Slope-derived deposits from the Cretaceous Interior Seaway, northwestern Colorado

Sedimentary Geology, 65 (1989) 69-85

69

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Slope-derived deposits from the Cretaceous Interior Seaway, northwestern Colorado JOHN

C. L O R E N Z

1 and CHRIS

A. MUHR

2

1 Division 6253, Sandia National Laboratories, Albuquerque, N M 87185 (U.S.A.) 2 Oak Ridge National Laboratory, Grand Junction, CO 81502 ( U.S. A.)

Received May 30, 1989; revised version accepted August 29, 1989

Abstract Lorenz, J.C. and Muhr, C.A., 1989. Slope-derived deposits from the Cretaceous Interior Seaway, northwestern Colorado. Sediment. Geol., 65: 69-85. Two types of mass-movement deposits are interbedded within the dark marine Upper Cretaceous Mancos Shale, in outcrops exposed on and near the slopes of Mount Garfield, 12 km east of Grand Junction, Colorado. These deposits are inferred to have formed at or seaward of the shallow-marine shelf edge, about 200 km from the contemporaneous shoreline. The first type is a thick (tens of meters), laterally restricted (200-400 m) deposit composed of rip-up clasts of Mancos Shale, rare sandstone and siltstone blocks, and a silty mudstone matrix. These deposits overlie abrupt, bedding-parallel basal contacts, and are interpreted to record submarine debris flows. The second type of deposit consists of landward-rotated blocks of Mancos Shale tens of meters in scale, adjacent to a listric fault scar, and overlying a similar sharp, planar base. These are interpreted as slump blocks. An associated third type of deposit consists of one or more 4- to 5-m-thick beds composed of laterally extensive, centimeter-scale, sandstone-shale laminations that are rippled and extensively burrowed and that may represent distal turbidite deposits. The mass-movement deposits are interpreted as rare examples of the sedimentary record of the offshore slope environment in the Cretaceous Interior Seaway. A twice-repeated vertical succession, with basal sandstones, intermediate mudstones, and overlying slump blocks or debris-flow deposits, records fluctuations in relative sea level, tectonism, and sedimentation rate during the Claggett transgression.

Introduction The Late Cretaceous Interior Seaway of North A m e r i c a was filled b y d e p o s i t s of e n v i r o n m e n t s t h a t r a n g e d f r o m " d e e p " m a r i n e to n o n - m a r i n e . T h e d e p o s i t s d e s c r i b e d here o c c u r w i t h i n the M a n c o s Shale, a g r a y - b l a c k to olive-gray, fissile m a r i n e shale t h a t constitutes a m a j o r p a r t o f the U p p e r C r e t a c e o u s s t r a t a in C o l o r a d o . V a r i o u s l y c a l c a r e o u s to silty to clay rich, the M a n c o s Shale ranges f r o m a b o u t 500 to 1500 m thick. I t is a b o u t 1200 m thick in t h e s t u d y a r e a ( L o h m a n , 1965). I n n o r t h w e s t e r n C o l o r a d o , it overlies the transgressive L o w e r C r e t a c e o u s D a k o t a S a n d s t o n e , a n d is overlain b y s h a l l o w - m a r i n e to n o n - m a r i n e regres0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

sive s t r a t a o f the C a m p a n i a n M e s a v e r d e G r o u p . T h e M a n c o s Shale is easily e r o d e d a n d forms b r o a d valleys o r l o w hills, except w h e r e it is p r o t e c t e d b y the o v e r l y i n g s a n d s t o n e s of the M e s a v e r d e G r o u p . I n these p r o t e c t e d areas, it f o r m s steep slopes such as t h o s e at Mt. G a r f i e l d , t h e r e b y offering excellent e x p o s u r e s for study. Many authors have schematically diagrammed shelf, slope, a n d b a s i n e n v i r o n m e n t s in shale s t r a t a in the I n t e r i o r Seaway, b a s e d o n A s q u i t h ' s (1970) r e c o n s t r u c t i o n o f the C r e t a c e o u s s e a f l o o r p r o f i l e in W y o m i n g . D e p o s i t s o f the shelf e n v i r o n m e n t are increasingly being recognized and described (e.g., Boyles a n d Scott, 1982; H o b s o n et al., 1982). H o w e v e r , t h e shelf edge is o f t e n difficult to locate,

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and outcrops of deposits containing slope facies have not been previously reported from the Interior Seaway. The deposits described here are inferred to have originated on slope environments within the Mancos Shale, although some of them may have been transported seaward to become interbedded with lower slope or even basin deposits. Two types of deposits that crop out on Mount Garfield have not been previously described from the Mancos Shale. They are interpreted here as submarine debris flows and slump blocks (Fig. 1). Some ambiguity in interpretation derives from the fact that these deposits are slightly more resistant to erosion than the surrounding Mancos Shale, and therefore hold up ledges (Fig. 2) on which talus and other modern erosion products accumulate (Muhr, 1980). It is locally difficult to distinguish the modern from the ancient deposits because of the slurry of mud that has coated these outcrops during rainstorms. Although the resistant Cretaceous beds were depicted as Cretaceous strata in sketches of the Book Cliffs in the earliest geologic studies of the area (Peale, 1876, 1878), more recently 'they have been inferred to be the remnants of relatively recent landslide deposits (Lohman, 1965, fig. 26), or the remains of Pleistocene outwash terraces or pediments (Young et al., 1981, p. 18). As will be shown, however, (1) the lower parts of these ledges below the recent talus are in situ Cretaceous beds within the Mancos Shale, and (2) the in situ beds are the result of submarine debris flows and slump blocks that originated on slopes beyond the distal edge of the wide, shallow shelf areas of the Interior Seaway. The deep-water/offshore marine sedimentary rocks characteristic of the basin environment in the Interior Seaway consist predominantly of limestones and shales. However, sandstone and siltstone deposits have been recognized within these otherwise fine-grained sedimentary rocks (e.g., Winn et al., 1985, 1987). Derived from sources on the shelf or shoreline, many of these are inferred to be turbidite deposits, which produced relatively coarse-grained lenses of sandstone within the shale deposits. Two such sandstone beds exist within the Mancos Shale on Mount Garfield. Sandstone turbidites have been de-

J.C. L O R E N Z A N D C . A . M U H R

scribed from the Mancos Shale in the Uinta basin to the west (Balsley, 1982), and equivalent deepwater sandstones occur in the Eagle basin to the east (Krystinik, 1983). Sandstones have not yet been described within the Mancos Shale in the Piceance Creek basin, although the nearby "Mancos B" (in the subsurface along the adjacent Douglas Creek Arch) has been variously described as a "turbidite-contourite" deposit (Witherbee et al., 1983) or as a "shelf-sand complex" (Cole and Young, 1987). Submarine debris-flow deposits and slump blocks on Mount Garfield

Debris-flow deposits Debris-flow deposits (Fig. 1) on Mount Garfield occur as lenses 200-400 m in apparent width and on the order of several tens of meters thick. The true dimensions are obscured by recent sloughing of the Mancos Shale, erosion, and by the capping of recent talus. The basal contacts of the debris-flow deposits are usually planar and parallel to the bedding of the underlying Mancos Shale (Fig. 3). The presence of rounded centimeter-scale rip-up clasts of Mancos Shale, abundant in the lower few meters (Fig. 4), might indicate that this is an erosional contact, although the absence of relief on this plane suggests that scour was negligible. Above the basal meter of small rip-up clasts, the debris-flow deposits consist of larger matrixsupported clasts of Mancos Shale and, more rarely, sandstone and centimeter-scale pieces of carbonaceous debris. The mudstone clasts range in size from centimeters to a few meters, and they are very poorly sorted. Contorted bedding within the larger blocks of Mancos indicates syndepositional plasticity and incomplete lithification. The basal planar contacts of the debris-flow deposits display linear features that suggest softsediment shear during transport and deposition. The strikes of the lineations, measured at the base of five different debris flows along the Book Cliffs, range from 155 ° to 65 °, although the basal lineations of any single lens are unidirectional. The surfaces with lineations have a gentle relief of only

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Fig. 1. Location map of Mount Garfield, near Grand Junction, northwestern Colorado (U.S.A.), and schematic sections through the offshore deposits on Mount Garfield. a few centimeters over widths of many meters, and the individual lineations are less than 0.1 m m in height and width, more resembling smeared-out,

clayey contacts. Sense of motion along the lineations is not indicated. The lineations are commonly highlighted by iron oxide staining.

Fig. 2. An erosion-resistant ledge holds up a caprock of modem talus (above level of arrow), further protecting the ledge from erosion. Vertical face of cliff is about 15 m high.

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J.C. LORENZ and C.A. MUHR

Fig. 3. A sharp, planar, bedding-parallel contact (arrow) is the most c o m m o n relationship between debris-flow deposits and the underlying Mancos Shale.

Only one steep-sided contact, inferred to be an erosional channel margin, was found. This outcrop suggests local erosive scouring, perhaps 5-10 m down into the Mancos Shale, by the debris flow, or else an earlier channel-forming process that left a hollow to be filled by a later debris flow. The only other lateral contact observed consists of an inclined plane marked by similar soft-sediment lineations. The plane dips 45-60 ° and strikes

at nearly right angles to the trend of the local basal lineations. Lineations on the dipping plane indicate dip-slip latest motion. This shear plane does not extend down into the Mancos Shale below the base of the debris flow. In this area, debris flow deposits appear to be about 50 m thick, but the contact with the overlying recent talus is obscured. A large block of intact Mancos Shale is present within the debris-flow mudstone conglomerate.

Slump blocks

Fig. 4. Intraelast conglomerate, showing some of the smaller rounded d a s t s of Mancos Shale, from the basal meter of a debris-flow deposit.

The upper ledge within the Mancos Shale on Mount Garfield (Fig. 1) is a bed consisting of large-scale (tens of meters) rotated blocks of Mancos, in which the originally horizontal bedcling now dips to the northwest, toward the paleoshoreline (Figs. 2, 5). The rotated blocks overlie a planar surface that displays basal lineations identical to those previously described below the debris flows, Most of these lineations are oriented at 150 ° , perpendicular to the strike of bedding in the rotated blocks, but becoming divergent outward from this trend near the margins of the deposit. This unit of rotated bedding is on the

SLOPE-DERIVED DEPOSITS F R O M T H E C R E T A C E O U S I N T E R I O R SEAWAY

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Fig. 5. Rotated blocks of Mancos Shale (A), within normally bedded Mancos Shale (B). C = modern talus; D = sandstones. Cliff face approximately 20 m high.

o r d e r of 20 m t h i c k a n d a b o u t 1 k m in l a t e r a l extent. Locally, the b a s a l m e t e r of the s l u m p - b l o c k unit consists of r o u n d e d clasts of M a n c o s Shale

similar to those f o u n d in the d e b r i s - f l o w deposits. I n a few o u t c r o p s , a d i s t i n c t 2 - m - t h i c k unit o f the u p p e r m o s t M a n c o s Shale b e l o w the s l u m p disp l a y s d i s r u p t i o n c o n s i s t i n g of n u m e r o u s , small,

Fig. 6. Curved contact (arrows) between the lateral margin of the rotated slump blocks (righ0 and the Mancos Shale. Smatler arrow denotes obscure secondary scar.

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J.C. L O R E N Z and C.A. M U H R

Fig. 7. Internal shear planes (dipping to left) within a rotated block of Mancos Shale (where bedding dips to the fight). Bentonite marker beds are offset.(Tracing from photograph.)

vertical fault planes with up to 10 cm of vertical offset, creating irregularities that were apparently sheared off by the slump block. One occurrence of centimeter-scale drag-induced contortion of the underlying Mancos Shale was found. In most cases, however, the inclined bedding planes abut undisturbed underlying Mancos Shale at a beddingparallel, sheared contact. The northwestern termination of the slumpblock unit consists of gently curving surfaces, concave upward and toward the rotated blocks (Fig. 6), and becoming coincident with the planar basal contact. Two subparallel curved surfaces can be distinguished, separated by 4 - 5 m toward the top but joining to a common surface near the base. One is marked by a 5-cm-thick zone of indurated, millimeter-scale, Mancos intraclast breccia; the other is obscured by weathering and surficial creep of the weathered shale. In several exposures of the slump-block bed, internal shearing perpendicular to the rotated bedding planes is present (Fig. 7), and is clearly outlined by offset bentonites. The shear planes are spaced about half a meter to a meter apart, and offsets are of similar scale. Some of the rock between the shear planes has deformed plastically as well, the bedding being deformed with the same sense of motion but without the discrete planes of

offset. Bedding dips toward the paleoshoreline, whereas the shear planes dip away from it and show a normal offset.

Interpretation Age of the deposits Several lines of evidence show that the slump blocks and debris-flow deposits are of Cretaceous age. First, the planar basal contacts with the Mancos Shale are dissimilar to the common highrelief Quaternary erosion surfaces in the area. Moreover, the planar horizon of the contacts can be seen to extend back into the mountainside parallel to the bedding in the Mancos Shale: they do not ramp up toward the escarpment as do obviously recent, talus-covered erosional slopes. The presence of numerous of these deposits at two well-defined and distinct stratigraphic horizons along 10 km of the Book Cliffs is further evidence that they did not form on an irregular, post-Cretaceous erosional surface. Secondly, the clasts within the debris-flow deposits are distinct from those produced by recent erosion and found in the modern talus that caps these resistant ledges, Sandstone blocks are rare in the debris-flow deposits, and are significantly smaller than those c o m m o n in the talus.

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SLOPE-DERIVED DEPOSITS F R O M T H E C R E T A C E O U S I N T E R I O R SEAWAY

Meter-scale clasts of Mancos Shale, such as occur in the Cretaceous debris-flow deposits, are not presently being formed by erosion. Moreover, whereas the centimeter-scale, modern shale clasts are tabular and reflect the fissility of the eroded shale, the Mancos clasts in this size range contained by the Cretaceous debris-flow deposits are subspherical and rounded (Fig. 4), indicating poor consolidation at the time of erosion and resedimentation. Finally, in order to test the hypothesis that these units are not recent in origin, we submitted five samples of debris-flow strata to two different laboratories for microfossil analysis. Results show no contamination of the samples by recent palynomorphs, and both laboratories concluded that the probable age of the samples is no younger than Maastrichtian. (Stratigraphic correlations suggest a Campanian age.) Moreover, foraminifera and palynomorph assemblages from debris-flow samples suggest a shallow-marine environment (and therefore possibly significant transport into deep-marine environments), but unfortunately the samples taken from the subjacent Mancos Shale

were barren, and no definitive environmental comparison can be made on this basis.

Origin of the deposits The upper bed (Fig. 1) is interpreted as a group of slump blocks that originated on an unstable slope on the Cretaceous sea floor (Fig. 8). They rotated along listric faults, leaving slump scars (Fig. 6), and slid seaward. The multiple fistric planes seen at the head of the slump are similar to those diagrammed by Prior and Coleman (1982, their fig. 4) in modern Golf of Mexico deposits. As the rotated blocks accommodated their shape to the flat basal contact, they locally developed internal shear planes (Fig. 8). Debris flows were probably derived from slumps, and the two types of deposits are believed to be proximal and distal facies equivalents, much as suggested by Morris (1971) for similar deposits in Arkansas, or as described by Hampton (1972) from laboratory experiments. The identical nature of the basal contacts of the two types of deposits shows that they are related. The Mount Garfield slump blocks cannot be traced laterally into de-

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bris-flow deposits due to erosion of equivalent distal strata, although blocks that contain the most abundant shear planes are those that occur farthest from the slump scar and these blocks may represent the early stages of disintegration and the transition from slump block to debris flow. The disintegrated slumps became fluidized and flowed downslope into deeper waters as debris flows, much as described by Prior and Coleman (1982) for slumps and mudflows off the modern Mississippi River. Most of the Cretaceous debrisflow deposits have non-erosive bases, perhaps having acted much like a slab avalanche, whereby a mass of material is carried along on top of a sliding surface characterized by poor adhesion (Armstrong et al., 1974). Similar mechanisms in submarine environments could be produced by layers of high pore pressure associated with rapid sedimentation and generation of gasses, as reported by Prior and Coleman (1982, p. 34) offshore from the Mississippi River delta system. Such systems are unstable and prone to failure even on slopes of less than one degree (Morgenstern, 1967; Gorsline, 1984). Locally, the debris flows may have become channelized as they flowed seaward. The channeling could be the result of the infilling of gullies and turbidite by-pass channels that exist on many modern submarine slopes (Stanley and Unrug, 1972), rather than the result of extensive erosion by the flow itself. Most of the intraclasts were probably derived from the original Mancos Shale slump material, rather than having been eroded directly from the Mancos Shale that the deposits now overlie. The debris-flow deposits are laterally restricted "pods", rather than linear features. Where they cap isolated outlying hilltops in front of the Book Cliffs escarpment, there is often no extension of the deposit evident in the adjacent hillside, despite the obviously planar-horizontal basal contacts that would project into the escarpment. This suggests that the flows were transported as discrete units and represent localized mounds of debris deposited not too distant from the slump scar at which they originated. Middleton and Hampton (1973) have described such localization of experimentally produced deposits, wherein debris flows "pulled apart" into restricted units separated en-

J.C. L O R E N Z and C.A. M U H R

tirely from the source area and the parent mass. Prior and Coleman (1982) also report that a zone of transport and non-sedimentation commonly exists between slump areas and the resulting distal mud-flow lobes off of the modern Mississippi River. The lineations at the basal contacts with the Mancos Shale record the direction of motion of the sediment during transport. Basal lineations in other formations have been attributed to a zone of shearing that occurs at the basal parts of submarine debris flows (Hampton, 1972; Middleton and Hampton, 1973; Rupke, 1978). The consistency in the orientation of these markings for a given debris-flow lens on Mount Garfield suggests collinear, laminar motion throughout the central part of the flow at the time of deposition. The lineations are dissimilar to the few detailed descriptions of ancient debris-flow sole markings that have been published (e.g., Stauffer, 1967). The difference between the characteristics reported here and those described from other debris-flow deposits is most likely due to the different compositions of the various deposits: those described here consist predominantly of soft mudstone intraclasts in a muddy matrix, whereas most other descriptions are of sandy or even conglomeratic debris flows, where tool marks were commonly impressed into the underlying strata. Debris-flow deposits are common along two stratigraphic horizons in the Mancos Shale northeast of Grand Junction. (Slump blocks have been recognized only at the southern limit of the upper horizon, on Mount Garfield.) The stratigraphic position of the two horizons--within dark shales, 140 and 270 m below the shoreline deposits that eventually prograded over the area--indicates that this sedimentation took place well offshore (although it does not provide an estimate of water depth). Estimates of the average width of the shallow-marine shelf in the interior Seaway are on the order of 100-200 km (e.g., Asquith, 1970; Boyles and Scott, 1982). Ammonites found in undisturbed Mancos Shale that is equivalent to the slump-block and debris-flow deposits on Mount Garfield have been identified as Baculites asperiformis and Baculites perplexus, respectively (identification courtesy of W.A. Cobban, personal

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SLOPE-DERIVED DEPOSITS FROM THE CRETACEOUS INTERIOR SEAWAY

communication, 1987). This coincides with the ammonite zones in the Mancos Shale for this part of the Book Cliffs reported by Gill and Hail (1975). Shorelines for these ammonite zones are mapped at about 200 km to the north-northwest (Gill and Cobban, 1973) and west (Fouch et al., 1983). Age- and facies-equivalent, shallow-marine shelf deposits of the Duffy Mountain Sandstone, located between the mapped shoreline and Mount Garfield, have been described (Boyles and Scott, 1982). Thus, the facies relationships and the distance offshore of the Mount Garfield deposits are compatible with interpretations of a shelf-edge slope environment. Multiple debris-flow lenses over a wide area at a given stratigraphic horizon may (1) indicate a common depth of erosion of several non-synchronous debris-flow events down to a resistant horizon within the Mancos Shale, (2) represent a regional synchronous response to short-term events such as a storm or an earthquake, or (3) record regional long-term events such as sea level changes or rapid sedimentation in response to tectonism and elevated erosion rates in the source area. The third choice is preferred here. The possible linkage between source-area tectonism and slumping/debris-flow occurrence will be discussed further below, but it may be noted that, as mapped in McGooky et al. (1972, fig. 40, p. 218), Mount Garfield lies in the center of a large embayment of the Cretaceous shoreline (Fig. 9). Sediments derived from source areas all around this embay-

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ment must have been concentrated at this depocenter, leading to locally elevated sedimentation rates and associated excess pore pressures, and creating optimum conditions for unstable slopes in this area. The presence of the large embayment may in fact help to explain the occurrence of slumps and debris flows on Mount Garfield, whereas they have not been reported from possible slope areas elsewhere in the area of the Interior Seaway.

Modern analogs Modern shelf-edge/slope slump blocks and debris flows are known primarily from indirect means, such as sonar and seismic lines. Slump scars of similar dimensions as the deposits described here have been described by Coleman et al. (1981), Prior and Coleman (1982) and Almagor and Garfunkel (1979), at the shelf edge and slope off of the Nile delta, Mississippi delta, and coast of Israel respectively. Almagor and Garfunkel report that the slumps usually degenerate into debris flows but that, occasionally, coherent rotated "blocks and slabs" of sediment are produced. Piper et al. (1985) have also suggested that slump blocks are facies equivalents to mud-clast debris flows on the Scotian shelf. Few of the slope deposits in the vicinity of Mount Garfield have retained the slump-block configuration. The bedding within the others was apparently destroyed as they turned into debris flows. This is consistent with Almagor and Garfunkel's modern observations, and with Hampton's (1972) suggestion that the ready availability of water in the submarine environment would reduce the strength of slumps such that most would become debris flows. Mudstone-intraclast debris-flow conglomerates from modern submarine environments have been described briefly from cores from several geologic settings. Coleman (1981) and Prior and Coleman (1982) describe mudflows, containing erratic blocks up to 30 m in diameter, that originate in the shallows near the mouth of the Mississippi River. One- to five-centimeter clay clasts from the deep-water environments off of the Mississippi River cone, and clay clasts associated with contorted bedding on the Amazon River cone, were inferred to be local debris-flow deposits by Davies

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J.C. L O R E N Z and C.A. M U H R

(1972), and Damuth and Embley (1981), respectively. Similarly, intraclast conglomerates on the lower continental rise beyond the shelf of northwest Africa are inferred to be debris-flow deposits that traveled hundreds of kilometers over nearly flat deep-water slopes (Embley, 1976; Jacobi, 1976; Embley and Jacobi, 1977). McGregor et al. (1984) noted "pebble-sized clayballs" in inferred turbidite deposits seaward of the U.S. Atlantic shelf, while Piper et al. (1985) and Stanley and Silverberg (1969) noted clay-clast slumps and debris flows on the shelf off of southeastern Canada. Finally, Normark and Gutmacher (1988) suggest that rounded to subangular centimeter-scale mud clasts in a deep-water canyon-turbidite system off California are "slide" deposits. Such descriptions are commonly brief due to lack of data, and few of them are from areas that are similar to the Mount Garfield area: i.e., tectonically inactive, distant from a direct fluvial source of sediment, probably relatively shallow, and at the edge of a wide shelf that was constructed primarily by progradational deposition. However, the Cretaceous debris flows compare favorably with Hampton's (1972) hypothetical characteristics of submarine debris flows, and resemble "cohesive flows" as described by Lowe (1982).

Sandstone deposits within the Mancos Shale on Mount Garfield

Description Two sandstone deposits are present within the Mancos Shale on Mount Garfield (Fig. 1). The upper sandstone is present at all horizons about 8 m below the slump-block ledge (see Figs. 1, 5), and is on the order of several kilometers in lateral extent. The lower sandstone is present approximately the same distance below a debris flow, although its extent and exact position are obscured by sloughing of the Mancos Shale. The upper sandstone is about 14 m thick, while the lower one is about 3 m thick. They both consist of amalgamations of 1- to 4-cm-thick beds of fine- to medium-grained sandstone, with each thin bed capped by a shale unit about 0.5 cm thick (Fig. 10). Sedimentary structures and small-scale vertical grain-size trends are obscured by burrowing that has destroyed much of the original bedding, and that produced a mottled sandstone/ mudstone mixture. However, sharp basal contacts are still evident locally, overlying disrupted but distinctly shaly laminae. A less intensely burrowed outcrop displays suggestions of current ripples. Individual thin beds can be traced for the entire

Fig. 10. Centimeter-scale,fining-upwardcouplets of burrowed sandstoneand shale.

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Fig. 11. Compositenature of the upper sandstone deposit: a lower, coarsening-upwardunit, a separation bedding plane marked by siderite concretions(arrow), and an upper unit. Photograph shows about 15 m of vertical exposure.

width of the main outcrop, and thus extend a minimum distance of 30 m. There are two subunits of the upper sandstone bed (Fig. 11). The two subunits can be traced as a couplet for at least 2 km laterally, without change in character except that they both thin to about half of their original thickness. Only poor exposure prevents further tracing, but a general color change at this horizon can be followed for at least 10 km. The lower subunit is a coarsening-upward siltstone to fine-grained sandstone sequence that ends abruptly at a planar horizon marked by concretions. The upper subunit consists of medium-grained sandstone. It is abruptly overlain by unburrowed, dark-gray to black Mancos Shale.

Interpretation Each thin, rippled, fining-upward sandstoneto-shale bed represents a sedimentation event. However, bioturbation occurred between each event, destroying sedimentary structures, and it may not be possible to strictly identify the environment of deposition of these deposits. The centimeter-scale fining-upward trends and the position of these beds below inferred slope de-

posits suggests a distal submarine turbidite fan, as described by turbidite models such as Walker's (1979), but other deep-water sedimentation models may apply. Winn et al. (1987) have described similar burrowed sandy facies in the Lewis Shale of Wyoming, inferring that they are turbidites, but Cole and Young (1987) have described a somewhat similar and possibly time-equivalent facies west of Mount Garfield as a shelf-sand complex. These beds are not winnowed shallow-marine deposits, which should contain remnants of oscillation ripples, crossbeds, or even hummocky crossbedding. The reworking necessary to winnow such thick sands from the Mancos Shale would not have allowed for the preservation of the observed abundant burrowing. Although these sandstones have some similarities to deep-water contourites and winnowed turbidites as described by LoveU and Stow (1981), the presence of the thin, obscured, fining-upward couplets argues against this interpretation. The coarsening-upward lower subunit of the upper sandstone suggests a prograding environment. The planar upper limit of this lower unit, marked by concretions, would indicate a hiatus in sedimentation. The overlying upper subunit is ini-

80

tially relatively coarse grained, and the first sedimentation events of this subunit may have carried away any Mancos mud deposited during the hiatus. The unburrowed Mancos Shale that abruptly overlies the sandstone indicates a rapid abandonment of this sedimentary environment, and possibly the deepening of the waters and/or anaerobic seafloor conditions. The thinner lower sandstone, located below the lower debris-flow ledge on Mount Garfield, is inferred to be of similar origin, although limited exposure makes interpretations difficult. It is only exposed naturally in a few gullies near the debrisflow deposits, but trenching has revealed its presence elsewhere. Biostratigraphic correlations based on ammonite data indicate that these sandstones were located too far offshore (about 200 km) to have been directly sourced from a delta front. The ageand facies-equivalent shelf deposits include the Duffy Mountain Sandstone to the northwest of Mount Garfield. Boyles and Scott (1982) have interpreted the Duffy Mountain Sandstone as a shallow-marine bar complex that migrated southwestward across the shelf. Ammonite data in Gill and Hail (1975) indicate that the distal, shallowmarine facies of the Castlegate Sandstone is also age equivalent to this interval of the Mancos Shale. If these sandstones are turbidites, they could have been fed by the migration of shelf sands into bypass channels in the slope. The bioturbation indicates that, although a considerable thickness of sand accumulated, the overall sedimentation rate was low. The repeated sedimentation of centimeter-scale, fining-upward beds suggests that the source of sand could have been storm-induced repetitive avalanches of shelf bars into a slope channel, and "spillover" of shelf sands into deeper basinal environments as described in modern systems by Hill (1984), and McGregor et al. (1984). Discussion

Slope environments The shelf-edge slope environment has been depicted schematically in the Western Interior Sea-

J.C. L O R E N Z and C.A. M U H R

way by numerous authors as a sedimentologically uncharacterized transition between the shelf and basin. Asquith (1970) recognized shelf-edge slopes in the Cretaceous shales of Wyoming as distinctive clinoforms that were made obvious only by the vertical exaggeration and proper correlation of subsurface data. However, the sedimentary characteristics of the primary deposits of this environment are not distinctive. Stanley and Unrug (1972) suggested that, although there are probably no deposits that are uniquely indicative of submarine slopes, a general facies assemblage of turbidites, hemipelagites, "pebbly mudstones", slumps, and channel deposits is common on slopes. This is the general assemblage that is present at Mount Garfield. Hubert et al. (1972), and Weimer and Land (1975) have described soft-sediment deformation features from Western Interior marine strata, but these are smaller in scale than those described here, and are inferred by the authors to have originated on prodelta slopes that prograded onto the shelf, much nearer to the shoreline. Levy (1985) briefly described a large subaqueous slump from Cretaceous strata in Wyoming, but again this is associated with near-shore, prodelta sedimentation. Slope deposits with deformational features have been described in ancient formations from other parts of the world (e.g., Morris, 1971; Mutti and Ricci-Lucchi, 1972; Surlyk, 1987), but most of these are near-shore slopes, without an associated shelf, that were created as much by the active local tectonism as by the depositional regime. Moreover, the debris flows described by these authors are composed of sand and pebbles derived from outside the basin, whereas the debris-flow clasts described here are locally derived intraclasts. Perhaps the formation with the greatest similarity to the Mount Garfield facies is the CambroOrdovician carbonate slope facies of central Nevada described by Cook (1979). The composition of this facies, at the edge of a depositionally formed shelf, is up to 50% gravity-displaced deposits: slides, slumps, and intraclast debris-flow deposits with bedding-parallel basal contacts. However, the early self-cementing capabilities of the carbonates allowed for steeper slopes, more angular clasts, and more restricted deposits.

SLOPE-DERIVED

DEPOSITS FROM THE CRETACEOUS

INTERIOR

81

SEAWAY

Several other slump-block/debris-flow, slope deposits similar to those Of Mount Garfield occur along a linear distance of 8-10 km in this part of the Book Cliffs. They have not been found in examinations of the Book Cliffs for 30 km northwest or 10 km southeast of the locality. The northwest trend of the Book Cliffs in this area is approximately normal to the strike of the Cretaceous shelf-edge slope, assuming that the slope was roughly parallel to the northeasterly trend of the paleoshoreline in southwestern Wyoming. (The strike of the shorelines that eventually prograded across Mount Garfield was also generally northeast-southwest: Warner, 1964.) This suggests that the Cretaceous offshore slope was on the order of 8-10 km in width in this area, which is compatible with slope widths reconstructed by Asquith (1970) in central Wyoming. Although the Mount Garfield area was located at the center of a large marine embayment (Fig. 9), the strike of the local slope was probably east-northeast, as defined by the lineations that indicate south-southeast paleotransport below most of the debris-flow and slump-block deposits. There is no definitive upper-slope/lower-slope differentiation apparent along the Mount Garfield-Book Cliffs trend. The slump blocks occur near the seaward limit of the inferred slope facies of the upper horizon, suggesting that they may have originated on the lower slope. However, it is difficult to generalize from this one occurrence, and a slope width of only 8-10 km would not demand significant differences between upper and lower slope environments. Asquith (1970) reconstructed the slope relief between the shelf and the basin to have been on the order of 400 m or more, but this relief is not apparent in differences in thicknesses of strata along the Book Cliffs. Winn et al. (1987) suggest that in southwestern Wyoming, the total water depth in basin areas was "never more than a few hundred meters deep" (p. 879), and, by inference, the shelf-to-basin relief must have been considerably less. The height of the slope in the Mount Garfield area was probably more in accordance with the general magnitudes reconstructed by Winn et al. An interesting point is that the lateral distribution of slope deposits along the Book Cliffs ap-

pears to be similar for both the upper and lower stratigraphic horizons. This suggests a certain amount of stability in the position of the slope, despite minor fluctuations in sea level and the sedimentation that separated the two horizons. This also suggests that the slope in this area was of significantly less relief than that reconstructed for the slope in Wyoming by Asquith (1970). However, the seaward limits of the non-marine facies of the subsequent regressive shoreline and delta-plain environments (the coal-bearing facies of the overlying Corcoran and Cozzette Sandstones) occur in the vicinity of Mount Garfield (Warner, 1964), suggesting that they were not capable of prograding into the deeper waters beyond the shelf edge.

Cyclic deposition There may be significance to the twice-repeated sedimentary sequence of sandstone, abruptly overlain by several meters of shale which, in turn, are covered by debris-flow deposits or slump blocks (Fig. 12). We construct one possible model for this sequence, based in part on the Shanmugam and Moiola (1982) model of the interplay between deep-sea sedimentation and changes in eustatic sea level. Initially, sandstone accumulated in the Mount Garfield area, in the basin seaward of the slope. An age-equivalent shelf-bar complex is known to have been active landward of this area (Boyles and Scott, 1982), and the equivalent non-marine facies record a rapid influx of sediment (Kirschbaum, 1986; Fouch et al., 1983). This is the time interval of the Claggett transgression, which is inferred to have been a regional, eustatic rise in sea level (Kanffman, 1977). The curves of Haq et al. (1987) suggest that several 25- to 50-m sea level excursions occurred during this time, but it is unlikely that these would have been of sufficient magnitude to have produced the observed sedimentation patterns independent of tectonic subsidence. Repeated source-area overthrust activity leading to nearby basin subsidence was common in the Cretaceous Western Interior, and provides an attractive mechanism for governing sedimentation, as it initially increased the relief of source areas

82

J.C. L O R E N Z a n d C,A. M U H R

(resulting in accelerated rates of erosion and sedimentation) while concurrently increasing water depth due to subsidence (caused by supracrustal overthrust loading). Elevated sea level combined with subsidence raised wave base off the shelf sea floor, trapping sand in near-shore areas and stopping much of the movement of the shelf sands. More easily transported clays, however, would have continued to migrate to the shelf and slope, being supplied in quantities commensurate with the inferred elevated source-area erosion rates. The upward transition from burrowed laminated sandstone and shale to unburrowed black shales is suggested to record the cutoff of the sand supply to this environment due to deepening of the waters, although the cause of

100 m of Mancos Shale to ~, shallow-marine Corcoran Sandstone

slump

blocks

"o

~ ~~

sandstone

%-i :::::: :_-::_::%:: :::% :-:% :_---5:

"

50 m

i---

:__-i_-ii

Mancos Shale

::_-:::

..

i---

@

--

0

_-Z-_

debris flow

sandstone

the abruptness of the change is not apparent. Rapid sedimentation of clays resulted in failure along parts of the slope, and the shales that had been deposited over the sandstones were in turn covered by slump blocks and debris-flow deposits. Mancos Shale subsequently continued to accumulate, but at a lower rate as source-area relief diminished. Sedimentation or other changes in relative sea level raised the surface of deposition in the shelf area, allowing for the re-establishment of a shelf-sand complex. There are two such sedimentation sequences on Mount Garfield, thus there must have been a repetition of the combination of tectonic activity, sedimentation rate, and sea level change that produced these sequences. Winn et al. (1987) suggest that, similarly, high sedimentation rates were associated with high subsidence rates during deposition of the (younger) Lewis Shale in southwestern Wyoming.

Conclusion Three different types of offshore marine deposits are present within the Mancos Shale on and near Mount Garfield, northwestern Colorado. Although sandy deposits are increasingly being recognized in offshore rocks from the Cretaceous Interior Seaway, descriptions of debris-flow deposits and slump blocks are rare. The Mancos Shale exposed on the sides of Mount Garfield contains examples of the sedimentary record of the shelf-edge slope environment of the seaway. This environment was located about 200 km offshore during early late Campanian time. The sedimentary record consists of sandstone sequences of possible turbidite origin, and associated slope-derived slump blocks and intraclast debris flows. The vertical succession of these deposits was probably controlled by a linked combination of eustatic changes in sea level and tectonically induced changes in rates of sedimentation and basin subsidence.

Acknowledgements

transgressive

600 m of Mancos Shale to Dakota Sandstone

Fig. 12. Cyclic deposition of strata on Mount Garfield.

We would like to thank W.A. Cobban for his help in the identification of ammonite specimens

SLOPE-DERIVEDDEPOSITSFROMTHE CRETACEOUSINTERIORSEAWAY found

during

this s t u d y ,

and

R.A.

Young

for

s h a r i n g his k n o w l e d g e o f the B o o k Cliffs. W e thank Union Pacific Resources Company and Oak Ridge National Laboratories for providing funds f o r t h e m i c r o f o s s i l analyses. W e w o u l d also l i k e to acknowledge stimulating discussions and thoughtful r e v i e w s b y L . F . K r y s t i n i k , M . A . K i r s c h b a u m , a n d L.E. S h e p h e r d . C r i t i c a l r e v i e w s b y G . d e V . Klein and an anonymous reviewer have strengthe n e d this p a p e r . T h i s w o r k w a s s u p p o r t e d in p a r t b y the U . S . D e p a r t m e n t of E n e r g y ' s W e s t e r n G a s Sands Subprogram, administered by the Morgantown Energy Technology Center, under Contract No. DE-AC04-76DP00789.

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Young, R.G., Keighin, C.W. and Campbell, J.A., 1981. Second day log from Grand Junction to Glenwood Canyon and return to Grand Junction. In: R.C. Epis and J.F. Callender (Editors), Western Slope, Colorado. N.M. Geol. Soc., 32nd Field Conf. Guideb., pp. 17-28.