Joe S. Creager

Sedimentary geology of the Columbia River Estuary

Abstract A multiyear study of the sedimentary geology of the Columbia River Estuary has provided valuable data regarding sediment distribution, bedform distribution, and suspended sediment distribution on spatial and temporal scales that permit delineation of sedimentary environments and insight into the sedimentary processes that have shaped the estuary. In comparison to other more-intensively studied estuaries in North America, the Columbia River estuary has relatively larger tidal range (maximum semidiurnal range of 3.6m) and large riverflow (6,700m3s−1). Variations in riverflow, sediment supply, and tidal flow occur over a range of time scales, making the study of modern processes, as they relate to long-term effects, particularly challenging. Analyses of more than 2000 bottom-sediment grab samples indicate that the bed material of the estuary varies in a relatively narrow range between 0 and 8 phi (1.0 and 0.0039mm) with an overall mean size of 2.5 phi (0.177mm). Sediment size decreases generally in the downstream direction. Sediments from the upriver channels are coarse (1.5–2.0phi; 0.25–0.35mm) and moderately sorted; sediments in the central estuary show wider range and variation in grain size and sorting (1.75–6.0phi; 0.016–0.3mm). Sediment from the entrance region has a mean size of 2.75phi (0.149mm) and is well sorted. Seasonal changes in sediment size distributions occur and are best delineated by those samples containing more than 10% mud (silt plus clay). Sediments containing a significant fine fraction generally occur only in the peripheral bays and in channels isolated from strong currents. Thin deposits of fine sediments are occasionally found in main channels, and the ephemeral nature of these sediments suggest that they may erode and produce the silty rip-up clasts that appear intermittently in the same regions. The distribution of bedforms of various size and shape has been mapped with side-scan sonar during three seasons and at various tidal stages. The presence of bedforms with wavelengths of 6–8m and alternating slip faces about 40cm high indicates that the deeper portion of the entrance region is dominated by tidally reversing lower flow regime sediment transport. Bedforms in the upper reaches of the estuary are much larger, with heights of up to 3m and wavelengths of up to 100m. These bedforms, and the smaller, superimposed bedforms, imply downstream transport under fluvial conditions. In the central estuary, bedforms in the deep portion of the main channels are oriented upriver while those on the shallow flanks of the channels are oriented seaward. The landward limit of upriver bedform transport varies seasonally in response to riverflow fluctuations. A complex array of sedimentary environments exists in the Columbia River estuary. Each environment is influenced by the relative importance of waves, fluvial currents, and tidal currents, as modified by the presence or absence of estuarine circulation, vegetation, or human activity. The importance of these enviroments to the ecosystem of the estuary is discussed in subsequent papers in this volume.

Clay mineral dispersal patterns in the north Bering and Chukchi Seas

Abstract Characterization of the clay mineralogic constituents of the μ m e.s.d. (equivalent spherical diameter) size fraction of contemporary marine and fluvial sediments from the north Bering Sea—Chukchi Sea region and adjacent land areas has resulted in delineation of suites dominated by broadly defined “illitic” and “expandable group” components, with lesser amounts of chlorite and kaolinite. Distribution patterns elucidated indicate net northward transport of sediments relatively enriched in the “expandable group” with predominant terrigen ous source from the Yukon River system. This material is distributed across the study area in a medial zone along a general north—south trend, with peripherally decreasing relative amounts of this “expandable group” component, and concomitant relative increases in the “illitic” components. These patterns are consistent with known physical oceanographic and regional geologic relationships, in terms of sediment sources, transport processes, and depositional mechanisms. The foregoing suggest, further, that consideration of the stratigraphic relationships of these clay mineral component-types, as a function of time, in relevant marine sediments north and south of the present Bering Strait, should be informative with respect to regional Quaternary paleogeography, as related to global sea level fluctuations during glacial and interglacial episodes.

Displacement of Yukon-derived sediment from Bering Sea to Chukchi Sea during Holocene time

Since Shpanberg Strait was opened by sea-level transgression about 12,000 B.P., one-third to one-half of the sediment load of the Yukon River has bypassed the northern Bering Sea to accumulate in the thick blanket of Holocene[1][1] sediment in the southern Chukchi Sea. Prior to the transgression of Norton Sound about 9500 B.P., more than half of the Yukon River sediment may have been bypassed to the Chukchi Sea. After about 5000 B.P., deposition of Yukon sediment significantly increased in the Bering Sea when the present Yukon subdelta was apparently formed in the southern part of Norton Sound. Even now, one-third of the Yukon load may be carried to the Chukchi Sea, because the continual strong northward circulation of the Alaskan Coastal Water advects some of the Yukon sediment plume, along with masses of sediment intermittently resuspended by storms. This major displacement of sediment 500 to 1,000 km from a river source has important implications for models of advective transport over shelves, paleogeographic reconstructions of sedimentary environments in epicontinental seas, and development of nonsubsiding deltas, such as the Yukon. [1]: #fn-1

Recent Sediments of the East Siberian Sea

The East Siberian Sea, one of the large epicontinental arctic seas off northeastern Siberia, is shallow and covered with ice most of the year. The sea bottom is monotonously flat except where intersected by two drowned river valleys. The surficial sediments are fine grained, often containing over 15% colloidal (finer than 11o) material. This is attributed to mechanical weathering in the arctic permafrost region, the low gradients of incoming rivers, and the low energy conditions that exist in the East Siberian Sea. The primary sources of sediment are the Indigirka and Kolyma Rivers and the New Siberian Island region. Sediment transport is generally easterly to northeasterly. On the basis of factor analysis of grain-size data and heavy mineral analysis of the 4o sand fraction, three distinctive sediment groups have been defined within regions corresponding roughly to the western, central, and eastern portions of the East Siberian Sea. Sediment derived from the New Siberian Islands dominates the shallow (10 to 15 m) western region. Currents here are stronger than average and silt is the dominant sediment type. The central plateau is dominated by material introduced by the Indigirka River, characterized by low concentrations of heavy minerals. Currents here are generally weaker than in the New Siberian Shoal region and sediments are typically clayey-silts. Zones of cleaner silts indicate locally more intense currents. The eastern third of the East Siberian Sea is characterized by relatively deep (30 to 50 m) irregular topography, and variable sediment texture and mineralogy. Winnowed sandy sediments and gravelly-sandy-muds possibly associated with ice rafting break the pattern of silty-clays and clayey-silts. The Kolyma River has introduced most of the sediment into this region, although local shore line sources are indicated clearly by the mineralogy. Ice rafting generally appears to be insignificant in the East Siberian Sea sediments.

Continental Shelf Sedimentation in an Arctic Environment

A factor analysis of 579 bottom sediment samples from the Continental Shelf in the Chukchi and northeastern Bering Seas identified three factors that “explain” 92 percent of the variation of ten granulometric variables. Factor I represents deposition of silts and clays by settling from the water column. Although Factor I is extensively distributed in the areas of quieter water, the extreme values occur where there is an abrupt reduction in transporting capacity. Factor II represents both the provenance of the sand and the deposition or modification of sands by nearshore processes. Factor III represents beach processes and also several processes producing a poorly sorted sediment. The silts and clays from the Yukon River cover the bottom of Norton Sound and are encroaching onto the relict sands of the Chirikov Basin. Together with the muds from other Alaskan rivers, the Yukon sediment is also transported into the Chukchi Sea, in both the coastal water and offshore water. The coastal flow leaves the coast at Point Hope, diverges, and enters the complex circulation in the Chukchi Basin that is controlled by regional winds. Depending upon the atmospheric pressure distribution, the currents may carry much of the sediment off the shelf down Herald Canyon or into the East Siberian Sea, or the bottom water and sediment may remain in residence on the shelf even to a point of minor stagnation of the circulation. Sufficient mud has been deposited to floor the basin. The compensation current from the East Siberian Sea may be a more significant sediment supplier than previously thought, and the northward flow of the Bering Strait current from Point Hope to Point Barrow may be sporadic. The nearshore sands of the southeastern Chukchi Sea are modern, wave sorted in places, and current deposited in other places. Along the Siberian coast and in the northeastern Chukchi Sea, the sands are relict and residual. Residual sediment also occurs on Herald Shoal. Although the seas are ice covered for 9 to 10 months annually, ice rafting is not a dominant sedimentary process. Deposition of fine sediment may occur during this time by settling from the homogeneous water.

Sea-level data for parts of the Bering-Chukchi shelves of Beringia from 19,000 to 10,000 14C yr B.P.

Abstract Sea-level changes in Beringia are especially significant because they affect the migration of land plants and animals between Asia and North America, and marine plants and animals between the Pacific and Arctic oceans. Previous studies of cores from the Bering and Chukchi shelves produced sea-level curves. Evaluation of these data suggests that nine of the radiocarbon-dated estimates of sea-level position are most reliable for the time period 19,000 to 10,000 yr B.P. The trend of these nine points is proposed as the basis for a regional sea-level curve for central Beringia. Constraints on the data must be noted, however, by anyone using them.

Sedimentary environments of the east-central Bering Sea continental shelf

Abstract A factor analysis of 180 bottom sediment samples from the east-central Bering Sea continental shelf identifies five factors that account for 95% of the variation in the 17 whole o size classes that were used as variables. Factor I represents coarse sediments that have been bypassed in areas of active water circulation. Factors II and III represent fine and very fine sands that have been hydraulically sorted, reworked, and mixed. Factor IV represents coarse to medium silt that has been segregated from areas of relatively high energy. Factor V represents both the production of sediments finer than medium silt and deposition within the lowest-energy environment in this area. Modern and palimpsest sediments are areally prevalent over this section of the shelf. Relict sediments occur in only a few small areas. The dispersal of sediments is affected by surface and tidal currents as well as wave action. Ice rafting is not an important geological agent. Data from the eastcentral Bering Sea shelf indicate that sediments on subarctic continental shelves are not necessarily characterized by an abundance of rocky sediments or gravel.

Yukon River: Evidence for extensive migration during the holocene transgression

The shift of the Yukon River, during the Holocene sea-level transgression, from south of Nunivak Island during the Wisconsin maximum to its present location (a distance greater than 300 kilometers) is indicated by remanent channels, distinct subbottom structures, deltaic sediments, and anomalous rates of sediment accumulation on the continental shelf of the east-central Bering Sea. These features were produced as the ancestral river migrated northward across the easternmost part of this area before 11,000 years ago.

Pleistocene drainage patterns on the floor of the Chukchi Sea

Abstract It is proposed that the lack of bathymetric continuity in a submarine valley crossing a continental shelf is the result of deltaic deposition and may therefore be used to recognize periods of a lesser rate of sea-level rise or a period of sea-level stillstand. Using this criterion three stillstands may be recognized in the bathymetry of the Chukchi Sea: ( 1 ) present sea level (past 3,000–6,000 years); ( 2 ) −18 to −24 fathoms (12,000 years B.P.) and ( 3 ) −29 fathoms (estimated between 13,000 and 17,000 years B.P.).

An Instrument System to Measure Boundary-Layer Conditions at the Sea Floor

Abstract An instrument system (data-collecting devices mounted on a tripod platform) has been built for measuring currents and sediment motion within 2 m of the sea floor. The platform is an aluminum tripod capable of sinking to depths of 200 m and returning to the surface. This platform contains its own lighting system and electrical power. Twelve shipboard controlled electrical outputs provide power for the sensing elements. The system is capable of: ( 1 ) providing continuous observation of the sea floor by means of underwater television; ( 2 ) continuously measuring the velocity profile (6 current meters and 1 direction vane) within 2 m of the bottom; ( 3 ) taking, on command, stereophotographs of the bottom for determination of bed configuration; ( 4 ) sampling the suspended sediment within 2 m of the bottom, and ( 5 ) measuring water depth. All data are transmitted continuously to shipboard and recorded on paper tape. This system has been used on several occasions in inshore waters for continuous periods up to 32 h.