.4 37 35.7 34
500 300 240 72 76 86 85 54
1475 1370 2163 1210 815 1685 1262 627
0–4 e 0–4 e 26 f 15 f 11 g 16 g 10–17 g, h 9–12 g, h, i
7 (10%) 0 (0%) 1 (2%) 1 (2%) 1 (1%) 2 (4%) 28 (10%) 28 (15%)
San Juan Patton Escarpment
33 32.2– 32.9 32.6 32.3 31.9
109 50–150
562 –
3–18 h 16–30 h
13 (13%) 3 (7%)
177 100 154
1770 j 357 k 2210
7–16 h, i 7–9 h 7i
6 (20%) 4 (6%) 14 (56%)
San Marcos Northeast Bank Little Joe
Dredgecollected erratics 2
1 1
13
Lithologies d
Me, Ss, Gr Gr, Me Ss Ss Gr, Me Gr, Ss Li, Me, Gr, Sv, Ss Li, Ss, Sv, Ch, Me, Co, Ga, Mu Ss, Li, Me, Gr Li, Mu, Sv, Ba, Ch, Me, Ss Ch, Ss, Li, Mu Co, Li, Ss, Sv Ss, Li, Me, Mu, Sv
a
Shallowest point from multibeam sonar surveys. Approximate 40Ar/39Ar radiometric ages, in millions of years. c Number of erratics collected, and percent of erratics/all rocks collected on the dives. d Ba = basalt, Ch = chert, Co = conglomerate, Ga = gabbro, Gr = granite, Li = limestone, Me = metamorphic, Mu = mudstone, Ss = sandstone, Sv = silicic volcanic; listed in descending order of abundance. e Clague et al. (2000). f Davis et al. (1998). g Davis et al. (2002). h Clague, unpublished. i Davis et al. (1995). j Shallowest point on incomplete multibeam coverage. k Shallowest point from single-beam transit crossings. b
in addition to in-situ lavas of the seamount (Fig. 2B, C), and was cemented by clays at the seamount. The erratics range in size from 1cm pebbles in conglomerate samples (Fig. 2B, C), to 34cm long (the
basalt from Patton Escarpment, T667-R19). The heaviest weighs 7.4kg (a limestone from the Taney Seamounts, T120-R33), and four others weigh more than 5kg. Sixty-two of the erratics are rounded to well-rounded (Fig. 2A–D, F, G), whereas only fifteen are angular. Rounded in-situ volcanic rocks were observed only near the summits on three of the seamounts, where they were exposed to wave-action when these volcanoes were islands (Paduan et al., 2004). Thirty-three of the sandstones and limestones are penetrated by borings ranging from 0.25 to 2cm across (Fig. 2D, E). Most of the erratics have manganese–iron oxide (Mn-oxide) coatings that are only a patina or stain (Fig. 2B, D–F). Others have none at all; just 15 rocks have Mn-oxide crusts 1 to 3cm thick (Fig. 2A, G). For comparison, the rocks of the seamounts typically have Mn-oxide crusts to 4cm thick. 4. Discussion
Fig. 3. Histogram of numbers and types of rocks collected by the ROV Tiburon and dredges combined.
Nearly 10% of the rocks collected with the ROV Tiburon, despite efforts to avoid collecting them, are
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erratics that differ from the rocks that formed the volcanoes. It should be emphasized that the sampling technique was selective, so the amount, sizes, roundness, and lithologies of the erratics collected are not necessarily representative of what may be present. 4.1. Characteristics of erratics The sedimentary to felsic lithologies of most of the erratics are similar to rocks usually considered continental in origin, and not typical of the basalt suites of seamounts or oceanic islands. These lithologies are represented onshore in California, Oregon, and Washington: granitic rocks in the Franciscan mélange and Sierra Nevada of California; sandstone, marble, and limestones in central California, Oregon, and Washington coast ranges; chert from the Monterey Formation of central California; gabbro from the Point Sal ophiolite in southern California and elsewhere in the coast ranges.
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The well-rounded character of many of the erratics is also not typical of seamount lavas, which are pillow lavas and massive or sheet flows, often with radial or columnar jointing and glassy, jagged margins. The joints form fractures along which the manipulator can break off the rocks. Cobbles and pebbles become rounded during transport in rivers or on beaches, and very wellrounded pebbles are typical of gravel beaches. The holes riddling some of the rocks appear to be worm or bivalve borings. Some of the erratics have Mn-oxide crusts almost as thick as the in-situ rocks. Mn-oxides precipitate slowly from seawater to coat exposed surfaces over time (e.g., Moore and Clague, 2004). The crust thickness offers a gauge of the time since arrival of each erratic, with the caveat that crusts can be eroded by currents or slope movement. The thicker crust on some erratics suggests that they had been on the ocean floor almost as long as the in-situ rocks, ruling out anthropogenic transport. Most, however, had thin crusts or merely a patina, which suggests that they had not been there as long. Erratics are easier to sample than in-situ volcanic rocks because of the thinner or absent Mn-oxide crusts. Some erratics look deceptively like in-situ rocks as hand specimens, and it is only after careful examination or analysis and comparison with all the other samples that their identity becomes apparent. Examples are the basalt with Rodriguez Seamount chemistry found on Patton Escarpment (T667-R19); T1008-R12 from the Vance Seamounts that resembled an aphyric basalt but thin section and chemical analyses proved to be a finegrained arkosic sandstone; and the oldest Ar–Ar dated rock from Davidson Seamount, L2-79-NC-D2-11, which is a plagioclase–phyric andesite with chemistry that falls well outside the range of all the other lavas of the seamount (Davis et al., 2002). 4.2. Transport mechanisms
Fig. 4. A) Kelp holdfast with attached pebbles on the beach in Moss Landing, California. B) Drift kelp being scavenged by large urchins at 1110 m on Rodriguez Seamount.
The erratics, with continental lithologies, rounded, bored, and variable Mn-oxide crusts, were transported to the volcanic seamounts from near-shore environments. Turbidity flows cannot have deposited these rocks so high above the abyssal plain. Erratics transported by icebergs during glacial maxima may be expected as far south as 45°N in the NE Pacific (Menard, 1964). Anthropogenic transport, as discarded fishing weights, ship ballast or debris, would be expected to be most prevalent near the coast (for example, a large piece of coal, sample T143-R2, was collected near Davidson Seamount). These mechanisms probably did not transport the abundant erratics we
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find so far offshore at these temperate latitudes. Several mechanisms are more plausible: 1. Kelp holdfasts that transported attached rocks as large as 40cm were documented from southern California (Emery, 1941; 1963); these rocks are typically rounded (Fig. 4A). Several species of giant kelp grow near shore along much of the western US, and numerous strands of kelp have been observed during our dives on the same seamounts where some erratics were found (Fig. 4B). 2. Roots of drift logs may transport entangled rocks (Emery, 1955); these rocks are typically angular. Large fallen trees often wash out to sea after winter storms, particularly in northern California, Oregon, and Washington, and pieces of driftwood have been collected from the seafloor off California by the ROV Tiburon. 3. Pinnipeds have been found on California beaches with rocks, often rounded and several cm long, in their stomachs and may transport them to where they forage (Emery, 1941, 1963). They swallow the rocks, possibly for diving ballast, grinding food, or for assuaging hunger pangs while fasting on shore, then disgorge them when they feed or die with them still in their stomachs (Emery, 1941; Taylor, 1993; Bryden, 1999). Pinnipeds haul out on beaches and rocky shores all along the west coast of the US, and migrate offshore hundreds of kilometers (Le Boeuf et al., 2000; Weise et al., 2006). For the seamounts off the US west coast, all of these mechanisms for rock transport are possibilities. Rounded rocks are consistent with kelp and pinniped transport, and angular rocks with driftwood transport. The distribution of erratics on the seamounts is consistent with their arriving randomly at the seafloor. The range in Mn-oxide crust thickness is consistent with sporadic arrival to the seamounts over time, starting when the seamounts formed. The three mechanisms are plausible over the entire history of the seamounts, since the earliest pinnipeds date to 23Ma (Berta et al., 1989), coniferous trees originated about 300Ma (Nadakavukaren and McCracken, 1985), and fossils of brown algae have been found in rocks over 400Ma in age (Chapman and Chapman, 1973). 4.3. Factors influencing distribution of erratics The abundance of erratics collected at the seamounts does not vary uniformly with distance from shore (Table 1). The distribution was probably influenced by
several factors. Drift logs have the potential to carry larger loads, remain afloat longer and travel farther than kelp (Emery, 1955), and some of the largest erratics were collected from the Taney and Vance Seamounts, farthest offshore. Prevailing ocean currents may advect more debris to some locations than others and carry lithologies typical of the shorelines upstream. An example is the basalt found on Patton Escarpment with Rodriguez Seamount chemistry (T667-R19), which may have been plucked from the coast of Rodriguez while it was an island (Paduan et al., 2004) and rafted south with the California Current for 200km. The currents at depth sweep pelagic sediments from the seamounts, which keeps the outcrops and erratics exposed for millions of years and may concentrate erratics in lag deposits, such as at the summit of a cone on Davidson Seamount where 6 of 14 cobbles collected were erratics (T429-R35 to -R40). The currents are unlikely to be so strong that they isolate seamounts from surrounding waters (Codiga and Eriksen, 1997). Sampling bias undoubtedly influenced the distribution of erratics collected. Obviously rounded rocks were deliberately not selected. Gravel-sized rocks rarely were sampled, but when they were, erratics were plentiful (e.g., the conglomerate of rounded pebbles in Fig. 2B, C; and the lag deposit above). More problematic, however, is that in many places erratics were more susceptible to collection due to their thinner Mn-oxide crusts, particularly on deeper or older seamounts completely blanketed by Mn-oxides. At Little Joe Seamount, the deepest, 4cm thick Mn-oxides camouflaged the erratics and made the in-situ rocks difficult to break free, and the greatest percentage of erratics were collected there (Table 1). At the Taney Seamounts, the oldest and also thickly encrusted, the most effective way to collect in-situ rocks was to take talus from the caldera walls, which is not an environment conducive to accumulation of erratics, and we collected few. In contrast, from the neovolcanic axes of the Gorda and Juan de Fuca Ridges, no erratics were among the 507 rocks collected on our dives with the ROV Tiburon. It is easy to sample in-situ rocks there, and erratics have had little time to accumulate on the young seafloor. 5. Conclusions The lithologies and other characteristics of the erratics are distinct from the volcanic rocks of the seamounts, and indicate that they are of distant origin, rounded in near-shore environments, and rafted out to the seamounts where they have accumulated over time. Transport mechanisms previously proposed for
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Fig. 5. World map showing distribution near the coast of pinnipeds (yellow; after King, 1983), kelp (red; Woodborne et al., 1989), and historic forest cover (green; after Bryant et al., 1997).
deposition of erratics in fine-grained sedimentary sequences are probably responsible for bringing erratics to these seamounts: kelp holdfasts, roots of drift logs, and pinnipeds. Winnowing of fine-grained sediments and lag deposition concentrate erratics on the seamounts. The generally thinner Mn-oxide crusts make erratics easier to collect by dredging or ROV: nearly 10% of our ROV-collected rocks are erratics, even though they were discriminated against. Transport of erratics is likely to occur wherever giant kelp, forests, or pinnipeds are found along the coast, which includes much of the ocean basin perimeter and many islands (Fig. 5), and is not restricted to high latitudes. In addition to the California coast, kelp transport has been documented off South Africa (Woodborne et al., 1989) and Macquarie Island, Southern Ocean (Smith and Bayliss-Smith, 1998), and pinniped swallowing of stones was reported at Macquarie Island (Bryden, 1999). These transport mechanisms have potentially been active for much of the age of the extant ocean basins. For seamounts older than 23 Ma, pinnipeds would not have been a mechanism, but gastroliths have been found in fossils of marine dinosaurs and crocodiles (summarized in Taylor, 1993), and teeth of a 110 Ma crocodilian were found on Allison Guyot in the mid-Pacific (Firth et al., 2006). The widespread occurrence of the transport mechanisms implies that erratics need to be considered throughout the oceans and as part of the geologic record for the entire history of the extant ocean basins. The tendencies for erratics to be concentrated on seamounts and more easily sampled than Mn-oxide-cemented insitu rocks make it likely that they will be present in
collections. Careful evaluation of many samples sometimes is necessary to distinguish the erratics among them. Failing to recognize erratics and including their lithologies in regional geologic reconstructions will confuse interpretations and lead to incorrect geologic complexity. Acknowledgements We thank the Captain and crew of the R/V Western Flyer, the Chief and pilots of the ROV Tiburon, and Warren Smith of the Scripps Geological Collection. We thank two anonymous reviewers for their helpful comments. This work was funded by the David and Lucile Packard Foundation. References Berta, A., Ray, C.E., Weiss, A.R., 1989. Skeleton of the oldest known pinniped, Enaliarctos mealsi. Science 244, 60–62. Bryant, D., Nielsen, D., Tangley, L., 1997. The last frontier forests: ecosystems and economies on the edge. World Resources Institute, Washington, DC. 57 pp. Bryden, M.M., 1999. Stones in the stomachs of southern elephant seals. Mar. Mamm. Sci. 15 (4), 1370–1373. Clague, D.A., Reynolds, J.R., Davis, A.S., 2000. Near-ridge seamount chains in the northeastern Pacific Ocean. J. Geophys. Res. 105 (B7), 16,541–16,561. Chapman, V.J., Chapman, D.J., 1973. The Algae, second ed. MacMillan Press, New York. 497 pp. Codiga, D.L., Eriksen, C.C., 1997. Observations of low-frequency circulation and amplified subinertial tidal currents at Cobb Seamount. J. Geophys. Res. 102 (C10), 22,993–23,008. Davis, A.S., Clague, D.A., 2000. President Jackson Seamounts, northern Gorda Ridge: tectonomagmatic relationship between on- and off-axis volcanism. J. Geophys. Res. 105 (B12), 27,939–27,956.
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Davis, A.S., Gunn, S.H., Bohrson, W.A., Gray, L.-B., Hein, J.R., 1995. Chemically diverse, sporadic volcanism at seamounts offshore southern and Baja California. Geol. Soc. Amer. Bull. 107 (5), 554–570. Davis, A.S., Gray, L.B., Clague, D.A., 1998. Non-hotspot ocean island volcanoes: examples from the central and NE Pacific Ocean. Evolution of ocean island volcanoes. Penrose Conference Series, Galapágos, Ecuador, Geol. Soc. Amer., vol. 27. Davis, A.S., Clague, D.A., Bohrson, W.A., Dalrymple, G.B., Greene, H.G., 2002. Seamounts at the continental margin of California: a different kind of oceanic intra-plate volcanism. Geol. Soc. Amer. Bull. 114 (3), 316–333. Doyle, L.J., Gorsline, D.S., 1977. Marine geology of Baja California continental borderland, Mexico. Am. Assoc. Pet. Geol. Bull. 61, 903–917. Emery, K.O., 1941. Transportation of rock particles by sea-mammals. J. Sediment. Petrol. 11, 92–93. Emery, K.O., 1955. Transportation of rocks by driftwood. J. Sediment. Petrol. 25 (1), 51–57. Emery, K.O., 1963. Organic transportation of marine sediments. In: Hill, M.N. (Ed.), The Sea. The Earth Beneath the Sea, vol. 3. Interscience Pub., New York, pp. 776–793. Emery, K.O., Tschudy, R.H., 1941. Transportation of rock by kelp. Geol. Soc. Amer. Bull. 52, 855–862. Eriksen, C.C., 1991. Observations of amplified flows atop a large seamount. J. Geophys. Res. 96 (C8), 15,227–15,236. Firth, J.V., Yancey, T., Alvarez-Zarikian, C., 2006. An 110 Ma crocodilian-bearing vertebrate assemblage preserved between basalt flows on a mid-Pacific seamount, ODP Site 865, Allison Guyot. Eos, Trans., Am. Geophys. Union 87 (52), V13A–0651. King, J.E., 1983. Seals of the World. Cornell University Press, New York. 240 pp. Le Boeuf, B.J., Crocker, D.E., Costa, D.P., Blackwell, S.B., Webb, P.M., Houser, D.S., 2000. Foraging ecology of northern elephant seals. Ecol. Monogr. 70 (3), 353–382.
Lonsdale, P., 1991. Structural patterns of the Pacific floor offshore of peninsular California. In: Dauphin, J.P., Simoneit, B.T. (Eds.), The Gulf and Peninsular Provinces of the Californias. Amer. Assoc. of Petroleum Geol. Mem., vol. 47, pp. 87–125. Marsaglia, K.M., Davis, A.S., Rimkus, K., Clague, D.A., 2006. Evidence for interaction of a spreading ridge with the outer California borderland. Mar. Geol. 229, 259–272. Menard, H.W., 1964. Marine Geology of the Pacific. McGraw-Hill, San Francisco. 271 pp. Moore, J.M., Clague, D.A., 2004. Hawaiian submarine manganese– iron oxide crusts — a dating tool? Geol. Soc. Amer. Bull. 116, 337–347. Nadakavukaren, M., McCracken, D., 1985. Botany: an Introduction to Plant Biology. West Publ. Co., San Francisco. 591 pp. Paduan, J.B., Clague, D.A., Davis, A.S., 2004. Evidence that three seamounts off southern California were ancient islands. Eos, Trans., Am. Geophys. Union 85 (47), V43E–1463. Smith, J.M.B., Bayliss-Smith, T.P., 1998. Kelp-plucking: coastal erosion facilitated by bull-kelp Durvillaea antarctica at subantarctic Macquarie Island. Antarct. Sci. 10 (4), 431–438. Taylor, M.A., 1993. Stomach stones for feeding or buoyancy? The occurrence and function of gastroliths in marine tetropods. Philos. Trans. R. Soc. Lond., Ser. B 341 (1296), 163–175. Weise, M.J., Costa, D.P., Kudela, R.M., 2006. Movement and diving behavior of male California sea lion (Zalophus californianus) during anomalous oceanographic conditions of 2005 compared to those of 2004. Geophys. Res. Lett. 33, L22S10. doi:10.1029/ 2006GL027113. Woodborne, M.W., Rogers, J., Jarman, N., 1989. The geological significance of kelp-rafted rock along the west coast of South Africa. Geo Mar. Lett. 9, 109–118.
Erratic continental rocks on volcanic seamounts off the US west coast Jennifer B. Paduan ⁎, David A. Clague, Alicé S. Davis Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, United States Received 18 May 2007; received in revised form 13 July 2007; accepted 18 July 2007
Abstract Sampling of volcanic seamounts with dredges and the remotely operated vehicle Tiburon recovered erratic rocks in surprising abundance as far as 500km offshore of the US West coast. The erratics usually have continental lithologies and appear to have been weathered in nearshore environments. They are probably transported by kelp holdfasts, drift logs, and pinnipeds to the seamounts, where they accumulate over time. The erratics are concentrated as lag deposits and kept from becoming buried in sediment by currents that sweep the seamounts. The erratics often have thinner manganese-oxide crusts than rocks of the seamounts because they were delivered to the seafloor more recently and manganese-oxide crusts precipitate over time. The thinner crusts make erratics easier to collect. While most of the erratics clearly did not originate by the volcanic processes that formed the seamounts, careful evaluation of some is necessary to distinguish them as erratics. Failure to recognize the presence of erratics may result in unrealistically complex interpretations of regional geology. © 2007 Elsevier B.V. All rights reserved. Keywords: seamount; erratics; pinniped; kelp holdfast; driftwood; remotely operated vehicle
1. Introduction The volcanic seamounts offshore of the western continental US (Fig. 1) offer an ideal setting for the study of erratics, rocks transported from elsewhere. Exploration of these seamounts, first by dredging and then with a remotely operated vehicle (ROV), established their volcanic origin and characteristic basaltic lithologies (Davis and Clague, 2000; Davis et al., 2002). However, sampling also recovered a great number of rocks with a wide range of continental lithologies that could not have
⁎ Corresponding author. Tel.: +1 831 775 1729; fax: +1 831 775 1620. E-mail addresses: [email protected] (J.B. Paduan), [email protected] (D.A. Clague), [email protected] (A.S. Davis). 0025-3227/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2007.07.007
originated through the volcanic processes that formed the seamounts. These rocks are recognized to be erratics. Continental material is transported offshore primarily by turbidity flows. The debris is deposited as distinctive sequences on continental slopes, submarine fans, and abyssal plains, with coarse materials carried shorter distances than fine particles (Menard, 1964). Erratic rocks have long been associated with rafting by icebergs in high latitudes, and as discarded ballast rock along fishing or shipping routes (i.e., dropstones). Emery (1941, 1955, 1963) and Emery and Tschudy (1941) proposed that rocks found sporadically in the Monterey Formation (fine-grained sedimentary sequences of the mid-latitude California continental margin) were transported attached to kelp holdfasts, tangled in roots of drift logs, or in the guts of pinnipeds. Among the accumulated
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sediments of the continental margin, observations of erratics are relatively rare, yet dredged materials have been used indiscriminately in support of complicated geological interpretations (e.g., Doyle and Gorsline, 1977). High current velocities on seamounts (Eriksen, 1991) and other sites lead to low sediment accumulation rates and more frequent erratics. Lonsdale (1991) mentioned that rounded granitic and metamorphic rocks found at seamounts off the US west coast were likely to be kelp-rafted erratics. We document here the widespread occurrence of erratics with a broad range of lithologies, sizes, and roundness at seamounts as far as 500km offshore. Some resemble in-situ lavas. With the increasing use of submersibles for sampling at seamounts and continental margins, the opportunity to be misled by erratics also increases. 2. Sample locations and methods Erratics were among rocks collected with dredges and the ROV Tiburon. Dredges collect rocks that are larger than the mesh size and that can be plucked from the seafloor; they cannot be selective. The hauls were not exhaustively or quantitatively searched for erratics. The ROV-collected rocks were picked up with the manipulator arm (Fig. 2A), and were selected based on what was large enough for analysis, small enough to fit in the sample drawer, and able to be broken loose from the manganese-encrusted outcrops. The rocks in-situ to the seamounts were the intended targets for collection on these dives, and obviously rounded rocks were deliberately avoided. Despite this discrimination, many erratics were collected on ROV dives between 1998 and 2007 on the seamounts and Patton Escarpment (Table 1). All samples were sawed open and examined as hand specimens. Roundness was estimated visually. Thin sections were made of some of the fine-grained erratics to confirm the lithologies. Basaltic erratics were analyzed by XRF and ICP-MS for major and trace elements. Erratics were collected with the ROV on the Vance, President Jackson, and Taney near-ridge seamounts off northern California and Oregon, on nine seamounts off the central to southern California margin, and on Patton Escarpment off southern California (Fig. 1). Dredge hauls from Davidson Seamount (L2-79-NC-D2), Pioneer Seamount (S5-79-NC-D13), the President Jackson Seamounts (L5-85-NC-31D), and Patton Escarpment (AG69-D3,-D4,-D9) contained erratics (Table 1). The near-ridge seamounts are chains of volcanic cones with mid-ocean ridge basalt (MORB) compositions that erupted sequentially near a mid-ocean ridge
Fig. 1. Locations of erratics collected with the ROV Tiburon (black dots) and selected from dredges (red squares). SJ = San Juan Seamount; SM = San Marcos Seamount; NEB = Northeast Bank; PE = Patton Escarpment, which is the edge of the continental shelf.
axis onto recently-formed oceanic crust (Clague et al., 2000; Davis and Clague, 2000). The California margin seamounts Gumdrop, Guide, Pioneer, Davidson, Rodriguez, San Juan, San Marcos, and Little Joe are NE–SW trending volcanic edifices composed of basaltic lavas ranging from tholeiite to trachyte erupted over millions of years onto much older oceanic crust (Davis et al., 2002); Northeast Bank is of similar age and origin, but erupted onto the continental shelf. These sites are 45.5 to 31.9°N latitude and from 50 to 500km from the nearest shore (Table 1). Patton Escarpment, the only non-seamount site in this study, is the NNW–SSE continental shelf edge off southern California. It is the relict accretionary prism of a Cretaceous subduction zone. Lithologies range from sedimentary to low-grade metamorphic rocks, with some MORB to andesitic volcanics (Marsaglia et al., 2006). 3. Results ROV Tiburon-collected samples include 108 rocks identifiable as erratics, which is 9.4% of the 1149 rocks
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Fig. 2. A) Granodiorite cobble T146-R17 from Davidson Seamount being collected with the manipulator arm of the ROV. B) Surface of conglomerate T666-R37 from Northeast Bank. C) Cross-section of T666-R37; rounded pebbles of sandstone, andesite, and rhyolite and an angular piece of limestone are visible, as well as basalt from the seamount. D) Cross-section of T670-R34, a rounded, bored marble from Rodriguez Seamount. E) Cross-section of T662-R27, a bored limestone from San Juan Seamount. F) T628-R7, a round cobble of andesite with an Mn-oxide patina from Rodriguez Seamount. G) Cross-section of T945-R2, a round cobble of sandstone with a thick Mn-oxide crust, from Davidson Seamount. Scale bars are 5 cm.
collected. Ninety-seven were from the California seamounts; 3 from Patton Escarpment, and 8 from the near-ridge seamounts (Table 1), representing 10.7%, 7.3%, and 2.9% of the total rocks collected on the dives, respectively. Erratics occurred in a random distribution along the dive tracks and with no relationship to depth. From the dredges an additional 17 rocks that had similar characteristics were found, for a total of 125 erratic rocks.
The erratics include an assortment of lithologies (Table 1, Figs. 2 and 3): sandstone, limestone, metamorphic rocks, silicic volcanics, granitic rocks, mudstone, chert, conglomerate, gabbro, and basalt. The basalt (T667-R19) was collected on Patton Escarpment, and the major and trace element composition indicates it is chemically similar to lavas from Rodriguez Seamount. A conglomerate from Northeast Bank (T666R37) contained pebbles of most of the above lithologies
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Table 1 Erratic collection data and lithologies Seamount
Latitude (°N)
Distance from land (km)
Summit depth (m) a
Age of seamount (Ma) b
Tiburon-collected erratics (%) c
Vance Pres. Jackson Taney Gumdrop Pioneer Guide Davidson Rodriguez
45.5 42.5 36.7 37.5 37.4 37 35.7 34
500 300 240 72 76 86 85 54
1475 1370 2163 1210 815 1685 1262 627
0–4 e 0–4 e 26 f 15 f 11 g 16 g 10–17 g, h 9–12 g, h, i
7 (10%) 0 (0%) 1 (2%) 1 (2%) 1 (1%) 2 (4%) 28 (10%) 28 (15%)
San Juan Patton Escarpment
33 32.2– 32.9 32.6 32.3 31.9
109 50–150
562 –
3–18 h 16–30 h
13 (13%) 3 (7%)
177 100 154
1770 j 357 k 2210
7–16 h, i 7–9 h 7i
6 (20%) 4 (6%) 14 (56%)
San Marcos Northeast Bank Little Joe
Dredgecollected erratics 2
1 1
13
Lithologies d
Me, Ss, Gr Gr, Me Ss Ss Gr, Me Gr, Ss Li, Me, Gr, Sv, Ss Li, Ss, Sv, Ch, Me, Co, Ga, Mu Ss, Li, Me, Gr Li, Mu, Sv, Ba, Ch, Me, Ss Ch, Ss, Li, Mu Co, Li, Ss, Sv Ss, Li, Me, Mu, Sv
a
Shallowest point from multibeam sonar surveys. Approximate 40Ar/39Ar radiometric ages, in millions of years. c Number of erratics collected, and percent of erratics/all rocks collected on the dives. d Ba = basalt, Ch = chert, Co = conglomerate, Ga = gabbro, Gr = granite, Li = limestone, Me = metamorphic, Mu = mudstone, Ss = sandstone, Sv = silicic volcanic; listed in descending order of abundance. e Clague et al. (2000). f Davis et al. (1998). g Davis et al. (2002). h Clague, unpublished. i Davis et al. (1995). j Shallowest point on incomplete multibeam coverage. k Shallowest point from single-beam transit crossings. b
in addition to in-situ lavas of the seamount (Fig. 2B, C), and was cemented by clays at the seamount. The erratics range in size from 1cm pebbles in conglomerate samples (Fig. 2B, C), to 34cm long (the
basalt from Patton Escarpment, T667-R19). The heaviest weighs 7.4kg (a limestone from the Taney Seamounts, T120-R33), and four others weigh more than 5kg. Sixty-two of the erratics are rounded to well-rounded (Fig. 2A–D, F, G), whereas only fifteen are angular. Rounded in-situ volcanic rocks were observed only near the summits on three of the seamounts, where they were exposed to wave-action when these volcanoes were islands (Paduan et al., 2004). Thirty-three of the sandstones and limestones are penetrated by borings ranging from 0.25 to 2cm across (Fig. 2D, E). Most of the erratics have manganese–iron oxide (Mn-oxide) coatings that are only a patina or stain (Fig. 2B, D–F). Others have none at all; just 15 rocks have Mn-oxide crusts 1 to 3cm thick (Fig. 2A, G). For comparison, the rocks of the seamounts typically have Mn-oxide crusts to 4cm thick. 4. Discussion
Fig. 3. Histogram of numbers and types of rocks collected by the ROV Tiburon and dredges combined.
Nearly 10% of the rocks collected with the ROV Tiburon, despite efforts to avoid collecting them, are
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erratics that differ from the rocks that formed the volcanoes. It should be emphasized that the sampling technique was selective, so the amount, sizes, roundness, and lithologies of the erratics collected are not necessarily representative of what may be present. 4.1. Characteristics of erratics The sedimentary to felsic lithologies of most of the erratics are similar to rocks usually considered continental in origin, and not typical of the basalt suites of seamounts or oceanic islands. These lithologies are represented onshore in California, Oregon, and Washington: granitic rocks in the Franciscan mélange and Sierra Nevada of California; sandstone, marble, and limestones in central California, Oregon, and Washington coast ranges; chert from the Monterey Formation of central California; gabbro from the Point Sal ophiolite in southern California and elsewhere in the coast ranges.
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The well-rounded character of many of the erratics is also not typical of seamount lavas, which are pillow lavas and massive or sheet flows, often with radial or columnar jointing and glassy, jagged margins. The joints form fractures along which the manipulator can break off the rocks. Cobbles and pebbles become rounded during transport in rivers or on beaches, and very wellrounded pebbles are typical of gravel beaches. The holes riddling some of the rocks appear to be worm or bivalve borings. Some of the erratics have Mn-oxide crusts almost as thick as the in-situ rocks. Mn-oxides precipitate slowly from seawater to coat exposed surfaces over time (e.g., Moore and Clague, 2004). The crust thickness offers a gauge of the time since arrival of each erratic, with the caveat that crusts can be eroded by currents or slope movement. The thicker crust on some erratics suggests that they had been on the ocean floor almost as long as the in-situ rocks, ruling out anthropogenic transport. Most, however, had thin crusts or merely a patina, which suggests that they had not been there as long. Erratics are easier to sample than in-situ volcanic rocks because of the thinner or absent Mn-oxide crusts. Some erratics look deceptively like in-situ rocks as hand specimens, and it is only after careful examination or analysis and comparison with all the other samples that their identity becomes apparent. Examples are the basalt with Rodriguez Seamount chemistry found on Patton Escarpment (T667-R19); T1008-R12 from the Vance Seamounts that resembled an aphyric basalt but thin section and chemical analyses proved to be a finegrained arkosic sandstone; and the oldest Ar–Ar dated rock from Davidson Seamount, L2-79-NC-D2-11, which is a plagioclase–phyric andesite with chemistry that falls well outside the range of all the other lavas of the seamount (Davis et al., 2002). 4.2. Transport mechanisms
Fig. 4. A) Kelp holdfast with attached pebbles on the beach in Moss Landing, California. B) Drift kelp being scavenged by large urchins at 1110 m on Rodriguez Seamount.
The erratics, with continental lithologies, rounded, bored, and variable Mn-oxide crusts, were transported to the volcanic seamounts from near-shore environments. Turbidity flows cannot have deposited these rocks so high above the abyssal plain. Erratics transported by icebergs during glacial maxima may be expected as far south as 45°N in the NE Pacific (Menard, 1964). Anthropogenic transport, as discarded fishing weights, ship ballast or debris, would be expected to be most prevalent near the coast (for example, a large piece of coal, sample T143-R2, was collected near Davidson Seamount). These mechanisms probably did not transport the abundant erratics we
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find so far offshore at these temperate latitudes. Several mechanisms are more plausible: 1. Kelp holdfasts that transported attached rocks as large as 40cm were documented from southern California (Emery, 1941; 1963); these rocks are typically rounded (Fig. 4A). Several species of giant kelp grow near shore along much of the western US, and numerous strands of kelp have been observed during our dives on the same seamounts where some erratics were found (Fig. 4B). 2. Roots of drift logs may transport entangled rocks (Emery, 1955); these rocks are typically angular. Large fallen trees often wash out to sea after winter storms, particularly in northern California, Oregon, and Washington, and pieces of driftwood have been collected from the seafloor off California by the ROV Tiburon. 3. Pinnipeds have been found on California beaches with rocks, often rounded and several cm long, in their stomachs and may transport them to where they forage (Emery, 1941, 1963). They swallow the rocks, possibly for diving ballast, grinding food, or for assuaging hunger pangs while fasting on shore, then disgorge them when they feed or die with them still in their stomachs (Emery, 1941; Taylor, 1993; Bryden, 1999). Pinnipeds haul out on beaches and rocky shores all along the west coast of the US, and migrate offshore hundreds of kilometers (Le Boeuf et al., 2000; Weise et al., 2006). For the seamounts off the US west coast, all of these mechanisms for rock transport are possibilities. Rounded rocks are consistent with kelp and pinniped transport, and angular rocks with driftwood transport. The distribution of erratics on the seamounts is consistent with their arriving randomly at the seafloor. The range in Mn-oxide crust thickness is consistent with sporadic arrival to the seamounts over time, starting when the seamounts formed. The three mechanisms are plausible over the entire history of the seamounts, since the earliest pinnipeds date to 23Ma (Berta et al., 1989), coniferous trees originated about 300Ma (Nadakavukaren and McCracken, 1985), and fossils of brown algae have been found in rocks over 400Ma in age (Chapman and Chapman, 1973). 4.3. Factors influencing distribution of erratics The abundance of erratics collected at the seamounts does not vary uniformly with distance from shore (Table 1). The distribution was probably influenced by
several factors. Drift logs have the potential to carry larger loads, remain afloat longer and travel farther than kelp (Emery, 1955), and some of the largest erratics were collected from the Taney and Vance Seamounts, farthest offshore. Prevailing ocean currents may advect more debris to some locations than others and carry lithologies typical of the shorelines upstream. An example is the basalt found on Patton Escarpment with Rodriguez Seamount chemistry (T667-R19), which may have been plucked from the coast of Rodriguez while it was an island (Paduan et al., 2004) and rafted south with the California Current for 200km. The currents at depth sweep pelagic sediments from the seamounts, which keeps the outcrops and erratics exposed for millions of years and may concentrate erratics in lag deposits, such as at the summit of a cone on Davidson Seamount where 6 of 14 cobbles collected were erratics (T429-R35 to -R40). The currents are unlikely to be so strong that they isolate seamounts from surrounding waters (Codiga and Eriksen, 1997). Sampling bias undoubtedly influenced the distribution of erratics collected. Obviously rounded rocks were deliberately not selected. Gravel-sized rocks rarely were sampled, but when they were, erratics were plentiful (e.g., the conglomerate of rounded pebbles in Fig. 2B, C; and the lag deposit above). More problematic, however, is that in many places erratics were more susceptible to collection due to their thinner Mn-oxide crusts, particularly on deeper or older seamounts completely blanketed by Mn-oxides. At Little Joe Seamount, the deepest, 4cm thick Mn-oxides camouflaged the erratics and made the in-situ rocks difficult to break free, and the greatest percentage of erratics were collected there (Table 1). At the Taney Seamounts, the oldest and also thickly encrusted, the most effective way to collect in-situ rocks was to take talus from the caldera walls, which is not an environment conducive to accumulation of erratics, and we collected few. In contrast, from the neovolcanic axes of the Gorda and Juan de Fuca Ridges, no erratics were among the 507 rocks collected on our dives with the ROV Tiburon. It is easy to sample in-situ rocks there, and erratics have had little time to accumulate on the young seafloor. 5. Conclusions The lithologies and other characteristics of the erratics are distinct from the volcanic rocks of the seamounts, and indicate that they are of distant origin, rounded in near-shore environments, and rafted out to the seamounts where they have accumulated over time. Transport mechanisms previously proposed for
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Fig. 5. World map showing distribution near the coast of pinnipeds (yellow; after King, 1983), kelp (red; Woodborne et al., 1989), and historic forest cover (green; after Bryant et al., 1997).
deposition of erratics in fine-grained sedimentary sequences are probably responsible for bringing erratics to these seamounts: kelp holdfasts, roots of drift logs, and pinnipeds. Winnowing of fine-grained sediments and lag deposition concentrate erratics on the seamounts. The generally thinner Mn-oxide crusts make erratics easier to collect by dredging or ROV: nearly 10% of our ROV-collected rocks are erratics, even though they were discriminated against. Transport of erratics is likely to occur wherever giant kelp, forests, or pinnipeds are found along the coast, which includes much of the ocean basin perimeter and many islands (Fig. 5), and is not restricted to high latitudes. In addition to the California coast, kelp transport has been documented off South Africa (Woodborne et al., 1989) and Macquarie Island, Southern Ocean (Smith and Bayliss-Smith, 1998), and pinniped swallowing of stones was reported at Macquarie Island (Bryden, 1999). These transport mechanisms have potentially been active for much of the age of the extant ocean basins. For seamounts older than 23 Ma, pinnipeds would not have been a mechanism, but gastroliths have been found in fossils of marine dinosaurs and crocodiles (summarized in Taylor, 1993), and teeth of a 110 Ma crocodilian were found on Allison Guyot in the mid-Pacific (Firth et al., 2006). The widespread occurrence of the transport mechanisms implies that erratics need to be considered throughout the oceans and as part of the geologic record for the entire history of the extant ocean basins. The tendencies for erratics to be concentrated on seamounts and more easily sampled than Mn-oxide-cemented insitu rocks make it likely that they will be present in
collections. Careful evaluation of many samples sometimes is necessary to distinguish the erratics among them. Failing to recognize erratics and including their lithologies in regional geologic reconstructions will confuse interpretations and lead to incorrect geologic complexity. Acknowledgements We thank the Captain and crew of the R/V Western Flyer, the Chief and pilots of the ROV Tiburon, and Warren Smith of the Scripps Geological Collection. We thank two anonymous reviewers for their helpful comments. This work was funded by the David and Lucile Packard Foundation. References Berta, A., Ray, C.E., Weiss, A.R., 1989. Skeleton of the oldest known pinniped, Enaliarctos mealsi. Science 244, 60–62. Bryant, D., Nielsen, D., Tangley, L., 1997. The last frontier forests: ecosystems and economies on the edge. World Resources Institute, Washington, DC. 57 pp. Bryden, M.M., 1999. Stones in the stomachs of southern elephant seals. Mar. Mamm. Sci. 15 (4), 1370–1373. Clague, D.A., Reynolds, J.R., Davis, A.S., 2000. Near-ridge seamount chains in the northeastern Pacific Ocean. J. Geophys. Res. 105 (B7), 16,541–16,561. Chapman, V.J., Chapman, D.J., 1973. The Algae, second ed. MacMillan Press, New York. 497 pp. Codiga, D.L., Eriksen, C.C., 1997. Observations of low-frequency circulation and amplified subinertial tidal currents at Cobb Seamount. J. Geophys. Res. 102 (C10), 22,993–23,008. Davis, A.S., Clague, D.A., 2000. President Jackson Seamounts, northern Gorda Ridge: tectonomagmatic relationship between on- and off-axis volcanism. J. Geophys. Res. 105 (B12), 27,939–27,956.
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