Search USGS⌕ Search

Geology topics

David W. Scholl

Publications and source records attributed to David W. Scholl.

35 records · Page 2Linked to original sources
USGS1994

Circum-North Pacific tectonostratigraphic terrane map

The companion tectonostratigraphic terrane and overlap assemblage of map the Circum-North Pacific presents a modern description of the major geologic and tectonic units of the region. The map illustrates both the onshore terranes and overlap volcanic assemblages of the region, and the major offshore geologic features. The map is the first collaborative compilation of the geology of the region at a scale of 1:5,000,000 by geologists of the Russian Far East, Japanese, Alaskan, Canadian, and U.S.A. Pacific Northwest. The map is designed to be a source of geologic information for all scientists interested in the region, and is designed to be used for several purposes, including regional tectonic analyses, mineral resource and metallogenic analyses (Nokleberg and others, 1993, 1994a), petroleum analyses, neotectonic analyses, and analyses of seismic hazards and volcanic hazards. This text contains an introduction, tectonic definitions, acknowledgments, descriptions of postaccretion stratified rock units, descriptions and stratigraphic columns for tectonostratigraphic terranes in onshore areas, and references for the companion map (Sheets 1 to 5). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). This map is the result of extensive geologic mapping and associated tectonic studies in the Russian Far East, Hokkaido Island of Japan, Alaska, the Canadian Cordillera, and the U.S.A. Pacific Northwest in the last few decades. Geologic mapping suggests that most of this region can be interpreted as a collage of fault-bounded tectonostratigraphic terranes that were accreted onto continental margins around the Circum- . orth Pacific mainly during the Mesozoic and Cenozoic (Fujita and ewberry, 1983; 1987; Parfenov, 1984, 1991; Howell, 1985; Watson and Fujita, 1985; Parfenov and Natal'in, 1984; Jones and others. 1987; Monger and Berg, 1987, Fujita and Cook. 1990; Zonenshain and others. 1990; Natal'in, 1991, 1993; Moore and others, 1992; Silberling and others, 1992; Nokleberg and others. 1992, 1993, 1994a; Parfenov and others. 1993; Plaflcer and Berg, 1994; Tabor, 1994). On the companion map and in the descriptions bel?w· terranes are interpreted according to inferred tectonic environments. These environments are (I) cratonai; (2) passive continental margin; (3) metamorphosed continental margin; (4) continental-margin arc; (5) island arc; (6) oceanic crust, seamount, and ophiolite; (7) accretionary wedge and subduction zone: (8) turbidite basin; and (9) metamorphic for terranes that are too highly-deformed and metamorphosed to de~ermine the original tectonic environment. For terranes with complex geologic histories, the chosen color indicates the tectonic environment most prevalent during this history of the terrane. The tectonic environments inf~rred for igneous rocks are both temporal and genetic. The temporal environments are preaccretion and postaccretion. The genetic environments are subduction-related, rift-related, and collisional (anatectic)- related. In addition to terranes, the map also depicts postaccretion units that include: (I) Cenozoic and Mesozoic overlap assemblages of sedimentary and volcanic rocks that are deposited across two or more terranes that formed generally after accretion of most terranes in the region; (2) Cenozoic and Mesozoic basinal deposits that occur within a terrane or on the craton; (3) plutonic rocks. The postaccretion igneous units are identified by age-lithologic abbreviations and by name. These overlap assemblages and basinal deposits formed mainly during sedimentation and magmatism that occurred after accretion of terranes to each other or to a continental margin. Overlap assemblages provide minimum ages on the timing of accretion of terranes. Some Cenozoic and Mesozoic overlap assemblages and basinal deposits, as well as fragments of terranes, are extensively offset by movement along postaccretion faults. In addition, in onshore areas, the map depicts major preaccretion plutonic rocks that are limited to individual terranes. and in offshore areas. the map depicts major oceanic plates,-ocean floor magnetic lineations. oceanic spreading ridges, and seamounts. The map consists of five sheets. Sheets I and 2 depict, at a scale of I :5.000.000. the tectonostratigraphic terranes. preaccretion plutonic rocks, and postaccretion Cenozoic and Mesozoic overlap sedimentary, volcanic. and plutonic assemblages, and basinal deposits for the Circum- orth Pacific including the Russian Far East, northern Hokkaido Island of Japan, Alaska. the Canadian Cordillera, part of the U.S.A. Pacific Northwest. and adjacent offshore areas. Sheet 3 provides the list of map units for Sheets I and 2. Sheet 4 is a index map showing generalized onshore terranes and overlap assemblages for onshore parts of the Circum-North Pacific at a scale of I: I 0,000,000. Sheet 4 is a guide to the more complicated onshore features depicted on Sheets I and 2. Sheet 5 is an index map showing the major geographic regions for the Circum-North Pacific. Significant differences exist between the representation of onshore and offshore geology on Sheets I and 2. These are: (I) compared to the onshore part of the map, the offshore part is depicted in a more schematic fashion because of more limited data and because the offshore terranes and early Cenozoic and older overlap assemblages generally are obscured by extensive late Cenozoic sedimentary cover that is not shown unless thicker than two kilometers; (2) marginal contacts of offshore Cenozoic and Cretaceous sedimentary basins do not match contacts of onshore Cenozoic and Cretaceous sedimentary units because offshore basins are limited to those regions with sediment thicknesses greater than two kilometers; (3) stratigraphic columns, included at the end of this explanation. are provided only for onshore terranes because the geology of offshore terranes is generally less well-known; and (4) for simplicity, the major onshore Cenozoic sedimentary basins are generally not defined and described separately because the onshore part of the map is designed to emphasize terranes and overlap volcanic assemblages that are crucial for both for tectonic and metallogenic analyses published elsewhere (Nokleberg and others, 1993, 1994a). Several key geologic sources were used in the compilation of the map. For Alaska. the basic outcrop pattern for the map is from Beikman (1980), Gehrels and Berg (1992, 1994). Barker and others ( 1994). Brew (1994), and Moli-Stalcup and others ( 1994b). The distribution of terranes is from Jones and others (1987) and Monger and Berg (1987), with modification by Grantz and other (1991 ). Worall (199 1 ), okleberg and others (1993, 1994a), the cited references, and the Alaskan co-authors of this report. For the Canadian Cordillera. the basic outcrop pattern is from Monger and Berg ( 1987), Wheeler and other (1988). and Wheeler and McFeeley ( 1991) with modifications by the Canadian authors. For the northern part of the Russian Far East. the basic outcrop pattern is from So unov (1985) with modifications by the Russian authors. For the outhern part of the Russian Far East, the basic outcrop pattern is from Krasny (1991) and Bazhanov and Oleinik ( 1986) with modification by the Russian authors. The Russian Far East part of the map is the first attempt to define and delineate terranes in that region. In their compilation. the Russian authors utilized the methodology of U.S.A. and Canadian geologists. Because this map is the first attempt to display the terranes. Cenozoic and Mesozoic overlap assemblages. basinal deposit , and plutonic belts of the Russian Far East. the Russian author will appreciate constructive sugge tions for improving the map.

Open-File Report↗
USGS1994

Geologic framework of the Aleutian arc, Alaska

The Aleutian arc is the arcuate arrangement of mountain ranges and flanking submerged margins that forms the northern rim of the Pacific Basin from the Kamchatka Peninsula (Russia) eastward more than 3,000 km to Cooke Inlet (Fig. 1). It consists of two very different segments that meet near Unimak Pass: the Aleutian Ridge segment to the west and the Alaska Peninsula-the Kodiak Island segment to the east. The Aleutian Ridge segment is a massive, mostly submerged cordillera that includes both the islands and the submerged pedestal from which they protrude. The Alaska Peninsula-Kodiak Island segment is composed of the Alaska Peninsula, its adjacent islands, and their continental and insular margins. The Bering Sea margin north of the Alaska Peninsula consists mostly of a wide continental shelf, some of which is underlain by rocks correlative with those on the Alaska Peninsula. There is no pre-Eocene record in rocks of the Aleutian Ridge segment, whereas rare fragments of Paleozoic rocks and extensive outcrops of Mesozoic rocks occur on the Alaska Peninsula. Since the late Eocene, and possibly since the early Eocene, the two segments have evolved somewhat similarly. Major plutonic and volcanic episodes, however, are not synchronous. Furthermore, uplift of the Alaska Peninsula-Kodiak Island segment in late Cenozoic time was more extensive than uplift of the Aleutian Ridge segment. It is probable that tectonic regimes along the Aleutian arc varied during the Tertiary in response to such factors as the directions and rates of convergence, to bathymetry and age of the subducting Pacific Plate, and to the volume of sediment in the Aleutian Trench. The Pacific and North American lithospheric plates converge along the inner wall of the Aleutian trench at about 85 to 90 mm/yr. Convergence is nearly at right angles along the Alaska Peninsula, but because of the arcuate shape of the Aleutian Ridge relative to the location of the plates' poles of rotation, the angle of convergence lessens to the west (Minster and Jordan, 1978). Along the central Aleutian Ridge, underthrusting is about 30° from normal to the volcanic axis. Motion between plates is approximately parallel along the western Aleutian Ridge. In this paper we briefly describe and interpret the Cenozoic evolution of the Aleutian arc by focusing on the onshore and offshore geologic frameworks in four of its sectors, two sectors each from the Aleutian Ridge and Alaska Peninsula-Kodiak Island segments (Fig. 1). We compare the geologic evolution of the segments and comment on the implications of some new, previously unpublished data.

Alaska↗
USGS1989

Late cretaceous pelagic sediments, volcanic ASH and biotas from near the Louisville hotspot, Pacific Plate, paleolatitude ∼42°S

Dredging on the deep inner slope of the Tonga Trench, immediately north of the intersection between the Louisville Ridge hotspot chain and the trench, recovered some Late Cretaceous (Maestrichtian) slightly tuffaceous pelagic sediments. They are inferred to have been scraped off a recently subducted Late Cretaceous guyot of the Louisville chain. In the vicinity of the Louisville hotspot (present location 50°26′S, 139°09′W; Late Cretaceous location ∼42°S, longitude unknown) Late Cretaceous rich diatom, radiolarian, silicoflagellate, foraminiferal and coccolith biotas, accumulated on the flanks of the guyot and are described in this paper. Rich sponge faunas are not described. ? Inoceramus prisms are present. Volcanic ash is of within-plate alkalic character. Isotope ratios in bulk carbonate δ 18 O − 2.63 to + 0.85, δ 13 C + 2.98 to 3.83) are normal for Pacific Maestrichtian sediments. The local CCD may have been shallower than the regional CCD, because of high organic productivity. In some samples Late Cretaceous materials have been mixed with Neogene materials. Mixing may have taken place on the flanks of the guyot during transit across the western Pacific, or on the trench slope during or after subduction and offscraping about 0.5 Ma.

Palaeogeography, Palaeoclimatology, Palaeoecology↗
USGS1982

Ancient plate boundaries in the Bering Sea region

Plate tectonic models of the Bering Sea suggest that the abyssal Bering Sea Basin is underlain by oceanic crust, a supposition supported by refraction and magnetic data. The oceanic crust is thought to be a remnant of the Kula(?) plate that was isolated within what is now the Bering Sea when the proto-Aleutian arc began to form between the Alaska Peninsula and Kamchatka in late Mesozoic or earliest Tertiary times. Prior to the formation of the Aleutian arc, the Kula(?) plate moved NW, directly underthrusting eastern Siberia; the plate’s eastern edge either obliquely underthrust or slid past the Bering Sea margin along a transform boundary. The Koryak Range in eastern Siberia is composed in part of mélange units that include Palaeozoic and Mesozoic allochthonous blocks juxtaposed within a matrix of Cretaceous sedimentary rocks. Structural trends suggest that these blocks were accreted into the Koryak area from the south along an ancient subduction zone formed by underthrusting of the Kula(?) plate. The base of the Bering Sea continental margin that extends from eastern Siberia to the Alaska Peninsula—the so-called Beringian margin—is underlain by a thick (7–10 km) sedimentary section along the base of the slope. Rocks dredged from the basement exposed farther up the slope (1500–2000 m deep) include shallow-water Upper Jurassic sandstone that is unconformably overlain by shallow-water Eocene to Miocene diatomaceous mudstone. Fauna in the dredge samples indicate that the shelf edge has subsided several kilometres since late Palaeogene time, perhaps in response to the cessation of motion relative to the adjacent oceanic plate and subsequent sediment loading of the oceanic plate. Uplift of the former plate boundary exposed in the Koryak Range occurred principally in late Cenozoic time, and collapse of the adjacent plate boundary, the Beringian margin, began in earliest Tertiary time and has continued to the present. Both tectonic events occurred after the site of active plate collision shifted south to near the present Aleutian Trench. We are uncertain as to why these two ancient, yet adjacent former plate boundaries should behave so differently, i.e. why one area was folded and uplifted while the other was extensionally deformed and subsided, both apparently in response to the cessation of convergent or strike-slip plate motion.

Geological Society, London, Special Publications↗
USGS1978

The Aleutian Basin, Bering Sea a frontier area for hydrocarbon exploration

The Aleutian Basin is the deep water (>3000 m) basin that lies north of the Aleutian Islands adjacent to the Bering Sea continental shelf. The basin, about the size of the state of Texas, is underlain by a 2-9 km-thick flat-lying sequence of mostly Cenozoic sediment and rock that includes diatomaceous silty clay interbedded with turbidities in the upper 1 km. Before 1974, geologic and geophysical investigations in the Aleutian Basin were directed at determining its geologic history; more recently, investigations have also been aimed at assessing the basin's hydrocarbon potential. The four major requirements for hydrocarbon accumulation may be present, namely, structural and stratigraphic traps, source rocks, reservoir beds, and an adequate thermal and sedimentation history. Energy resource investigations by the US Geological Survey indicate that: (1) numerous structural features (gentle folds, diapirs, basement ridges) are present in the central and eastern parts of the basin, (2) acoustic features (VAMP's _ Velocity AMPlitude features) that may be due to trapped gases within the sedimentary section are common (over 350 identified) in the central basin, (3) concentrations of organic gases, primarily methane, in the upper 1-3 m beneath the seafloor are very small, they increase with depth, and they are highest in areas near VAMP's, and (4) both the thermal gradient and the sediment thickness are sufficiently great to allow hydrocarbon maturation at depth, if source rocks are present. Adverse conditions in the Aleutian Basin such as excessive water depths and severe weather pose difficult technical problems for the recovery of hydrocarbons that may be present. The data nevertheless suggest the basin is a promising site for hydrocarbon accumulations and therefore warrants further exploration.

Conference Paper↗
USGS1978

VAMPs—Possible hydrocarbon-bearing structures in Bering Sea Basin: Geologic notes

Narrow (1 to 2 km) subsurface columns of concave reflection horizons are common time-base seismic profiles collected in the Bering Sea basin. The columns of recorded downflexures are thought to be velocity pulldowns and commonly are associated with one or more arched or gently domed high-amplitude reflection horizons about 100 m higher in the section. Inferred from this association is that subsurface deposits characterized by anomalously low acoustic velocity are present. We refer to the velocity-anomaly and reflection-amplitude association as a velocity-amplitude feature, or VAMP, and speculate that VAMPs are deep-seated “bright spots” underlain by a strong velocity pulldown possibly caused by gas-charged deposits.

AAPG Bulletin↗
USGS1976

Plate tectonic model for the evolution of the eastern Bering Sea Basin

The eastern Bering Sea Basin, composed of the Aleutian and Bowers Basins, is flanked to the north by Mesozoic foldbelts that probably represent zones of plate subduction in Mesozoic time. Present plate subduction occurs 400 to 1,000 km farther south, at the Aleutian Trench. North-south magnetic lineations that formed at an oceanic spreading ridge, probably in Mesozoic time (117 to 132 m.y. ago), have been identified in the Aleutian Basin. The orientation and age of those anomalies can be explained by reconstructing Kula-Farallon Pacific plate motions during late Mesozoic–early Tertiary time. In Mesozoic time, subduction of the Kula plate occurred north of the Aleutian Trench near the present location of the Bering Sea continental margin. At about 70 m.y. B.P. (Late Cretaceous), the zone of subduction shifted south to the present location of the Aleutian Trench, thereby trapping a fragment of oceanic plate imprinted with north-south magnetic lineations within the eastern Bering Sea Basin. A stable basin framework has prevailed behind the Aleutian arc since early Tertiary time.

GSA Bulletin↗
USGS1976

Structure and evolution of Bering Sea shelf south of St. Lawrence Island

The virtually featureless Beringian shelf south of St. Lawrence Island is underlain structurally by at least 14 basins. Encompassing a total area of more than 300,000 sq km, most of the basins are either elongate structural sags, grabens, or half (asymmetric) grabens beneath the outer shelf. The regional trend of these basins is northwest, parallel with that of the continental margin. Two of the basins, St. George and Navarin, contain 7 to 10 km of Upper Cretaceous(?) and Cenozoic sedimentary strata. A major divergence in dip of beds in the upper half of the sedimentary section may reflect an abrupt shelf-wide change in the rate of sedimentation and/or subsidence, probably during the Miocene. The outer sub-shelf basement grabens and adjacent ridges (horsts) are bounded by high-angle normal faults that exhibit growth-type structure. St. Matthew basin, an elongate, southwest-trending feature of the inner shelf, lies along the offshore expression of the Kaltag fault of western Alaska. The Kaltag fault, like the Denali fault in southwestern Alaska, does not extend to the outer Bering Sea shelf but ends or turns parallel with the margin within the inner shelf. The inner shelf is underlain by a broad basement high, Nunivak arch, the seaward half of which is characterized by an arcuate belt of high-frequency and high-amplitude magnetic anomalies. This zone of intense magnetic anomalies along the shelf is probably the signature of a Mesozoic magmatic arc that extends from southwestern Alaska to eastern Siberia and consists of Jurassic to Cretaceous plutonic and volcanic rocks. We speculate that this magmatic arc resulted from oblique convergence and subduction in the Mesozoic between the Kula(?) and North American plates along the eastern Beringian margin. Folding and uplift in the area of the present outer shelf occurred contemporaneously with magmatism along the inner shelf. Plate convergence apparently ceased by the end of the Mesozoic or t e beginning of the Cenozoic. Subsequently, the foldbelt underlying the outer shelf was eroded extensively and rifted extensionally to form large, deep basins. On the average, the shelf has subsided more than 1.5 km. Subsidence and sediment burial of the eroded orogen formed the modern Beringian shelf.

AAPG Bulletin↗
USGS1976

Preliminary residual magnetic map of the eastern Bering Shelf and parts of western Alaska

Residual magnetic anomalies for the offshore data were calculated by subtracting the Earth's main dipole field, adjusted for secular variations (based on the International Geomagnetic Reference Field (IGRF), epoch 1965, Fabiano and Peddie, 1969), from the observed values of the surveys listed below. The effects of diurnal variations and magnetic storms have been ignored. Individual marine magnetic profiles have been upward continued to an elevation of 1 km (Robinson, 1970; Henderson, 1970) on the assumption that the magnetic sources are two-dimensional in a direction normal to the ship's track. The upward continued profiles were computed by convolving each profile with an optimum (7, 15 or 30 points) upward continuation operator. Additional errors of unknown magnitude result from the use of two-dimensional analytic techniques in a three-dimensional field. However, on the average, values at crossings differ by only 10 percent.

Alaska↗
USGS1970

Peru-Chile Trench sediments and sea-floor spreading

The hypotheses of sea-floor spreading and plate tectonics require the removal of sediment from oceanic trenches either by crustal underthrusting or by folding against the base of a continental or insular margin. Accordingly, over a period of time the volume of sediment removed by way of spreading must be equal to the difference between the observable volume of undeformed terrigenous deposits in a trench and the volume contributed to it by continental erosion. To assess possible sediment loss from the central Chilean segment (23°–44° S.) of the Peru-Chile Trench, we have compared the volume of terrigenous deposits overlying the land, the continental margin, and filling the trench with that expected from continental denudation. Our data indicate that an episode of sediment removal occurred at the base of the margin and adjacent deep-sea floor in Late Cretaceous and perhaps earlymost Tertiary time and may imply spreading. Nearly 100 × 10 3 km 3 of deposits of Tertiary age, chiefly Eocene to Pliocene, have accumulated on the margin, and perhaps an additional 5 × 10 3 km 3 in the trench. This amount of offshore sediment could be supplied by fairly low rates (3 cm/10 3 yrs) of Tertiary erosion. However, many uncertainties in our denudation-sedimentation budget make it impossible to determine whether or not sediment reaching the base of the margin was removed tectonically in Tertiary time. Between 27° and 44° S., the trench contains nearly 70 × 10 s km 3 of turbidite deposits that we believe accumulated during late Cenozoic periods of glacially lowered sea level. The volume of turbidites in the trench is virtually equal to that expected from continental erosion, which is estimated to have probably been no greater than 5 cm/10 3 yr for the arid region between 27° and 31°, and 50 cm/10 3 yr for the humid and partially glaciated region from 36° to 42°. During this time of rapid erosion and trench filling, magnetic data indicate that convergence of lithospheric plates was taking place below the trench at a rate between 5 and 10 cm/yr. If turbidite deposits were swept from the trench at these rates, then continental denudation must have been exceedingly rapid: 20–40 cm/10 3 yr for the arid zone, and 110–165 cm/10 3 yr for the partially glaciated region. If more conventional estimates of erosion are valid, then either (1) late Cenozoic underthrusting has not taken place (or at a rate much slower than that implied by geophysical data), or (2) underthrusting at the prescribed rates has not involved the removal of a significant volume of sediment from the trench.

Antofagasta↗
USGS1970

Eocene age of the Adak ‘Paleozoic (?)’ rocks, Aleutian Islands, Alaska

In 1948, several specimens identified as the plant genus Annularia, a primitive horsetail of Pennsylvanian or Permian age, were found in tuffaceous sandstone exposed near the northern end of Adak Island, Alaska. These beds form the basal part of the Andrew Lake Formation, a newly named sequence of marine sedimentary rocks that is more than 850 m thick, and, in the main, consists of northwest-dipping tuffaceous sandstone, siltstone, shale, and siliceous siltstone and shale interbedded with basaltic flows or penecontemporaneous(?) sills (or both) a few tens of meters thick. This formation rests depositionally(?) on the Finger Bay Volcanics, the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks that form the bulk of Adak Island. Mollusks, foraminifers, sponge spicules, and fish scales and skeletal remains occur in the lower 350 m of the section immediately overlying the basal “Annularia”-bearing beds. Included in this fauna is the pecten Pro-peamussium (cf. P. stanfordensis Arnold), of probable Eocene age; the associated foraminiferal fauna is provincially considered to be of late Eocene (Narizian) age, and the fish scales are similar to those found in the Narizian and Refugian (Eocene and Oligocene) of California. Examination of the matrix surrounding specimens of “Annularia” revealed a substantial dinoflagellate flora—establishing that the “Annularia”- bearing beds are themselves marine units of middle or late Eocene age. The Andrew Lake Formation probably accumulated in a perched basin along the crestal region of an early Tertiary Aleutian ridge. Accordingly, there is no evidence for a Paleozoic Aleutian ridge. There is only scant evidence that the ridge existed in Mesozoic time.

Alaska↗
USGS1969

Modern coastal mangrove swamp stratigraphy and the ideal cyclothem

The general stratigraphy of the “ideal” cyclothem of Late Paleozoic age can be recognized in a modern succession of sedimentary units underlying the coastal mangrove swamps of southwestern Florida. Because coal deposition is associated with the formation of cyclothems, this stratigraphic similarity has geologic importance with respect to coal formation. The lower part of the succession in Florida consists of nonmarine sediments, the middle part of brackish-water or fresh-water mangrove peat, and the upper part of brackish-water and marine units. This sequence of sediments records a relative rise in sea level. In comparison, the lower part of the ideal cyclothem consists basically of nonmarine units, the central sedimentary member is coal, and the upper units are brackish-water and marine sediments. The ideal cyclothem is thought to have formed in part in a deltaic environment and to record a periodic fluctuation in terrigenous sediment supply and a relative rise in sea level. In contrast, southwestern Florida has essentially no deltas, as most of its paralic sediments are derived from coastal sources. In view of this, the stratigraphic similarity noted above must reflect a partial duplication of sedimentary environments brought about by a relative rise in sea level across a low coastal platform supporting peat-depositing paralic and fresh-water swamps and forests. This conclusion tends to support the point of view that the coal member of some cyclothems formed in a swampy environment penecontemporaneously with a relative rise in sea level. The coal member, therefore, is in part a transgressive unit.

Florida↗
USGS1969

Cretaceous, Tertiary, and early Pleistocene rocks from the continental margin in the Bering Sea

Rocks dredged from the continental margin in eastern Bering Sea in and near the Pribilof Canyon indicate that the acoustic basement represents the upper surface of thoroughly lithified turbidite beds of graywacke and siltstone of Late Cretaceous age. The stratified sequence covering the acoustic basement is gently deformed and includes marine clastic and diatomaceous sediments ranging in age from middle or late Miocene through early Pleistocene. Dense argillite, siltstone, and calcareous sandstone of early Tertiary age in and near the Zhemchug Canyon probably represent an older part of this sequence. The main layered sequence accumulated above the acoustic basement in shallow water, and, because the older beds now lie as much as 1000 m below sea level, the continental margin must have undergone considerable subsidence during late Tertiary and Quaternary time. A rich pollen flora indicates that the shoreline lay only a few tens of kilometers away from the site of the Pribilof Canyon during late Miocene time.

Alaska↗