Cretaceous Continental Margin Sedimentation ...

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J. CASEY MOORE

Board of Studies in Earth Sciences, University of California, Santa Cruz, California 95060

Cretaceous Continental Margin Sedimentation, Southwestern Alaska Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.

ABSTRACT Cretaceous deep-water sedimentary rocks are discontinuously exposed or have been dredged along 1,650 km of the outer continental margin of the Alaska PeninsulaBering Sea Shelf; these rocks have been studied intensively in the outer Shumagin and Janak Islands, located on the continental shelf near the southwestern end of the Alaska Peninsula. Here, the sequence is comprised of monotonous sections of thin (4 cm) to thick (10 m) bedded sandstone and mudstone, showing grading, convolute lamination, and groove and flute casts. Petrographic studies indicate that the sandstone beds are lithic arenite with greater than 40 percent volcanically derived framework grains. The rocks have been subject to mild zeolite-grade metamorphism. Over 500 measurements of sole markings in the Shumagin and Sanak Islands show maxima to the southwest and west-northwest respectively, with minor lateral feed from the north. The data indicate turbidity current deposition in an elongate trough confined to the crescent trend of the 1,650 km belt of exposure. These ponded deep-water sediments occurring in an arcuate basin at a continental margin are interpreted to have been deposited in an ancient oceanic trench that bordered a Cretaceous volcanic arc. The deep-water rocks, a flysch deposit, comprise the central part of a Triassic to early Tertiary eugeosyncline exposed along the Shumagin-Kodiak Shelf. GEOLOGIC SETTING A thick sequence of Cretaceous sandstone and mudstone constitutes a major proportion of the continental shelf adjacent to the Alaska

Peninsula (Fig. 1). The Aleutian volcanic arc borders this sedimentary belt on the north; it is flanked by the Aleutian Trench to the south. The geologic history of these sandstone and mudstone beds is of particular interest because of their intimate location within this arctrench system. Near the western end of the Alaska Peninsula (about 55° N „ 162° W., Fig. 1) the Cretaceous sedimentary rocks intersect the Aleutian volcanic arc. These Cretaceous rocks have been studied intensively in two separate areas in this region, the outer Shumagin and Sanak Islands (Figs. 2 and 3). This report presents data and interpretation of the depositional framework of these rocks: current patterns, basin geometry, composition, provenance, mechanism of transport, and environment of deposition. Field work occupied six months, during the summers of 1969 and 1970. Detailed studies were supplemented by reconnaissance in other areas along the belt of late Mesozoic sedimentary rocks. Previous investigators in the Shumagin and (or) Sanak Islands include Grantz (1963) and Burk (1965). Burk applied the name Shumagin Formation to these exposures, and both workers correlated these rocks with the extensive belts of "slate and graywacke" on Kodiak Island and the Kenai Peninsula to the northeast. Burk (1965) correctly identified the rocks as turbidity current deposits and recognized their overwhelming volcanic provenance. On the basis of structural trends, Burk suggested that the Shumagin Formation extended from the Sanak Islands along the Bering Shelf edge (Fig. 1). A coeval shallow-water sequence is exposed on the Alaska Peninsula, northwest of the Shumagin-Kodiak Shelf (Fig. 1). These are dominantly clastic rocks deposited in shallow marine or nonmarine environments (Burk, 1965). They are more fossiliferous and much less intensely deformed than their equivalents on the Shumagin-Kodiak Shelf.

Geological Society of America Bulletin, v. 84, p. 595-614, 13 figs., February 1973 595

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Figure 1. Generalized geologic map of Alaska Peninsula, after Burk (1965) and Moore (1969).

STRATIGRAPHIC RELATIONS T h e thickness, age, and regional relations of the Shumagin Formation are difficult to define due to the monotonous lithology, intense deformation, and scarcity of fossils. Nevertheless, the range of these basic geologic characteristics can be described. Thickness T h e tight folds, pervasive faults, and absence of correctable paleontologic or lithologic marker horizons hinders estimates of stratigraphic thickness. Thickness measurements in this study were confined to relatively homoclinal sections between the axes of major folds. A 2,900-m section was measured by Jacob's staff along the n o r t h shore of Yvonne Harbor, Nagai Island, outer Shumagin Islands. A 4,100m section was measured off the m a p from a homoclinal section between two major folds inland from Northeast Bight, Nagai Island, outer Shumagin Islands. A 1,470-m section was measured along the coast south-southwest of Sanak Harbor in the Sanak Islands. The 4,1C0m section was estimated along a well-exposed ridge without obvious major folding or repetition by faulting. T h e above sections suggest

3 to 4 km as a reasonable minimum estimate of stratigraphic thickness of the Shumagin Formation. It is in sharp contrast to the 30,000-m section from the correlative Kodiak Formation. This figure was calculated by averaging dips over the :ype section on Kodiak Island (Moore, 1969). The author seriously doubts the validity of the 30-km thickness due to the complex deformation of the Kodiak Formation. Age The only age designation of the Shumagin Formation is based on sparse macrofossil collections. A number of samples have been examined for Foraminifera with negative results (Burk, 1965, p. 67). Six sample.; collected during this study were barren of coccoliths (Steven Percival, written commun., 1970). Inoceramus collections from various localities in the Shumagin Islands have been tentatively assigned to the Late Cretaceous (D. L. Jones, oral commun., 1970). These fossils are identical to those collected by Burk (1965) and G r a n t z (1963), which at that time were not definitely identified. N o fossil material was found in the Sanak Islands, but these exposures are tentatively assumed to be Late Cretaceous on the

Downloaded from gsabulletin.gsapubs.org on September 15, 2015 CRETACEOUS CONTINENTAL MARGIN SEDIMENTATION, ALASKA

Figure 2. Outer Shumagin Islands, showing distribution of Cretaceous sandstone and mudstone (blank with stippled margins) and early Tertiary granodiorite (hachured).

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Figure 3. Sanak Islands, showing distribution of chert-pillow lava-graywacke sequence (black), Cretaceous sandstone and mudstone (blank with stippled margins), and early Tertiary granodiorite (hachured). Quaternary alluvium not shown. basis of lithologic correlation with the outer Shumagin Islands. A brief reconnaissance during the summer of 1970 showed that the Kodiak Formation, Kodiak Island, is lithologically indistinguishable from the Shumagin Formation. Inoceramus kusiroensis of probably Maestrichtian age is known from "slate and graywacke" exposures on Kodiak Island and the Kenai Peninsula (S. Clark and D. L. Jones, oral commun., 1972). Samples of graded beds and siltstone lithologically similar to parts of the Shumagin Formation have been dredged from the acoustic basement of the Bering Sea Shelf edge near the Pribilof Islands (Fig. 1; Hopkins and others,

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1969). These samples have been assigned a Late Cretaceous age on the basis of benthonic Foraminifera (Hopkins and others, 1969). Inoceramus prisms, too fragmented for age assignment, were abundant in the dredged specimens. This correlation suggests that equivalents of the Shumagin Formation may constitute a significant portion of the acoustic basement at the edge of the Bering Shelf northwest of the Sanak Islands. According to available data, sedimentary rocks in the outer Shumagin and Sanak Islands may be tentatively assigned a Late Cretaceous age. These exposures and their stratigraphic equivalents to the northeast and northwest define an arcuate belt of deep-water sediment with a lateral extent of more than 1,650 km. Regional Relations T h e base of the Shumagin Formation is exposed only in a small area of the southwestern Sanak Islands. On Long Island, Sanak Islands, a sequence of pillow lava, bedded chert, and graded beds conformably underlie the Shumagin Formation. The areal extent of this exposure is roughly 15 sq km. These rocks are in fault contact to the northeast with the main body of Sanak Island. The age of the chertpillow lava-graded bed sequence is unknown. The top of the Shumagin Formation is not exposed. Older or younger rocks are exposed only in small isolated patches in the outer Shumagin and Sanak Islands. For this reason, it is worthwhile to review the stratigraphic relations of the Kodiak Islands in order to understand the position of the Shumagin Formation within the stratigraphic framework of the continental shelf. Kodiak Island is composed of sedimentary rocks bounded by northeast-trending faults. Stratigraphic units are successively younger to the southeast toward the present shelf edge. Triassic-Jurassic (?) chert, graded beds, ultramafic rocks, and pillow lava make up the northwest side of the island, Cretaceous sandstone and mudstone form a broad central core, and Paleocene and Eocene graded beds are exposed successively southeastward of the Cretaceous rocks (Moore, 1969). T h e simplest interpretation of this arrangement would be successive deposition of younger rocks at the seaward edge of the continental margin. The sedimentary sequence of the Kenai Peninsula, including the belt of slate and graywacke, decreases in age

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Figure 4. (A) Massive sandstone bed (about 10 m thick) intercalated with graded beds. (B) Folded intermediate beds. (C) Close-up of thin graded beds. (D) Photomicrograph of voclanic arenite, characteristic of massive sandstone and intermediate beds, crossed

niçois, width of view 3 mm. Predominant grain types are volcanic fragments, quar.z and plagioclase. Calcitefilled fractures cross-cut view from lower right to upper left.

southwestward. Correlations with the above stratigraphic arrangements suggest that the Shumagin Formation accumulated at the edge of the Cretaceous continental margin.

local indistinct lamination and rare channeling). Overlying lutite layers are generally < 15 cm thick. T h e basal contact of the sandstone beds is sharp, and the upper contact is generally distinct. T h e units are ;haracteristically even bedded. Most beds show no change in thickness for several hundred meters along strike. Intraformational mudstone breccia commonly occurs at the base or as discrete layers in the massive sand beds. T h e angular clasts range in diameter from i few centimeters to a few tens of centimeters. T h e mudstone clasts were derived by reworsing of substrata and were obviously only partly consolidated, as indicated by plastic deformation and sandstone dike injection into the clasts. Second generation breccia occurs locally, indicating relative uplift in parts of the trough during continuing deposition elsewhere. Contorted mudstone layers u p to 3 m ir. length are included in some of the massive sandstone beds. Pebble deposits of extrabasinal origin occur at the base of some massive sandstone beds.

PRINCIPAL ROCK TYPES T h e lithology of the Shumagin Formation consists of interbedded sandstone and mudstone (Figs. 4 and 5). T h e deposits are composed basically of massive sandstone beds and thin graded beds, with a continuous spectrum of rock types between them. Conglomeratic and volcanic ash deposits occur in minor amounts. Massive Sandstones T h e massive sandstone beds form thick prominent sedimentary units. T h e thickness ranges from < 1 m to > 20 m, but generally is between 1 and 5 m. The beds are characterized by high sandstone/shale ratios, very poor or nonexistent grading, and a general lack of internal structure (Fig. 4a; exceptions include


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surfaces vary from medium to dark gray (basal sandstone and siltstone) to grayish black (mudstone), weathering to lighter grays and buffs. Calcareous cement is occasionally present in the basal layers. Intermediate Bed Types fe^l

MUD STONE

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SANDSTONE,SHOWING CHANNELING,MUDSTONE CLASTS, AND CROSS BEDDING, FROM TOP TO BOTTOM

Figure 5. Representative stratigraphic section from Shumagin Formation, Yvonne Harbor, Nagai Island, Shumagin Islands. Section to left illustrates gross interbedding of graded beds and massive sandstone. Section to right shows details of graded beds.

Within some of the massive sandstone thin (5 cm) elongate ( > 3 m) mudstone slabs are oriented parallel to the basal contact. Such mudstone layers grade laterally into a zone of intraformational breccia. These undeformed slabs are interpreted as remnants of once continuous mudstone beds. C u t and fill structures are also present in some sandstone beds. Although the undeformed mudstone slabs and internal channeling are rare in the massive sandstone beds, they suggest that the larger beds represent succession of turbidity current deposits rather than a discrete event. The massive sandstone beds are highly indurated. The color of fresh dry surfaces ranges from medium-light gray to mediumdark gray; weathering causes lighter grayish, greenish, or brownish hues. Most sandstone is of medium grain size; small scale textural properties are difficult to determine macroscopically due to obscure grain boundaries. T h i n Graded Beds Thin graded beds form a second lithologic end member of the Shumagin Formation (Fig. 4C). They are characterized by sand:shale ratios always less than one—generally 1:3 or 1:4. Most of these units are 5 to 20 cm in thickness and evenly bedded. T h e basal layer ranges in grain size from fine sand to silt and grades upward to clay-size particles. Upper and lower contacts are sharp; the internal contact between sand or silt and clay may be distinct to gradational. Colors on fresh dry

By definition, the intermediate bed types include all mixtures of the coarse and fine end members described above. All variations exist, making the description of an average bed difficult. However, a typical intermediate bed may be 20 to 60 cm thick, with basal sandstone of medium-fine grain size, and a sand ¡shale ratio of 1:1 to 6:1. Generally the beds show welldeveloped internal and external structures. The complete spectrum of the intermediate bed types suggests lateral changes between the facies end members. This gradation could not be determined in the field due to lack of extensive exposures along strike. The basal and mid-portions of many intermediate beds are macroscopically and microscopically identical to the massive sandstone. Conglomerate and Ash Beds Scattered conglomerate and ash beds compose a small percentage ( < < 0.5 percent) of the Shumagin Formation. Conglomerate, excluding the intraformational breccia, composes well-defined units one to several meters thick. The beds characteristically exhibit an open framework with 15 to 40 percent rounded clasts as much as 20 cm in diameter. The pebbles are limy siltstone and sandstone, chert, and volcanic and plutonic rocks in variable proportions. The matrix varies from mudstone to poorly sorted sandstone. These conglomeratic deposits are similar to the pebbly mudstone of Crowell (1957) and probably represent thoroughly mixed slump deposits. Thin ( < 7 cm) light greenish-gray volcanic ash beds are exposed at two localities in the Shumagin Islands. These beds were not found elsewhere and were of no value in correlation. Proportions and Distribution of Facies Exposures of the Shumagin Formation average approximately 35 percent massive sandstone, 35 percent small graded beds, and 30 percent intermediate bed types, with extreme local variations. Conglomerate and ash deposits account for < 0.5 percent of total sediment volume. N o large concentrations of massive sandstone

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beds or small graded beds were noted in the Sanak Islands. In the outer Shumagin Islands, however, a lateral variation occurs from sections dominated by massive sandstone on the northwest to areas with abundant thin graded beds on the southeast. Massive units occur most commonly along the shoreline of northwestern Nagai Island, whereas small graded beds are abundant along the southwestern limit of sedimentary exposure—southern Xagai Island, Turner, Bendel and Spectacle Islands, and southern Big Koniuji Island. This lithologic distribution may represent either a lateral facies change or a vertical stratigraphic variation, or a combination of both. It is impossible to decide between the alternatives due to the intense deformation and lack of correlative marker horizons. PETROGRAPHY Petrographic studies of the Shumagin Formation included the inspection of more than 150 thin sections, resulting in 61 modal analyses. All point counted sections were from the basal part of sandstone beds. Analyses consisted of 300 to 500 counts per section on a 2 to 3 cm rectangular grid. These point counts were of a reconnaissance nature designed to indicate general composition and any obvious variation in provenance. The counted specimens were evenly distributed over exposures in the 700sq-mi shelf area in the outer Shumagin and Sanak Islands. T h e sample frequency and number of counts appear adequate in view of the close clustering of the data. Textural Characteristics

cite commonly forms turbid secondary cement. Quartz and calcite fill crosscutting veins. Most of the sandstone studied lies at or near the boundary between arenite and wacke (Williams and others, 1954). Therefore, all of these rocks may nor be textural graywacke, but certainly they are compositional graywacke with unstable lithic fragments more abundant than feldspar or quartz. T h e sandstone is not texturally immature (that is, with a bimodal grain-size distribution) but is nevertheless extremely immature compositionally, with a large proportion of volcanic lithic detritus (Fig. 4D). The increased proportion of matrix observed in sheared specimens is consistent with Cummins's (1962) view that much of the interstitial matrix in the classic graywacke is the result of alteration of larger detrital particles. In fact, the sandstone of the Shumagin Formation locally exhibits this transition—the volcanic and sedimentary particles show the effects of gradual breakdown to irresolvable interstitial fine material. Hawkins and Whetten (1969) have shown experimentally that graywacke matrix minerals may be produced from lithic sands at 250°C and 1 kb. Apparently, some of the matrix component of the Shumagin sandstone is secondary. In most rocks, then, the original detrital matrix was less than 10 percent. This value compares favorably with the 0.5 to 10 percent interstitial o u d common to deepsea sands (Kuenen, 1967). Geosynclinal graywacke is commonly bimodal, containing sand-size framework grains and 30 to 50 percent matrix. The occurrence of graywacke in ancient turbidity current deposits contrasts its absence in modern turbidities (deep-sea sand). Possibly the large volumes of graywacke, characteristic of mountain belts, originated as deep-sea sand, subsequently altered by diagenetic processes.

The textural components of the Shumagin Formation average 87 percent framework grains ( > us mm), 10 percent matrix ( < f c mm), and 3 percent cement (Fig. 6). In. practice, differentiating framework grains and matrix was very tedious due to continuous size variation. T h e detrital framework grains are angular to subangular and moderately sorted. Grain sizes range from coarse to fine o SHUMAGIN MASSIVE a sand (average medium) and fine sand to silt for INTERMEDIATE SANDSTONE the massive and intermediate units, and thin SANAK MASSIVE Et INTER«DIATE SANDSTONE graded beds, respectively. T h e highly indurated sandstones make mechanical size analysis SHUMAGIN a SANAK THIN GRADED BEDS practically impossible. However, microscopic observations show a generally unimodal, positively skewed grain size distribution. Matrix varies between 0 and 20 percent Figure 6. Textural characteristics of 61 point (Fig. 6). The greater amounts generally occur counted Shumagin Formation sandstone samples; note in specimens showing pervasive shearing. Cal- minor matrix and cement components.

Downloaded from gsabulletin.gsapubs.org on September 15, 2015 CRETACEOUS CONTINENTAL MARGIN SEDIMENTATION, ALASKA Composition The composition of the framework grains of the massive sandstone units, intermediate sandstone, and small graded beds has been studied in detail. Results of systematic point counting are plotted on a triangular diagram (Fig. 7) with end members of quartz + chert + quartzite (Q), total feldspar (F), and lithic fragments (L), roughly following the scheme of Dickinson (1970). In addition, ratios of these main constituents are tabulated (Table 1). These ratios are summarized for all point counted rocks in Table 1 and quoted for individual bed types in text. The petrology of a number of pebble and conglomeratic layers has also been studied microscopically. Massive and Intermediate Facies. The sandstone of the massive and intermediate facies is identical macroscopically and microscopically, and therefore considered together in this section. The sandstone is primarily composed of volcanic rock fragments, plagioclase, and quartz. Minor constituents include potassium feldspar, micas, heavy minerals, plutonic, metamorphic, and sedimentary rock fragments. Modal analyses were completed on 33 massive and 14 intermediate Shumagin-Sanak sandstone units. The average composition (main components are defined in preceeding paragraph) of 27 Shumagin rocks is Q21F28L51 and Q11F30L59 for 20 Sanak specimens. A plot of data from both island groups (Fig. 7) shows overlapping compositional fields, predominance of lithic fragments, and a greater proportion of feldspar relative to quartz. Plagioclase is much more abundant than potassium feldspar ( P : F = 0.92 Shumagin, 0.97 Sanak). Both microcline and sanidine are present, indicating respective plutonic and volcanic sources. Two types of plagioclase are QUARTZ+CHERT +OTZITE

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present, one fresh and unaltered and another highly sericitized. Volcanic rock fragments account for > 94 percent of all lithic material (V:L = 0.94 Shumagin, 0.98 Sanak); or, on the average, 51 percent of the framework grains and 44 percent of total rock volume are composed of volcanic lithic material. The matrix of the sandstone probably contains some altered volcanic lithic detritus. Most of the volcanic fragments show a pilotaxitic texture with felted groundmass. Devitrified glass in some fragments picked u p the yellow stain used to detect potassium feldspar. Outlines of glass shards could be seen in a few sections. The minor remaining lithic material consists of roughly equal amounts of igneous and sedimentary debfis, with rare metamorphic fragments. Plutonic rock fragments consist of intergrowths of quartz, microcline, and plagioclase. Lowgrade schistose siltstone makes u p most metamorphic lithic grains. Sedimentary debris is characterized by both shaley and arenaceous material. Shale particles, showing intricate deformation and squeezing between adjacent grains, were apparently derived from disintegration of intraformational clasts. These are not tabulated with the sedimentary rock fragments, since they are of intrabasinal origin and not suitable for determining true provenance. The ratio of chert to total quartzose grains is low ( C : Q = 0.13 Shumagin, 0.21 Sanak). Chert grains with included quartz veinlets were apparently eroded from a source area which had undergone some fracturing and solution. Quartz sandstone and quartzite fragments are extremely rare. Detrital micas (biotite > chlorite > muscovite) comprise < 1 percent (range 0 to 4 percent) of the massive and intermediate sandstone. Muscovite is always fresh; biotite may be partially altered to chlorite. All micas were distinguished by irregular extinction and kinking due to compaction. Opaque minerals make up < 1 percent of rock volume. Heavy minerals

TABLE 1. RATIOS AND STANDARD DEVIATIONS OF COMPONENTS OF THE END MEMBERS OF FIGURE 7

Components V:L P:F C:Q

Figure 7. Framework grain compositions from the 61 sandstone samples in Figure 6. Thin graded beds show slight enrichment in feldspar and quartz.

V:L B ratio lithic rock P:F « ratio C:Q • ratio

Number of measurements 61 61 61

Mean ratio .96 .97 .15

Standard deviation 0.04 0.05 0.12

of volcanic rock fragments to total unstable fragments. of plagioclase to total feldspar. of chert + quartzite to total siliceous grains.


J. CASEY MOORE

present in trace amounts include zircon, sphene(?), olivine, epidote, hornblende, and clinopyroxene. The apparent resemblance of the Shurragin and Sanak massive sandstones is strongly reinforced by similarity in petrographic modal composition, thus confirming their correlation on a lithologic basis. The abundant volcanic rock fragments indicate a dominantly extrusive source. Moreover, the predominant pilotaxiticfelted texture of this variety of lithic detritus suggests an andesitic source for both areas. This source accounts for at least 44 percent of rock volume on the basis of framework grains, and probably 60 percent considering contributions of feldspar, heavy minerals, and clays of volcanic origin. Granitic terrains made a significant but secondary contribution, judging f r o m the plutonic rock fragments, sericitized plagioclase, microline, and mica. T h e chert, reworked sedimentary material, and sparse metamorphic fragments indicate a minor but clearly defined nonigneous provenance. The quartz veins confined in chert fragments suggest a sedimentary to metasedimentary source which had undergone burial and some deformation. Volumetric contribution from plutonic and sedimentary + metamorphic sources is estimated to be 20 percent for each category (total 40 percent). T h e above provenance estimate applies to 65 percent of the Shumagin Formation (massive and intermediate beds). T h i n Graded Beds. Modal analyses of 14 thin sections from the basal portion of thin graded beds in the Shumagin and Sanak Islands indicate lithic fragments, plagioclase, and quartz as predominant detrital fragments. Potassium feldspar, micas (dominantly biotite), igneous debris, and metamorphic and sedimentary rock fragments are of secondary importance. T h e average composition of the 14 point counted sections is Q13F39L42. The scatter of analyses overlaps the respective compositional plots of massive and intermediate sandstones from both island groups. The ratio of volcanic to total lithic fragments is high (V:L = 0.96; the volcanic fraction composes about 40 percent of the framework grains or about 35 percent of total rock volume). T h e exact nature of many volcanic rock fragments is commonly difficult to determine. Most of these grains can be recognized only by a few microlites within an aphanitic, cloudy groundmass. Matrix, sedimentary grains, and volcanic fragments may not always

be accurately differentiated in these finegrained rocks. The plagioclase to total feldspar (P:F) ratio of 0.98 reflects the extreme rarity of potassium feldspar. Plagioclase occurs both sericitized and fresh, usual.y in the same thin section. Average chert to total siliceous (C:Q) ratio is 0.10. Chert again occurs with crosscutting quartz veins, terminating at the grain boundaries. Detrital micas (0 to 4 percent, avg 1.5 to 2 percent) are dominantly biotite with some chlorite and muscovite. Fragments of long-tube pumice are present in a number of the thin graded beds. The plot of modal analyses (Fig. 7) shows the clear overlap of the compositional fields of the massive and intermediate sandstones with the thin graded beds. This similarity strongly suggests a common provenance for the two facies. T h e average composition of the thin graded beds with lespect to the massive and intermediate sandstcne shows a decrease in lithic fragments which is roughly balanced by an increase in feldspar (massive and intermediate sandstone = Q17F29L54; thin graded beds = Q19F39L42). T h e additional feldspar in the thinner beds could have been derived from the breakdown of pilotaxitic volcanic fragments due to weathering, transport, or diagenetic alteration. T h e abundance of extrusive detritus over other types of lithic material proves that a large proportion of the thin graded beds were derived from a volcanic source (roughly 60 percent based on rock fragments, feldspars, and clays). The contribution from plutonic and sedimentary and metamorphic terrains is estimated at 20 percent for each category (total 40 percent). T h e provenance of the thin graded beds is identical to that of the massive and intermediate facies and applies to the entire Shumagin Formation. Conglomerate and Ash Beds. Although the conglomerate layers and ash beds are volume trically insignificant, they provide another measure of Shumagin Formation's source area. T h e most common pebbles (excluding obvious intraformational clasts) are chert, volcanic fragments (flow and pyroclastic), sedimentary rocks (wacke, arenite, and mudstone), and plutonic debris. T h e volcanic fragments are typically pilotaxitic with a felted groundmass. C h e r t pebbles are cut by veins of quartz which terminate at the grain

Downloaded from gsabulletin.gsapubs.org on September 15, 2015 CRETACEOUS CONTINENTAL MARGIN SEDIMENTATION, ALASKA boundary. The matrix of most conglomerate deposits is a volcanic arenite-wacke very similar to the typical massive and intermediate sandstones. The few ash beds consist of small broken feldspar crystals in a devitrified matrix showing the wispy remains of glass shards and long-tube pumice. Significantly, these ash beds are macroscopically and microscopically similar to portions of the submarine volcanic deposits of the Miocene Tokiwa Formation, Japan (Richard Fiske, oral commun., 1971). The ash beds provide the best documentation of concurrent volcanism during time of deposition of the Shumagin Formation. Glass shards in massive sandstone and long-tube pumice in thin graded beds also indicate a primary volcanic source. T h e gross lithology of the Shumagin Formation is not like subaqueous pyroclastic flows which have been described from the state of Washington and from Japan (Fiske, 1963; Fiske and Tokihiko, 1964). However, the highly angular, slightly altered volcanic lithic fragments and indicators of contemporaneous volcanism suggest a source area of freshly extruded lava and pyroclastic flows adjacent to the site of deposition. Active volcanoes are located within 25 km of submarine canyons which lead directly into the existing Aleutian Trench (Nichols and Perry, 1966). This suggests the obvious analogy of transporting volcanic sediments directly into a deep-sea environment with little abrasional modification. BURIAL M E T A M O R P H I S M The sandstone beds of the Shumagin Formation are highly indurated and very compact. Some of the matrix between sand-sized grains may have formed diagenetically, as previously discussed. The pervasive slaty cleavage apparently resulted from mechanical rotation of platy minerals during dewatering of the sediments (Maxwell, 1962). Other indications of diagenesis are present in thin section. The most widespread microscopic diagenetic effect is replacement of plagioclase by calcite. This alteration affects both individual detrital grains and phenocrysts in volcanic rock fragments. Some plagioclase is also altered to prehnite, laumontite, and possibly albite. These minerals are rare and only occur in specimens without calcareous cement and (or) calcareous alteration products, consistent with patterns of burial metamorphism in parts of

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the Great Valley Sequence of California (Dickinson and others, 1969). In contrast to the altered plagioclase, sheaths of detrital brown biotite remain fresh. Whole-rock x-ray studies confirm the optical identifications of prehnite and laumontite. X-ray data also indicate that well-crystallized illite (muscovite) and chlorite are the dominant clay minerals in the upper parts of the graded beds. The secondary zeolite minerals and wellordered clay minerals suggest that the Shumagin Formation has undergone mild burial effects. According to standard terminology, the rocks have been metamorphosed to the zeolite facies (laumontite-prehnite-quartz facies of Winkler, 1967). Distribution of zeolitic alteration appears to be nonstratigraphic. S E D I M E N T A R Y STRUCTURES Internal Structures Internal size variation is the most prominent sedimentary structure of the thin graded and intermediate bed types. The upward decrease in grain size may be regular and continuous, or abrupt, allowing clear separation of sandstone and mudstone layers. Size fluctuations and lamination commonly occur within an individual graded unit. Cross-bedding and convolute lamination are found in both the intermediate beds and the thin graded beds. Cross-bedding is commonly well developed in the basal part of the thin graded beds and in the middle or upper levels of the intermediate bed types. The succession of internal structures is consistent with the sequence reported by Bouma (1962) in various European flysch deposits. The complete sequence was seen locally in the intermediate graded beds. The massive sandstone beds appear to consist of a composite of Bouma's A intervals; whereas, the thin graded beds are usually composed of his C-D-E or D-E intervals. External Structures The only significant external sedimentary structures in the Shumagin Formation are sole markings and trace fossils (discussed under a separate heading). Sole markings are subdivided into two groups: scour marks and tool marks. Scour marks are formed by sediment-laden turbulent eddies, whereas tool marks are attributed to the dragging of some object by the turbulent flow. Both indicate directions of


J. CASEY MOORE

Figure 8. (A) Large flute casts on base of massive sandstone bed 2 m thick. (B) Flute casts, base of thin graded bed; scale is 15 cm in length. (C) Groove cast,

paleocurrent flow. Sole markings characterize turbidity current deposits and are not associated with contour (geostrophic) current deposits (Heezen and others, 1966). Scour Marks. Flute casts are the most prominent scour mark. T h e y range from a spectacular maximum of > 2 m X 75 cm X 20 cm (length-width-depth) to a minimum cf approximately 1 cm X 0.5 cm X 0.25 cm (Fig. 8). T h e largest flute casts occur on the sole of a massive sandstone bed < 2 m in thickness (Fig. 8A). This particular bed contained z large slab of highly contorted mudstone about 150 cm in length, apparently eroded by the same flow which scoured the large flute casts. T h e smallest flutes are associated with the thin graded beds. Longitudinal furrows and ridges are alternately developed and continuous over large areas of the basal sand bed. The furrows are rounded in section and may terminate at one end in a convex beak (interpreted as the upcurrent direction). The wave length of these structures (ridge-to-ridge distance in cross section) varies from a few millimeters (small graded beds) to 3 cm (massive sandstone).

base of massive sandstone. (E>) Trace fossil (Helminthoida crassa), base of Intermediate bed; scale is 15 cm in length.

Another type of sccur mark includes erosional depressions formed around the upcurrent side of obstacles. Tool M a r k s . Groove casts (Fig. 8C) are the most abundant tool marks; brush, bounce, and prod marks are of secondary importance. The usage follows Dzulynski and Walton (1965) and includes features commonly called drag marks or drag casts. Groove casts range in width from a few millimeters u p to 25 cm with maximum d e p t h to 1.5 cm. T h e largest are associated with the massive sandstone beds; narrow groove casts occur on beds of any size. Groove casts generally are continuous for the length of the outcrop. Brush, boun:e, and prod marks occur on the soles of all types of beds; maximum observed dimensions are u p to 3 cm wide and 10 cm in length. Deformational Structures T h e deformational structures described here include only those obviously of sedimentary origin. The larger scale tectonic deformation (which may have occurred while the sediments were semiconsolidated) is c.iscussed in another paper.


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Slump deposits average about 1 m in thickness; some have well-developed, measurable folds. The internal character varies from overfolded beds to extreme cases of contorted sandstone suspended in argillaceous matrix. In all cases, the slumps are exposed between undeformed beds. A particularly large slump is exposed on the north shore of Elma Island in the Sanak Islands. It is 135 m thick, containing blocks of graded beds u p to 8 X 20 m. A somewhat sheared matrix surrounds the large sandstone masses, suggesting that mass flow never reached a highly fluid state. N o exotic blocks appear in the slump, all inclusions having equivalent rock types in the Shumagin Formation. T h e slump is overlain by a massive 40 m sandstone bed with well-developed flute casts. Other deformational structures include load casts, load casted sole markings, and sand balls.

study. Field procedure involved direct measurement of the azimuth of the current feature and the dip of the respective basal bedding surface. All corrections for tectonic tilt were made on a stereonet by simple rotation around a horizontal axis. However, in certain areas of the Shumagin Islands, fold axes plunged at angles > 20° requiring two rotations (Ramsey, 1961). Arithmetic statistics were used exclusively in the analysis of paleocurrent data. Linear current indicators are by definition, never dispersed >180°. Directional current features in the Shumagin Formation are generally confined to one quadrant, always within two quadrants (dispersion