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The Hilt Bed, an Upper Cretaceous compound basin–plain seismoturbidite in the Hornbrook Forearc Basin of southern Oregon and northern California, USA
Sedimentary Geology 135 (2000) 51–63 www.elsevier.nl/locate/sedgeo
The Hilt Bed, an Upper Cretaceous compound basin–plain seismoturbidite in the Hornbrook Forearc Basin of southern Oregon and northern California, USA T.H. Nilsen* Consulting Geologist, 215 Club Drive, San Carlos, CA 94070, USA Received 14 March 1998; accepted 7 July 1999
Abstract The Hilt Bed is a thick, laterally continuous marker bed of fine- to medium-grained sandstone that crops out for a NW–SE along-strike distance of about 50 km on the southwestern flank of the Cretaceous Hornbrook forearc basin of northern California and southern Oregon. The Hilt Bed forms part of the Blue Gulch Mudstone Member of the Hornbrook Formation and is everywhere underlain and overlain by basin–plain mudstone that contains thin interbeds of turbidite sandstone and siltstone. The bed is as thick as 4.71 m in the central part of the outcrop belt and thins progressively to the NW and SE. It was derived from erosion of igneous and metamorphic basement rocks of the Klamath Mountains, which flank the basin to the SW. Paleocurrents measured from flute casts at the base of the bed indicate sediment transport to the NE. The Hilt Bed can be divided into a compound succession of nine Bouma divisions, all of which are generally present in outcrop. The succession consists in ascending stratigraphic order of Bouma abababcde divisions. The basal a division is typically the thickest and coarsest layer, with overlying divisions generally thinner and more variable in thickness. The bed appears to have been deposited as a compound turbidite by at least three separate turbidity currents that appear to have been generated either by the same triggering event or by a rapid succession of related events. Only the uppermost part of the bed thus contains a complete Bouma sequence. The unidirectional paleocurrent indicators, lateral continuity and extent of the three flows that make up the bed, incompleteness of the two lower Bouma sequences, and absence of interbedded shale or bioturbated levels suggest that the three turbidity currents reached the basin floor at almost the same time to yield the bed. The compound nature of the Hilt Bed suggests that a major seismic event triggered the three large flows, which probably originated in either a single submarine canyon or three adjacent canyons. 䉷 2000 Published by Elsevier Science B.V. Keywords: turbidite; seismoturbidite; basin plain; forearc basin; Cretaceous; California and Oregon
1. Introduction The sedimentary deposits of the Cretaceous Hornbrook Formation of southern Oregon and northern California are thought to fill a late Mesozoic forearc * Fax: ⫹1-650-591-1763. E-mail address: [email protected] (T.H. Nilsen).
basin that extended northward into the Mitchell area of central Oregon and southward into the Great Valley of north-central California (Fig. 1). The strata record infill of the sedimentary basin by facies that range in origin from alluvial fan to deep marine. A widespread marker bed, named the Hilt Bed by Nilsen (1983, 1993) for its type area near Hilt, Oregon, is present in the upper part of the stratigraphic succession. The
0037-0738/00/$ - see front matter 䉷 2000 Published by Elsevier Science B.V. PII: S0037-073 8(00)00062-2
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Fig. 1. Regional map showing outcrop locations of the Hornbrook Formation, major physiographic provinces, and adjacent outcrop areas of Cretaceous strata in dark pattern (from Nilsen, 1993).
bed extends in outcrop for at least 50 km along strike and may have been deposited as a seismoturbidite. The bed is unusual in that it appears to consist of three different beds deposited in rapid succession, infilling a large part of the deep-marine basin–plain (Nilsen, 1993). The purpose of this paper is to describe the bed and its regional setting, discuss the mechanisms of its deposition, and compare it with other well-described seismoturbidites in the geological literature.
2. Regional setting Lower and Upper Cretaceous sedimentary strata
cropping out in Siskiyou County, California, and Jackson County, Oregon, were named the Hornbrook Formation by Peck et al. (1956). They measured a composite type section 815 m thick near Hornbrook, California (Fig. 2). Additional stratigraphic and paleontologic data were obtained by Jones (1959), Elliott (1971), McKnight (1971), Nilsen (1984a,b) and others (see summary of previous work in Nilsen, 1993, pp. 5–10). The outcrop distribution of the Hornbrook Formation was defined in geologic maps prepared by Elliott (1971), Hotz (1971, 1977), Beaulieu and Hughes (1977), Nilsen et al. (1983), Wagner and Saucedo (1987), and Nilsen (1993). Along its southwestern flank, the Hornbrook Formation rests unconformably on older Paleozoic,
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Fig. 2. Simplified geologic map of the Hornbrook Formation, showing distribution of major stratigraphic units (from Nilsen, 1993). 53
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Fig. 3. Composite measured section of the Hornbrook Formation showing stratigraphic position of Hilt Bed (from Nilsen, 1993).
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Triassic, and Jurassic accreted terranes and plutons that form the core of the Klamath Mountains geologic province (Irwin, 1966, 1994; Hotz, 1971). The Hornbrook Formation crops out in a series of generally low-lying valleys on the northeastern flank of the Klamath Mountains, where it forms a NW-striking and NE-dipping homoclinal succession (Fig. 2). The outcrops of the Hornbrook Formation are offset by numerous NE-striking cross faults that result in displacement of stratigraphic boundaries and local difficulties in stratigraphic assignment (Nilsen et al., 1983; Nilsen, 1993). On its NE flank, the Hornbrook Formation is overlain with angular unconformity by Tertiary volcanic and sedimentary rocks of the Cascade Range (McKnight, 1971; Vance, 1984; Christiansen and Yeats, 1992). Stratigraphic relations and geophysical data indicate that the Hornbrook Formation extends northeastward in the subsurface beneath the Cascade Range. A single petroleum exploration well drilled through the western margin of the Cascade Range in the Shasta Valley area yielded a thickness of 1405 m (Nilsen, 1993). The lateral continuity of most of its members in outcrop suggests that the Hornbrook Formation originally extended much farther to the north and southeast, and that it was physically connected with coeval strata in the Redding area to the south and the Mitchell area to the north (Fig. 1).
3. Stratigraphic framework The Hornbrook Formation was subdivided by Nilsen (1984c) into a series of members that were subsequently formally defined by Nilsen (1993) in more detail (Figs. 2 and 3). The thickness of the formation varies along strike from published values of 815–1490 m. The series of members generally define an upward-deepening succession typical of an actively subsiding basin. The nonconformable basal contact is locally marked by reddish paleosols, a breccia composed of angular blocks of basement rocks, and conglomeratic grus composed of untransported granitic detritus derived from underlying plutonic rocks. Alluvial-fan and fluvial deposits generally overlie the basal residuum and are in turn abruptly overlain by cross-stratified and fossiliferous nearshore marine sandstone. In most sections, offshore
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hummocky cross-stratified shallow-marine sandstone overlies the nearshore deposits and grades upward into bioturbated and locally fossiliferous marine outer-shelf and slope siltstone and mudstone (Fig. 3). Along most of the outcrop belt, turbidite sandstone and associated conglomerate of the Rocky Gulch Sandstone Member stratigraphically overlie the siltstone and mudstone. This member, which is about 190 m thick, appears to have formed a sandstonerich slope or base-of-slope apron along the NE flank of the Klamath Mountains, where a line source for the sands appears to have characterized the basin margin. The Blue Gulch Mudstone forms the uppermost, thickest, and finest-grained member of the Hornbrook Formation (Figs. 2 and 3). It is at least 850 m thick and consists mostly of a stack of deep-marine mudstone with thin interbeds of siltstone and very fine and finegrained sandstone turbidites. It appears in general to deepen upward into a succession of deep-marine basin–plain turbidites. Although it is relatively uniform in character along the outcrop belt, in the Hornbrook area it contains a lens of hummocky cross-stratified shallow-marine sandstone in its lower part that is about 85 m thick and assigned to the Rancheria Gulch Sandstone Beds. The lens indicates local uplift and shoaling in this part of the basin. The Hilt Bed is a prominent marker bed of sandstone that forms the boundary between the middle and upper parts of the Blue Gulch Mudstone Member in its type section (Fig. 3; Nilsen, 1993). In its type section, the Hilt Bed lies about 458 m above the base of the Blue Gulch Mudstone Member and 388.5 m below the unconformable upper contact with Tertiary strata. The Hilt Bed is a unique thicker and coarser grained bed of sandstone within the middle and upper parts of the Blue Gulch Mudstone Member.
4. The Hilt Bed 4.1. Distribution and thickness The Hilt Bed forms a prominent marker bed within the Blue Gulch Mudstone Member for about 50 km along strike from the Shasta Valley northwestward to the Ashland area (Fig. 2). It is impossible to determine whether the bed extends farther to the NW and SE because, in these areas, the middle and upper parts
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where two excellent roadcuts provide clear and easily observed sections (Fig. 4). The bed can generally easily be traced in outcrop and on aerial photographs to the NW and SE of the type area. Where locally obscured by closely spaced NE-striking cross faults, its continuity may be uncertain. The bed is thickest in the middle part of the outcrop belt near Bailey Hill, where it has a maximum measured thickness of 4.71 m (Fig. 5, section 10). It generally thins progressively to the NW and SE of the Bailey Hill area to 2.69 m and 1.61 m, respectively, at the most distal measured sections. However, it is locally thinner, as in sections 15 and 16 between Hilt and Ashland (Fig. 5). 4.2. Sedimentology
Fig. 4. (A) Type section; and (B) reference section of the Hilt Bed of the Hornbrook Formation (from Nilsen, 1993). (a)–(e) correspond to divisions that Bouma (1962) defined for graded turbidite beds.
of the Blue Gulch Member are mostly covered by Quaternary alluvium and Tertiary volcanic rocks. The type section and reference section for the bed were defined by Nilsen (1993) in the Hilt area,
The Hilt Bed appears to be a laterally continuous basin–plain megaturbidite that records a major depositional triggering event in the evolution of the Hornbrook basin (Fig. 6a and b). However, in contrast to many previously described basin–plain megaturbidites, the bed appears to be compound in nature, consisting of at least three subunits made up of separate Bouma (1962) sequences that record at least three triggering subevents. The type section clearly shows that the bed is graded, has an erosive base, and contains abundant shale rip-up clasts in its lower part. Instead of containing a single complete Bouma
Fig. 5. Measured sections and paleocurrent orientations for the Hilt Bed (from Nilsen, 1984c). Arrows on map portion of figure represent paleocurrent directions measured principally from flute casts at the base of the bed.
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Fig. 6. Photographs of the Blue Gulch Mudstone Member and Hilt Bed in its type section and adjacent areas. (a) Type section of the Hilt Bed east of Hilt Store and Hilt overpass on Interstate Highway 5. (b) Exposures of NW-dipping Hilt Bed near Hornbrook. (c) Upper subunit of the Hilt Bed showing parallel-stratified Bouma b division and current-ripple-marked Bouma c division, east of Hilt. (d) Clastic sandstone dikes in Blue Gulch Mudstone Member along Dark Hollow Road at north end of outcrop area, offset by subhorizontal and subvertical faults (see Nilsen et al., 1984b, pp. 40–41); inked lines along some of the faults.
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Fig. 6. (continued)
abcde sequence typical of many siliciclastic megaturbidites, it consists of three stacked beds or subunits, each of which has an erosive base, a basal a division, and well-defined grading.
In the type section (Fig. 4A), the lower subunit consists of a massive basal a division of fine- to medium-grained sandstone overlain by a parallel-stratified b division of fine-grained sandstone. The middle
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subunit, which erosionally truncates the lower subunit, consists of a second massive a division of finegrained sandstone that grades upward into a second parallel-stratified b division of very fine grained sandstone. The superposition of the two graded beds or subunits without the presence in the lower subunit of the upper finer grained parts of the Bouma sequence suggests that the two turbidite events followed one another relatively closely in time and space. The upper subunit of the Hilt Bed in the type section and in almost all other measured sections consists of a complete Bouma abcde sequence that grades upward into overlying typical basin–plain mudstone of the Blue Gulch Mudstone Member (Fig. 4A). The upper subunit erosionally truncates the middle subunit. The superposition of this graded subunit directly on the middle subunit without the presence in the middle subunit of the upper finer grained parts of the Bouma sequence suggests again that these two turbidite events followed one another relatively closely in time and space. The complete Bouma sequence of the upper subunit indicates that this turbidity current carried both fineand coarse-grained sediment. In the upper subunit, the Bouma a division consists of massive fine-grained sandstone, the b division of parallel-stratified very fine grained sandstone, the c division of very fine grained sandstone with current-ripple lamination, and the d division of parallel-laminated very fine grained sandstone and siltstone with abundant mica flakes and finely comminuted plant debris (Fig. 6c). The e division consists of mudstone that is generally difficult to distinguish from the overlying mudstone of the Blue Gulch Mudstone Member, and its indeterminate thickness is not included in the thickness measurements of the Hilt Bed. The general upward succession of Bouma divisions that were fully developed in the three subunits of the Hilt Bed form an abababcde sequence. However, there is some regional variability in the succession of Bouma divisions. For example, in the reference section, the uppermost a division is missing and the upper subunit thus consists of a bcde sequence (Fig. 4). The lowermost a division is generally the thickest, most coarse-grained, and most resistant part of the bed. The uppermost divisions are generally thinner, finer grained, less resistant, and locally covered by
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soil, colluvium, and landslide deposits derived from the overlying mudstone. The persistent presence of erosional bases, flute casts and other sole markings at the base of the bed, grading, Bouma sequences, massive and parallel stratification in each of the subunits, and current ripple markings in the upper subunit suggest that the Hilt Bed was deposited by turbidity currents below storm wave base. Foraminifers collected from the underlying and overlying mudstones indicate a late Santonian to late Campanian age for the middle part of the Blue Gulch Mudstone Member and deposition in bathyal or greater depths (Nilsen, 1993). 4.3. Depositional system The stratigraphic framework of the Hornbrook Formation suggests deposition of the Blue Gulch Mudstone Member in a basin–plain setting. Almost all of the criteria used by Mutti and Ricci Lucchi, 1972 to define the basin–plain facies association are present in the middle and upper parts of the member, including the following: • made up dominantly of mudstone, with a relatively uniform and generally low sandstone-to-shale ratio; • interbedded turbidites are generally fine grained, thin, and well-graded, with base-cut-out Bouma sequences typical; • interbedded turbidites are laterally continuous, with little or no change in thickness or grain size observable at outcrop scale; • paleocurrent directions are relatively uniform in orientation, to the N, NW, and NE (Fig. 3); • upward-fining or upward-coarsening bodies of sandstone typical of submarine fans and submarine canyons are absent; • the planar contacts and parallel layering of successive beds indicate that the basin floor was relatively flat, yielding a layer-cake architecture for this part of the Blue Gulch Mudstone Member. These observations suggest that the Hilt Bed, as a single layer within the Blue Gulch Mudstone Member, was deposited in the same basin–plain setting, only by a larger event or succession of events. Sole markings from the base of the lower subunit of
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the Hilt Bed indicate fairly uniform transport of sediments toward the NE (Fig. 5). Flow directions from primary current lineation and current ripple markings in the middle and upper units also indicate sediment transport toward the NE, but are not shown on Fig. 5. There are no indications from any of the outcrop sections of reversals of flow direction within the Hilt Bed, ruling out an alternative hypothesis that the thick, compound bed may have resulted from reflection of a single flow within a confined basin (i.e. Pickering and Hiscott, 1985; Marjanac, 1990; Edwards et al., 1994). Regional paleogeographic reconstructions suggest that the original axis of the Hornbrook basin was not confined, but deepened and continued northward into central Oregon, the direction that many basin–plain turbidites flowed, as shown on Fig. 3. There are also no indications of current reworking of the top of the bed by contour currents (i.e. Shanmugam et al., 1993; Ito, 1997). Although Nilsen (1984c) noted some turbidite beds that appeared to be reworked by contour currents in the Hornbrook Formation, these beds are confined to the underlying turbidite apron deposits of the Rocky Gulch Sandstone Member, and have not been noted in the Blue Gulch Mudstone Member. The petrography of the Hilt Bed suggests that it was derived from the Klamath Mountains to the SW (Nilsen, 1984c, 1993). Petrographic analyses of thin sections of sandstone from each of the Bouma a divisions of the bed indicate a similar composition and thus derivation of each subunit from the same provenance. The grain size of the lower Bouma a division is generally slightly coarser than that of the upper a divisions, indicating that the coarsest sediment was carried by the first flow. The lower subunit is also generally thicker and more widespread than the sandy and silty portions of the middle and upper subunits (Fig. 5). The abundance of mudstone rip-up clasts in the lower Bouma a division and the general lack of clasts in the upper two a divisions suggests that flow of the initial turbidity current resulted in relatively uniform scour and erosion the muddy basin floor, incorporating eroded clasts in the lower subunit. Because the two subsequent flows apparently overrode the previously deposited sand, they generally lack significant amounts of shale rip-up clasts. The compound nature of the bed suggests that a major seismic event may have triggered three turbid-
ity currents, producing a megaturbidite that records three closely spaced events. The lower subunit is thickest in the middle part of the outcrop area and appears to have a convex-upward shape in strike cross section when the base of the bed is used as the datum (Fig. 5). The upper subunits of the bed appear to be thicker to the NW and SE, around the margins of the lower subunit. They may have filled in the topographically lower parts of the basin floor following deposition of the Bouma a and b divisions of the lower subunit. Unfortunately, the lack of three-dimensional control on the geometry of the Hilt Bed prevents analysis of more proximal and distal deposits. The time between the three depositional events represented by the three subunits was probably quite short, perhaps several hours, with deposition of the complete Bouma sequence and fines of the upper subunit taking much longer. The Hilt Bed thus could have been deposited in response to a single earthquake that triggered movement of sediment from three separate source areas, probably sands stored either in or at the heads of submarine canyons or in topographic lows along a line source on the Klamath Mountains margin to the SW. In this scenario, sands from the most proximal canyon or topographic low would have reached the basin floor first, deposited a convex-upward bed, followed shortly by arrival of the two later flows, which deposited their sediment around the margins of the first bed. Alternatively, the flows could have been generated by three closely spaced earthquakes that successively triggered three separate turbidity currents from the same submarine canyon or topographic low. In this scenario, the first seismic event would have resulted in entrainment and movement of the largest volume of sediment and subsequent events in lower-volume flows that resulted in deposition around the edges of the first flow. Proof of a seismic triggering event for the Hilt Bed, as for all putative ancient seismoturbidites, is more difficult than defining the seismic trigger for modern seismoturbidites or ancient seismites (Shiki, 2000). A succession of nonseismically triggered landslides within or at the heads of one or more submarine canyons could have also generated the flows. A rapid series of retrogressive failures associated with headward migration of the active landslide scarp
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could trigger three flows with the characteristics of the Hilt Bed subunits. Activation of submarine landslides could result from storm activity, sediment instability, accelerated creep, abrupt change in relative sea level, fluid expulsion, and other processes besides earthquakes. Without preservation of the more proximal parts of the depositional system, it is clearly difficult to be certain of the triggering mechanism(s). The overall setting and depositional characteristics of the Hilt Bed, however, argue for its origin by a seismic triggering event(s). The absence of any comparable beds within the upper 650 m of the Blue Gulch Mudstone suggests that a large and singular event in the basin generated the bed rather than more common events such as landslides, storms, or large flood-generated periods of hyperpycnal flow (i.e. Milliman and Syvitski, 1991; Mulder and Syvitski, 1995). The normal grading at the base of the Hilt Bed, as well as other sedimentary features, are generally not characteristic of hyperpycnal flow deposits (Mulder et al., 1998). Seismoturbidites appear to be most typical of basin–plain settings, where they commonly form megaturbidites with a variety of sedimentary characteristics (Pilkey, 1988). Thick and fine-grained muds have been identified as of both seismic (Anastasakis and Piper, 1991) and aseismic origin (e.g. Stanley, 1981; Cita et al., 1984). Very thick detrital carbonate beds that include megabreccia have been interpreted as seismoturbidites in several siliciclastic-dominated basins (Johns et al., 1981; Mutti, 1984; Seguret et al., 1984; Marjanac, 1987; Rosell and Wieczorek, 1989; Slaczka and Walton, 1992). Similar beds with breccias made up of basement clasts have also been interpreted to be of seismic origin (i.e. Kleverlaan, 1987). Submarine landslides triggered by seismic events may also generate seismoturbidites deposited on basin plains (e.g. Kastens, 1984; Bourrouilh, 1987; Piper and Shor, 1988). However, none of these types of megabeds resemble the Hilt Bed. Marjanac (1987, 1991) has described compound megaturbidites in the extensional Paleogene flysch basin of Dalmatia. He describes these calcarenitic beds, referred to as composite turbidites, as typically a few meters thick and made up of 5–10 graded units within an overall graded bed. He inferred rapid deposition, with surging flow or retrogressive flow
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sliding, rather than seismic activity, as responsible for the repetitive nature of the beds. Siliciclastic sandstone megaturbidites that have been interpreted as seismoturbidites are less common, perhaps because they resemble and typically have Bouma sequences that are similar to turbidites derived from flows triggered by other processes. Mutti et al., (1984) suggested that ancient seismoturbidites can be inferred where repetitive megaturbidites have been deposited that occupy a significant area and volume of the fill of tectonically active basins such as trenches, strike-slip, and rift basins. The Hilt Bed, in contrast, is a singular compound megaturbidite with uncertain lateral extent deposited in a moderately active forearc basin. There do not seem to be any strong arguments, however, against a seismic trigger for the Hilt Bed. A single large seismic event in a tectonically quiet basin most easily explains the characteristics of the bed. Modern seismoturbidites are characteristic of both tectonically active as well as tectonically quiet basins (i.e. Piper and Shor, 1988). With only a twodimensional view of the basin, the total volume and area of the basin occupied by the Hilt Bed are uncertain but possibly very large. Intrusive and injected sandstone in the form of dikes, sills, and more irregular bodies, as well as faults that are clearly synsedimentary in origin, are locally abundant in both the older Ditch Creek Siltstone Member (Nilsen, 1984b, Fig. 13C) and the Blue Gulch Mudstone Member (Nilsen et al., 1984b; Fig. 21A and B). These features (Fig. 6d) suggest the presence of at least some seismic activity during deposition of the Hornbrook Formation. Examinations of the modern forearc and trench system off the Oregon coast has also resulted in definition of several major seismic events tied to turbidite deposits (Adams, 1990).
5. Summary and conclusions The Hilt Bed forms a multiple or compound turbidite bed with a Bouma abababcde sequence deposited on the basin–plain of a Late Cretaceous forearc basin. The compound nature of the bed suggests that its three subunits were produced by three separate turbidity currents generated by either the same triggering event or by three separate but closely spaced events.
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The absence of bioturbation or interbedded shale between or within the three subunits, the base-cutout nature of the lower two subunits, and the lateral continuity and extent of the Hilt Bed suggest that it was deposited by three turbidity currents that flowed onto the basin floor in rapid succession at almost the same time. A seismic event (or events) is the most likely triggering agent for the flows. Acknowledgements I thank numerous co-workers and field assistants for their help during various stages of my work on the Hilt Bed, including Greg Barats, Monty Elliott, Jan Zigler, Mike Gaona, Ralph Golia, Donna Balin, David Jones, Bill Sliter, and Bill Purdom. I thank Hans Nelson and Emiliano Mutti for helpful and thought-provoking reviews, although I do not necessarily agree with some of their interpretations of the data. Donn Gorsline provided excellent editorial assistance when needed. References Adams, J., 1990. Paleoseismicity of the Cascade subduction zone: evidence from turbidites off the Oregon–Washington margin. Tectonics 9, 569–583. Anastasakis, G.C., Piper, D.J.W., 1991. The character of seismoturbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece. Sedimentology 38, 717–733. Beaulieu, J.D., Hughes, P.W. 1977. Land use geology of central Jackson County, Oregon. Oregon Dept. Geol. and Min. Ind. Bull. 94, 87pp. Bouma, A.H., 1962. Sedimentology of some flysch deposits, Elsevier, Amsterdam (168pp.). Bourrouilh, R., 1987. Evolutionary mass flow-megaturbidites in an interplate basin: example of the North Pyrenean basin. GeoMar. Lett. 7, 69–81. Christiansen, R.L., Yeats, R.S., 1992. Post-Laramide geology of the U.S. Cordilleran region. In: Burchfiel, B.C., Lipman, P.W., Zoback, M.L.(Eds.). The Cordilleran Orogen: Conterminous U.S. Geol. Soc. Am. Geology of North America. v.G-3, Chap. 7, pp. 261–406. Cita, M.B., Beghi, C., Camerlenghi, A., Kastens, K.A., McCoy, F.W., Nosetto, A., Parisi, E., Scolari, F., Tomadin, L., 1984. Turbidites and megaturbidites from the Herodotus Abyssal Plain (eastern Mediterranean) unrelated to seismic events. Mar. Geol. 55, 79–101. Edwards, D.A., Leeder, M.R., Best, J.L., 1994. On experimental reflected density currents and interpretation of certain turbidites. Sedimentology 41, 347–461.
Elliott, M.A., 1971. Stratigraphy and petrology of the Late Cretaceous rocks near Hilt and Hornbrook. Siskiyou County, California, and Jackson County, Oregon. PhD thesis, Oregon State University, Corvallis, Oregon, 171pp. Hotz, P.E., 1971. Plutonic rocks of the Klamath Mountains, California and Oregon. US Geol. Surv. Prof. Paper. 684-B, 20pp. Hotz, P.E., 1977. Geology of the Yreka quadrangle. Siskiyou County, California. US Geol. Surv. Bull. 1436, 72pp. Irwin, W.P. 1966. Geology of the Klamath Mountains province. In: Bailey, E.H. (Editor), Geology of Northern California. Calif. Div. Mines Bull. 79, pp. 19–38. Irwin, W.P., 1994. Geologic map of the Klamath Mountains, California and Oregon. US Geol. Surv. Inv. Ser. Map I-2148, scale. 1:500,000. Ito, M., 1997. Spatial variation in turbidite-to-contourite continuums of the Kiwada and Otadai Formations in the Boso Peninsula, Japan: an unstable bottom-current system in a PlioPleistocene forearc basin. . J. Sed. Res. 67, 571–582. Johns, D.R., Mutti, E., Rosell, J., Seguret, M., 1981. Origin of a thick, redeposited carbonate bed in Eocene turbidites of the Hecho Group, south-central Pyrenees, Spain. Geology 9, 161– 164. Jones, D.L., 1959. Stratigraphy of Upper Cretaceous rocks in the Yreka-Hornbrook area, northern California. Geol. Soc. Am. Bull. 70, 1726–1727 (abs.). Kastens, K.A., 1984. Earthquakes as a triggering mechanism for debris flows and turbidites on the Calabrian Ridge. Mar. Geol. 55, 13–33. Kleverlaan, K., 1987. Gordo megabed: a possible seismite in a Tortonian submarine fan, Tabernas basin, Province Almeria, southeast Spain. Sediment. Geol. 51, 165–180. Marjanac, T., 1987. Ponded megabeds and some characteristics of the Eocene Adriatic basin (Middle Dalmatia, Yugoslavia). Mem. Soc. Geol. It. 40, 241–249. Marjanac, T., 1990. Reflected sediment gravity flows and their deposits in flysch of Middle Dalmatia, Yugoslavia. Sedimentology 37, 921–929. Marjanac, T., 1991. Importance of megabeds for reconstruction of Paleogene flysch basin in Split Hinterland (Middle Dalmatia). Geol. Vjesnik 44, 201–213. McKnight, B.K., 1971. Petrology and sedimentation of Cretaceous and Eocene rocks in the Medford-Ashland region, southwestern Oregon. PhD thesis, Oregon State University, Corvallis, Oregon, 177pp. Milliman, J.D., Syvitski, J.P.M., 1991. Geomorphic/tectonic control of sediment discharge to the ocean: the importance of small mountainous rivers. J. Geol. 100, 525–544. Mulder, T., Syvitski, J.P.M., 1995. Turbidity currents generated at river mouths during exceptional discharges to the world oceans. J. Geol. 103, 299–385. Mulder, T., Syvitski, J.P.M., Skene, K.L., 1998. Modeling of erosion and deposition by turbidity currents generated at river mouths. J. Sed. Res. 68, 124–137. Mutti, E., Ricci Lucchi, F., 1972. Le torbiditi dell’Appennino settentrionale: Introduzione all’analisi di facies. Mem. Soc. Geol. Ital. 11, 161–199. Mutti, E., Ricci Lucchi, F., Seguret, M., Zanzucchi, G., 1984.
T.H. Nilsen / Sedimentary Geology 135 (2000) 51–63 Seismoturbidites: a new group of resedimented deposits. Mar. Geol. 55, 103–116. Nilsen, T.H., 1983. The Hilt Bed, a compound seismoturbidite in the Upper Cretaceous Hornbrook Formation, Oregon and California. Geol. Soc. Am. Abs. with Prog. 15, 653 (abs.). Nilsen, T.H., 1984a. Tectonics and sedimentation of the Upper Cretaceous Hornbrook Formation, Oregon and California. In: Crouch, J.K., Bachman, S.B. (Eds.). Tectonics and Sedimentation along the California Margin. Pacific Sec., Soc. Econ. Paleont. and Mineral., 38, pp. 101–117. Nilsen, T.H., 1984b. Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Pacific Sec., Soc. Econ. Paleont. and Mineral., 42, 257pp. Nilsen, T.H.,1984c. Stratigraphy, sedimentology, and tectonic framework of the Upper Cretaceous Hornbrook Formation, Oregon and California. In: Nilsen, T.H. (Editor). Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Pacific Sec., Soc. Econ. Paleont. and Mineral., 42, pp. 51–88. Nilsen, T.H., 1993. Stratigraphy of the Cretaceous Hornbrook Formation, southern Oregon and northern California. US Geol. Surv. Prof. Paper, 1521, 89pp. Nilsen, T.H., Barats, G.M., Elliott, M.A., Jones, D.L., 1983. Geologic map of the outcrop area of the Hornbrook Formation, Oregon and California. US Geol. Surv. Open-File Rept., scale 1:62,500, pp. 83–373. Nilsen, T.H., Elliott, M.A., Purdom, W.B.,1984. Description of field trip stops and roadlog, Upper Cretaceous Hornbrook Formation, southern Oregon and northern California. In: Nilsen, T.H. (Editor). Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Pacific Sec., Soc. Econ. Paleont. and Mineral., 42, pp. 9–41. Peck, D.L., Imlay, R.W., Popenoe, W.P., 1956. Upper Cretaceous rocks of parts of southwestern Oregon and northern California. Am. Assoc. Petrol. Geol. Bull. 40, 1968–1984.
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Pickering, K.T., Hiscott, R.N., 1985. Contained (reflected) turbidity currents from the Middle Ordovician Cloridorme Formation, Quebec, Canada: an alternative to the antidune hypothesis. Sedimentology 32, 373–394. Pilkey, O.H., 1988. Basin plains; Giant sedimentation events. Geol. Soc. Am. Spec. Paper 229, 93–99. Piper, D.J.W., Shor, A.N., Hughes Clarke, J.E., 1988. The Grand Banks Earthquake, slump and turbidity current. Geol. Soc. Am. Spec. Paper 229, 77–92. Rosell, J., Wieczorek, J., 1989. Main features of megaturbidites in the Eocene of southern Pyrenees. Ann. Soc. Geol. Poloniae 59, 3–16. Seguret, M., Labaume, P., Madariaga, R., 1984. Eocene seismicity in the Pyrenees from megaturbidites of the South Pyrenean Basin (Spain). Mar. Geol. 55, 117–131. Shanmugam, G., Spalding, T.D., Rofheart, D.H., 1993. Process sedimentology and reservoir quality of deep-marine bottomcurrent reworked sands (sandy contourites): an example from the Gulf of Mexico. Am. Assoc. Petrol. Geol. Bull. 77, 1241– 1259. Slaczka, A., Walton, E.K., 1992. Flow characteristics of Metresa: an Oligocene seismoturbidite in the Dukla Unit, Polish Carpathians. Sedimentology 39, 383–392. Stanley, D.J., 1981. Unifites: structureless muds of gravity flow origin in Mediterranean basins. Geo-Mar. Lett. 1, 77–83. Vance, J.A.,1984. The lower Western Cascades Volcanic Group in northern California. In: Nilsen, T.H. (Editor). Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Pacific Sec., Soc. Econ. Paleont. and Mineral., 42, pp. 195– 196. Wagner, D.L., Saucedo, G.J. (compilers), 1987. Geologic map of the Weed quadrangle. Calif. Div. Mines and Geol. Reg. Map Ser., 4A, scale. 1:250,000.
The Hilt Bed, an Upper Cretaceous compound basin–plain seismoturbidite in the Hornbrook Forearc Basin of southern Oregon and northern California, USA T.H. Nilsen* Consulting Geologist, 215 Club Drive, San Carlos, CA 94070, USA Received 14 March 1998; accepted 7 July 1999
Abstract The Hilt Bed is a thick, laterally continuous marker bed of fine- to medium-grained sandstone that crops out for a NW–SE along-strike distance of about 50 km on the southwestern flank of the Cretaceous Hornbrook forearc basin of northern California and southern Oregon. The Hilt Bed forms part of the Blue Gulch Mudstone Member of the Hornbrook Formation and is everywhere underlain and overlain by basin–plain mudstone that contains thin interbeds of turbidite sandstone and siltstone. The bed is as thick as 4.71 m in the central part of the outcrop belt and thins progressively to the NW and SE. It was derived from erosion of igneous and metamorphic basement rocks of the Klamath Mountains, which flank the basin to the SW. Paleocurrents measured from flute casts at the base of the bed indicate sediment transport to the NE. The Hilt Bed can be divided into a compound succession of nine Bouma divisions, all of which are generally present in outcrop. The succession consists in ascending stratigraphic order of Bouma abababcde divisions. The basal a division is typically the thickest and coarsest layer, with overlying divisions generally thinner and more variable in thickness. The bed appears to have been deposited as a compound turbidite by at least three separate turbidity currents that appear to have been generated either by the same triggering event or by a rapid succession of related events. Only the uppermost part of the bed thus contains a complete Bouma sequence. The unidirectional paleocurrent indicators, lateral continuity and extent of the three flows that make up the bed, incompleteness of the two lower Bouma sequences, and absence of interbedded shale or bioturbated levels suggest that the three turbidity currents reached the basin floor at almost the same time to yield the bed. The compound nature of the Hilt Bed suggests that a major seismic event triggered the three large flows, which probably originated in either a single submarine canyon or three adjacent canyons. 䉷 2000 Published by Elsevier Science B.V. Keywords: turbidite; seismoturbidite; basin plain; forearc basin; Cretaceous; California and Oregon
1. Introduction The sedimentary deposits of the Cretaceous Hornbrook Formation of southern Oregon and northern California are thought to fill a late Mesozoic forearc * Fax: ⫹1-650-591-1763. E-mail address: [email protected] (T.H. Nilsen).
basin that extended northward into the Mitchell area of central Oregon and southward into the Great Valley of north-central California (Fig. 1). The strata record infill of the sedimentary basin by facies that range in origin from alluvial fan to deep marine. A widespread marker bed, named the Hilt Bed by Nilsen (1983, 1993) for its type area near Hilt, Oregon, is present in the upper part of the stratigraphic succession. The
0037-0738/00/$ - see front matter 䉷 2000 Published by Elsevier Science B.V. PII: S0037-073 8(00)00062-2
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Fig. 1. Regional map showing outcrop locations of the Hornbrook Formation, major physiographic provinces, and adjacent outcrop areas of Cretaceous strata in dark pattern (from Nilsen, 1993).
bed extends in outcrop for at least 50 km along strike and may have been deposited as a seismoturbidite. The bed is unusual in that it appears to consist of three different beds deposited in rapid succession, infilling a large part of the deep-marine basin–plain (Nilsen, 1993). The purpose of this paper is to describe the bed and its regional setting, discuss the mechanisms of its deposition, and compare it with other well-described seismoturbidites in the geological literature.
2. Regional setting Lower and Upper Cretaceous sedimentary strata
cropping out in Siskiyou County, California, and Jackson County, Oregon, were named the Hornbrook Formation by Peck et al. (1956). They measured a composite type section 815 m thick near Hornbrook, California (Fig. 2). Additional stratigraphic and paleontologic data were obtained by Jones (1959), Elliott (1971), McKnight (1971), Nilsen (1984a,b) and others (see summary of previous work in Nilsen, 1993, pp. 5–10). The outcrop distribution of the Hornbrook Formation was defined in geologic maps prepared by Elliott (1971), Hotz (1971, 1977), Beaulieu and Hughes (1977), Nilsen et al. (1983), Wagner and Saucedo (1987), and Nilsen (1993). Along its southwestern flank, the Hornbrook Formation rests unconformably on older Paleozoic,
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Fig. 2. Simplified geologic map of the Hornbrook Formation, showing distribution of major stratigraphic units (from Nilsen, 1993). 53
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Fig. 3. Composite measured section of the Hornbrook Formation showing stratigraphic position of Hilt Bed (from Nilsen, 1993).
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Triassic, and Jurassic accreted terranes and plutons that form the core of the Klamath Mountains geologic province (Irwin, 1966, 1994; Hotz, 1971). The Hornbrook Formation crops out in a series of generally low-lying valleys on the northeastern flank of the Klamath Mountains, where it forms a NW-striking and NE-dipping homoclinal succession (Fig. 2). The outcrops of the Hornbrook Formation are offset by numerous NE-striking cross faults that result in displacement of stratigraphic boundaries and local difficulties in stratigraphic assignment (Nilsen et al., 1983; Nilsen, 1993). On its NE flank, the Hornbrook Formation is overlain with angular unconformity by Tertiary volcanic and sedimentary rocks of the Cascade Range (McKnight, 1971; Vance, 1984; Christiansen and Yeats, 1992). Stratigraphic relations and geophysical data indicate that the Hornbrook Formation extends northeastward in the subsurface beneath the Cascade Range. A single petroleum exploration well drilled through the western margin of the Cascade Range in the Shasta Valley area yielded a thickness of 1405 m (Nilsen, 1993). The lateral continuity of most of its members in outcrop suggests that the Hornbrook Formation originally extended much farther to the north and southeast, and that it was physically connected with coeval strata in the Redding area to the south and the Mitchell area to the north (Fig. 1).
3. Stratigraphic framework The Hornbrook Formation was subdivided by Nilsen (1984c) into a series of members that were subsequently formally defined by Nilsen (1993) in more detail (Figs. 2 and 3). The thickness of the formation varies along strike from published values of 815–1490 m. The series of members generally define an upward-deepening succession typical of an actively subsiding basin. The nonconformable basal contact is locally marked by reddish paleosols, a breccia composed of angular blocks of basement rocks, and conglomeratic grus composed of untransported granitic detritus derived from underlying plutonic rocks. Alluvial-fan and fluvial deposits generally overlie the basal residuum and are in turn abruptly overlain by cross-stratified and fossiliferous nearshore marine sandstone. In most sections, offshore
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hummocky cross-stratified shallow-marine sandstone overlies the nearshore deposits and grades upward into bioturbated and locally fossiliferous marine outer-shelf and slope siltstone and mudstone (Fig. 3). Along most of the outcrop belt, turbidite sandstone and associated conglomerate of the Rocky Gulch Sandstone Member stratigraphically overlie the siltstone and mudstone. This member, which is about 190 m thick, appears to have formed a sandstonerich slope or base-of-slope apron along the NE flank of the Klamath Mountains, where a line source for the sands appears to have characterized the basin margin. The Blue Gulch Mudstone forms the uppermost, thickest, and finest-grained member of the Hornbrook Formation (Figs. 2 and 3). It is at least 850 m thick and consists mostly of a stack of deep-marine mudstone with thin interbeds of siltstone and very fine and finegrained sandstone turbidites. It appears in general to deepen upward into a succession of deep-marine basin–plain turbidites. Although it is relatively uniform in character along the outcrop belt, in the Hornbrook area it contains a lens of hummocky cross-stratified shallow-marine sandstone in its lower part that is about 85 m thick and assigned to the Rancheria Gulch Sandstone Beds. The lens indicates local uplift and shoaling in this part of the basin. The Hilt Bed is a prominent marker bed of sandstone that forms the boundary between the middle and upper parts of the Blue Gulch Mudstone Member in its type section (Fig. 3; Nilsen, 1993). In its type section, the Hilt Bed lies about 458 m above the base of the Blue Gulch Mudstone Member and 388.5 m below the unconformable upper contact with Tertiary strata. The Hilt Bed is a unique thicker and coarser grained bed of sandstone within the middle and upper parts of the Blue Gulch Mudstone Member.
4. The Hilt Bed 4.1. Distribution and thickness The Hilt Bed forms a prominent marker bed within the Blue Gulch Mudstone Member for about 50 km along strike from the Shasta Valley northwestward to the Ashland area (Fig. 2). It is impossible to determine whether the bed extends farther to the NW and SE because, in these areas, the middle and upper parts
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where two excellent roadcuts provide clear and easily observed sections (Fig. 4). The bed can generally easily be traced in outcrop and on aerial photographs to the NW and SE of the type area. Where locally obscured by closely spaced NE-striking cross faults, its continuity may be uncertain. The bed is thickest in the middle part of the outcrop belt near Bailey Hill, where it has a maximum measured thickness of 4.71 m (Fig. 5, section 10). It generally thins progressively to the NW and SE of the Bailey Hill area to 2.69 m and 1.61 m, respectively, at the most distal measured sections. However, it is locally thinner, as in sections 15 and 16 between Hilt and Ashland (Fig. 5). 4.2. Sedimentology
Fig. 4. (A) Type section; and (B) reference section of the Hilt Bed of the Hornbrook Formation (from Nilsen, 1993). (a)–(e) correspond to divisions that Bouma (1962) defined for graded turbidite beds.
of the Blue Gulch Member are mostly covered by Quaternary alluvium and Tertiary volcanic rocks. The type section and reference section for the bed were defined by Nilsen (1993) in the Hilt area,
The Hilt Bed appears to be a laterally continuous basin–plain megaturbidite that records a major depositional triggering event in the evolution of the Hornbrook basin (Fig. 6a and b). However, in contrast to many previously described basin–plain megaturbidites, the bed appears to be compound in nature, consisting of at least three subunits made up of separate Bouma (1962) sequences that record at least three triggering subevents. The type section clearly shows that the bed is graded, has an erosive base, and contains abundant shale rip-up clasts in its lower part. Instead of containing a single complete Bouma
Fig. 5. Measured sections and paleocurrent orientations for the Hilt Bed (from Nilsen, 1984c). Arrows on map portion of figure represent paleocurrent directions measured principally from flute casts at the base of the bed.
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Fig. 6. Photographs of the Blue Gulch Mudstone Member and Hilt Bed in its type section and adjacent areas. (a) Type section of the Hilt Bed east of Hilt Store and Hilt overpass on Interstate Highway 5. (b) Exposures of NW-dipping Hilt Bed near Hornbrook. (c) Upper subunit of the Hilt Bed showing parallel-stratified Bouma b division and current-ripple-marked Bouma c division, east of Hilt. (d) Clastic sandstone dikes in Blue Gulch Mudstone Member along Dark Hollow Road at north end of outcrop area, offset by subhorizontal and subvertical faults (see Nilsen et al., 1984b, pp. 40–41); inked lines along some of the faults.
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Fig. 6. (continued)
abcde sequence typical of many siliciclastic megaturbidites, it consists of three stacked beds or subunits, each of which has an erosive base, a basal a division, and well-defined grading.
In the type section (Fig. 4A), the lower subunit consists of a massive basal a division of fine- to medium-grained sandstone overlain by a parallel-stratified b division of fine-grained sandstone. The middle
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subunit, which erosionally truncates the lower subunit, consists of a second massive a division of finegrained sandstone that grades upward into a second parallel-stratified b division of very fine grained sandstone. The superposition of the two graded beds or subunits without the presence in the lower subunit of the upper finer grained parts of the Bouma sequence suggests that the two turbidite events followed one another relatively closely in time and space. The upper subunit of the Hilt Bed in the type section and in almost all other measured sections consists of a complete Bouma abcde sequence that grades upward into overlying typical basin–plain mudstone of the Blue Gulch Mudstone Member (Fig. 4A). The upper subunit erosionally truncates the middle subunit. The superposition of this graded subunit directly on the middle subunit without the presence in the middle subunit of the upper finer grained parts of the Bouma sequence suggests again that these two turbidite events followed one another relatively closely in time and space. The complete Bouma sequence of the upper subunit indicates that this turbidity current carried both fineand coarse-grained sediment. In the upper subunit, the Bouma a division consists of massive fine-grained sandstone, the b division of parallel-stratified very fine grained sandstone, the c division of very fine grained sandstone with current-ripple lamination, and the d division of parallel-laminated very fine grained sandstone and siltstone with abundant mica flakes and finely comminuted plant debris (Fig. 6c). The e division consists of mudstone that is generally difficult to distinguish from the overlying mudstone of the Blue Gulch Mudstone Member, and its indeterminate thickness is not included in the thickness measurements of the Hilt Bed. The general upward succession of Bouma divisions that were fully developed in the three subunits of the Hilt Bed form an abababcde sequence. However, there is some regional variability in the succession of Bouma divisions. For example, in the reference section, the uppermost a division is missing and the upper subunit thus consists of a bcde sequence (Fig. 4). The lowermost a division is generally the thickest, most coarse-grained, and most resistant part of the bed. The uppermost divisions are generally thinner, finer grained, less resistant, and locally covered by
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soil, colluvium, and landslide deposits derived from the overlying mudstone. The persistent presence of erosional bases, flute casts and other sole markings at the base of the bed, grading, Bouma sequences, massive and parallel stratification in each of the subunits, and current ripple markings in the upper subunit suggest that the Hilt Bed was deposited by turbidity currents below storm wave base. Foraminifers collected from the underlying and overlying mudstones indicate a late Santonian to late Campanian age for the middle part of the Blue Gulch Mudstone Member and deposition in bathyal or greater depths (Nilsen, 1993). 4.3. Depositional system The stratigraphic framework of the Hornbrook Formation suggests deposition of the Blue Gulch Mudstone Member in a basin–plain setting. Almost all of the criteria used by Mutti and Ricci Lucchi, 1972 to define the basin–plain facies association are present in the middle and upper parts of the member, including the following: • made up dominantly of mudstone, with a relatively uniform and generally low sandstone-to-shale ratio; • interbedded turbidites are generally fine grained, thin, and well-graded, with base-cut-out Bouma sequences typical; • interbedded turbidites are laterally continuous, with little or no change in thickness or grain size observable at outcrop scale; • paleocurrent directions are relatively uniform in orientation, to the N, NW, and NE (Fig. 3); • upward-fining or upward-coarsening bodies of sandstone typical of submarine fans and submarine canyons are absent; • the planar contacts and parallel layering of successive beds indicate that the basin floor was relatively flat, yielding a layer-cake architecture for this part of the Blue Gulch Mudstone Member. These observations suggest that the Hilt Bed, as a single layer within the Blue Gulch Mudstone Member, was deposited in the same basin–plain setting, only by a larger event or succession of events. Sole markings from the base of the lower subunit of
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the Hilt Bed indicate fairly uniform transport of sediments toward the NE (Fig. 5). Flow directions from primary current lineation and current ripple markings in the middle and upper units also indicate sediment transport toward the NE, but are not shown on Fig. 5. There are no indications from any of the outcrop sections of reversals of flow direction within the Hilt Bed, ruling out an alternative hypothesis that the thick, compound bed may have resulted from reflection of a single flow within a confined basin (i.e. Pickering and Hiscott, 1985; Marjanac, 1990; Edwards et al., 1994). Regional paleogeographic reconstructions suggest that the original axis of the Hornbrook basin was not confined, but deepened and continued northward into central Oregon, the direction that many basin–plain turbidites flowed, as shown on Fig. 3. There are also no indications of current reworking of the top of the bed by contour currents (i.e. Shanmugam et al., 1993; Ito, 1997). Although Nilsen (1984c) noted some turbidite beds that appeared to be reworked by contour currents in the Hornbrook Formation, these beds are confined to the underlying turbidite apron deposits of the Rocky Gulch Sandstone Member, and have not been noted in the Blue Gulch Mudstone Member. The petrography of the Hilt Bed suggests that it was derived from the Klamath Mountains to the SW (Nilsen, 1984c, 1993). Petrographic analyses of thin sections of sandstone from each of the Bouma a divisions of the bed indicate a similar composition and thus derivation of each subunit from the same provenance. The grain size of the lower Bouma a division is generally slightly coarser than that of the upper a divisions, indicating that the coarsest sediment was carried by the first flow. The lower subunit is also generally thicker and more widespread than the sandy and silty portions of the middle and upper subunits (Fig. 5). The abundance of mudstone rip-up clasts in the lower Bouma a division and the general lack of clasts in the upper two a divisions suggests that flow of the initial turbidity current resulted in relatively uniform scour and erosion the muddy basin floor, incorporating eroded clasts in the lower subunit. Because the two subsequent flows apparently overrode the previously deposited sand, they generally lack significant amounts of shale rip-up clasts. The compound nature of the bed suggests that a major seismic event may have triggered three turbid-
ity currents, producing a megaturbidite that records three closely spaced events. The lower subunit is thickest in the middle part of the outcrop area and appears to have a convex-upward shape in strike cross section when the base of the bed is used as the datum (Fig. 5). The upper subunits of the bed appear to be thicker to the NW and SE, around the margins of the lower subunit. They may have filled in the topographically lower parts of the basin floor following deposition of the Bouma a and b divisions of the lower subunit. Unfortunately, the lack of three-dimensional control on the geometry of the Hilt Bed prevents analysis of more proximal and distal deposits. The time between the three depositional events represented by the three subunits was probably quite short, perhaps several hours, with deposition of the complete Bouma sequence and fines of the upper subunit taking much longer. The Hilt Bed thus could have been deposited in response to a single earthquake that triggered movement of sediment from three separate source areas, probably sands stored either in or at the heads of submarine canyons or in topographic lows along a line source on the Klamath Mountains margin to the SW. In this scenario, sands from the most proximal canyon or topographic low would have reached the basin floor first, deposited a convex-upward bed, followed shortly by arrival of the two later flows, which deposited their sediment around the margins of the first bed. Alternatively, the flows could have been generated by three closely spaced earthquakes that successively triggered three separate turbidity currents from the same submarine canyon or topographic low. In this scenario, the first seismic event would have resulted in entrainment and movement of the largest volume of sediment and subsequent events in lower-volume flows that resulted in deposition around the edges of the first flow. Proof of a seismic triggering event for the Hilt Bed, as for all putative ancient seismoturbidites, is more difficult than defining the seismic trigger for modern seismoturbidites or ancient seismites (Shiki, 2000). A succession of nonseismically triggered landslides within or at the heads of one or more submarine canyons could have also generated the flows. A rapid series of retrogressive failures associated with headward migration of the active landslide scarp
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could trigger three flows with the characteristics of the Hilt Bed subunits. Activation of submarine landslides could result from storm activity, sediment instability, accelerated creep, abrupt change in relative sea level, fluid expulsion, and other processes besides earthquakes. Without preservation of the more proximal parts of the depositional system, it is clearly difficult to be certain of the triggering mechanism(s). The overall setting and depositional characteristics of the Hilt Bed, however, argue for its origin by a seismic triggering event(s). The absence of any comparable beds within the upper 650 m of the Blue Gulch Mudstone suggests that a large and singular event in the basin generated the bed rather than more common events such as landslides, storms, or large flood-generated periods of hyperpycnal flow (i.e. Milliman and Syvitski, 1991; Mulder and Syvitski, 1995). The normal grading at the base of the Hilt Bed, as well as other sedimentary features, are generally not characteristic of hyperpycnal flow deposits (Mulder et al., 1998). Seismoturbidites appear to be most typical of basin–plain settings, where they commonly form megaturbidites with a variety of sedimentary characteristics (Pilkey, 1988). Thick and fine-grained muds have been identified as of both seismic (Anastasakis and Piper, 1991) and aseismic origin (e.g. Stanley, 1981; Cita et al., 1984). Very thick detrital carbonate beds that include megabreccia have been interpreted as seismoturbidites in several siliciclastic-dominated basins (Johns et al., 1981; Mutti, 1984; Seguret et al., 1984; Marjanac, 1987; Rosell and Wieczorek, 1989; Slaczka and Walton, 1992). Similar beds with breccias made up of basement clasts have also been interpreted to be of seismic origin (i.e. Kleverlaan, 1987). Submarine landslides triggered by seismic events may also generate seismoturbidites deposited on basin plains (e.g. Kastens, 1984; Bourrouilh, 1987; Piper and Shor, 1988). However, none of these types of megabeds resemble the Hilt Bed. Marjanac (1987, 1991) has described compound megaturbidites in the extensional Paleogene flysch basin of Dalmatia. He describes these calcarenitic beds, referred to as composite turbidites, as typically a few meters thick and made up of 5–10 graded units within an overall graded bed. He inferred rapid deposition, with surging flow or retrogressive flow
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sliding, rather than seismic activity, as responsible for the repetitive nature of the beds. Siliciclastic sandstone megaturbidites that have been interpreted as seismoturbidites are less common, perhaps because they resemble and typically have Bouma sequences that are similar to turbidites derived from flows triggered by other processes. Mutti et al., (1984) suggested that ancient seismoturbidites can be inferred where repetitive megaturbidites have been deposited that occupy a significant area and volume of the fill of tectonically active basins such as trenches, strike-slip, and rift basins. The Hilt Bed, in contrast, is a singular compound megaturbidite with uncertain lateral extent deposited in a moderately active forearc basin. There do not seem to be any strong arguments, however, against a seismic trigger for the Hilt Bed. A single large seismic event in a tectonically quiet basin most easily explains the characteristics of the bed. Modern seismoturbidites are characteristic of both tectonically active as well as tectonically quiet basins (i.e. Piper and Shor, 1988). With only a twodimensional view of the basin, the total volume and area of the basin occupied by the Hilt Bed are uncertain but possibly very large. Intrusive and injected sandstone in the form of dikes, sills, and more irregular bodies, as well as faults that are clearly synsedimentary in origin, are locally abundant in both the older Ditch Creek Siltstone Member (Nilsen, 1984b, Fig. 13C) and the Blue Gulch Mudstone Member (Nilsen et al., 1984b; Fig. 21A and B). These features (Fig. 6d) suggest the presence of at least some seismic activity during deposition of the Hornbrook Formation. Examinations of the modern forearc and trench system off the Oregon coast has also resulted in definition of several major seismic events tied to turbidite deposits (Adams, 1990).
5. Summary and conclusions The Hilt Bed forms a multiple or compound turbidite bed with a Bouma abababcde sequence deposited on the basin–plain of a Late Cretaceous forearc basin. The compound nature of the bed suggests that its three subunits were produced by three separate turbidity currents generated by either the same triggering event or by three separate but closely spaced events.
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The absence of bioturbation or interbedded shale between or within the three subunits, the base-cutout nature of the lower two subunits, and the lateral continuity and extent of the Hilt Bed suggest that it was deposited by three turbidity currents that flowed onto the basin floor in rapid succession at almost the same time. A seismic event (or events) is the most likely triggering agent for the flows. Acknowledgements I thank numerous co-workers and field assistants for their help during various stages of my work on the Hilt Bed, including Greg Barats, Monty Elliott, Jan Zigler, Mike Gaona, Ralph Golia, Donna Balin, David Jones, Bill Sliter, and Bill Purdom. I thank Hans Nelson and Emiliano Mutti for helpful and thought-provoking reviews, although I do not necessarily agree with some of their interpretations of the data. Donn Gorsline provided excellent editorial assistance when needed. References Adams, J., 1990. Paleoseismicity of the Cascade subduction zone: evidence from turbidites off the Oregon–Washington margin. Tectonics 9, 569–583. Anastasakis, G.C., Piper, D.J.W., 1991. The character of seismoturbidites in the S-1 sapropel, Zakinthos and Strofadhes basins, Greece. Sedimentology 38, 717–733. Beaulieu, J.D., Hughes, P.W. 1977. Land use geology of central Jackson County, Oregon. Oregon Dept. Geol. and Min. Ind. Bull. 94, 87pp. Bouma, A.H., 1962. Sedimentology of some flysch deposits, Elsevier, Amsterdam (168pp.). Bourrouilh, R., 1987. Evolutionary mass flow-megaturbidites in an interplate basin: example of the North Pyrenean basin. GeoMar. Lett. 7, 69–81. Christiansen, R.L., Yeats, R.S., 1992. Post-Laramide geology of the U.S. Cordilleran region. In: Burchfiel, B.C., Lipman, P.W., Zoback, M.L.(Eds.). The Cordilleran Orogen: Conterminous U.S. Geol. Soc. Am. Geology of North America. v.G-3, Chap. 7, pp. 261–406. Cita, M.B., Beghi, C., Camerlenghi, A., Kastens, K.A., McCoy, F.W., Nosetto, A., Parisi, E., Scolari, F., Tomadin, L., 1984. Turbidites and megaturbidites from the Herodotus Abyssal Plain (eastern Mediterranean) unrelated to seismic events. Mar. Geol. 55, 79–101. Edwards, D.A., Leeder, M.R., Best, J.L., 1994. On experimental reflected density currents and interpretation of certain turbidites. Sedimentology 41, 347–461.
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