Geology topics

C.K. Sommerfield

Publications and source records attributed to C.K. Sommerfield.

3 recordsLinked to original sources

USGS2009

Postglacial sedimentary record of the Southern California continental shelf and slope, Point Conception to Dana Point

Sedimentary strata on the Southern California shelf and slope (Point Conception to Dana Point) display patterns and rates of sediment accumulation that convey information on sea-level inundation, sediment supply, and oceanic transport processes following the Last Glacial Maximum. In Santa Monica Bay and San Pedro Bay, postglacial transgression is recorded in shelf deposits by wave-ravinement surfaces dated at 13-11 ka and an upsection transition from coastal to shallow-marine sediment facies. Depositional conditions analogous to the modern environment were established in the bays by 8-9 ka. On the continental slope, transgression is evidenced in places by an increase in sediment grain size and accumulation rate ca. 15-10 ka, a consequence of coastal ravinement and downslope resedimentation, perhaps in conjunction with climatic increases in fluvial sediment delivery. Grain sizes and accumulation rates then decreased after 12-10 ka when the shelf flooded and backfilled under rising sea level. The Santa Barbara coastal cell contains the largest mass of postglacial sediment at 32-42 ?? 10 9 metric tons, most of which occurs between offshore Santa Barbara and Hueneme Canyon. The San Pedro cell contains the second largest quantity of sediment, 8-11 ?? 10 9 metric tons, much of which is present on the eastern Palos Verdes and outer San Pedro shelves. By comparison, the mass of sediment sequestered within the Santa Monica cell is smaller at ??6-8 ?? 10 9 metric tons. The postglacial sediment mass distribution among coastal cells reflects the size of local fluvial sediment sources, whereas intracell accumulation patterns reflect antecedent bathymetric features conducive for sediment bypass or trapping. ?? 2009 The Geological Society of America.

Special Paper of the Geological Society of America↗

USGS2005

Latest Holocene evolution and human disturbance of a channel segment in the Hudson River Estuary

The latest Holocene sedimentary record of a cohesive channel and subtidal shoal in the lower Hudson River Estuary was examined to elucidate natural (sea-level rise, sediment transport) and anthropogenic (bulkheading, dredging) influences on the recent morphodynamic evolution of the system. To characterize the seafloor and shallow subbottom, ??? 100 km of high-resolution seismic reflection profiles (chirp) were collected within a 20-km reach of the estuary and correlated with sediment lithologies provided by eight vibracores recovered along seismic lines. Sediment geochronology with 137Cs and 14C was used to estimate intermediate and long-term sedimentation rates, respectively, and historical bathymetric data were analyzed to identify regional patterns of accretion and erosion, and to quantify changes in channel geometry and sediment volume. The shoal lithosome originated around 4 ka presumably with decelerating eustatic sea level rise during the latest Holocene. Long-term sedimentation rates on the shoal (2.3-2.6 mm/yr) are higher than in the channel (2 mm/yr) owing to hydrodynamic conditions that preferentially sequester suspended sediment on the western side of the estuary. As a result, the shoal accretes oblique to the principal axis of tidal transport, and more rapidly than the channel to produce an asymmetric cross-section. Shoal deposits consist of tidally bedded muds and are stratified by minor erosion surfaces that seismic profiles reveal to extend for 10s of meters to kilometers. The frequency and continuity of these surfaces suggest that the surficial shoal is catastrophically stripped on decadal-centennial time scales by elevated tidal flows; tidal erosion maintains the shoal at a uniform depth below sea level and prevents it from transitioning to an intertidal environment. Consequently, the long-term sedimentation rate approximates the rate of sea-level rise in the lower estuary (1-3 mm/yr). After the mid 1800s, the natural geometry of the lower Hudson River Estuary changed rapidly in response to engineering works that forced the channel to self-deepen. Analysis of historical bathymetric data indicates that the channel lost an estimated 3 ?? 106 tons of sediment between ca. 1939 and 2002 (50,000 tons/yr average) by subaqueous erosion, increasing in depth by as much as 4 m in places. Erosion appears to have been concurrent with systematic bulkheading of the shoreline after ca. 1865, which decreased the estuary surface area by ??? 19% overall. Evidently, self-deepening of the channel is a morphodynamic adjustment to reestablish equilibrium cross-sectional area, yet the state of this change locally and elsewhere in the estuary is unknown. Subaqueous erosion documented in this study is a significant source of sediment with implications to the sediment budget and environmental quality of the Hudson River Estuary. ?? 2005 Elsevier B.V. All rights reserved.

Marine Geology↗

USGS2001

Sedimentary Carbon, Sulfur, and Iron Relationships in Modern and Ancient Diagenetic Environments of the Eel River Basin (U.S.A.)

Depositional and diagenetic controls on the distributions of carbon, sulfur, and iron (C-S-Fe) in modern sediments and upper Pleistocene mudrocks of the Eel River Basin (ERB), northern California continental margin, were investigated using a combination of geochemical, radioisotopic, and sedimentological methods. A mass balance based on down-core profiles of porewater and solid-phase constituents and diagenetic modeling suggests that only 12-30% of the pyrite-S produced via SO 4 -2 reduction during burial is retained in modern shelf and upper slope deposits of the ERB. Bioturbational reoxidation of initially reduced S is inferred to be the major control on S preservation, on the basis of an observed inverse relationship between pyrite-S retention and biological mixing intensity, estimated from profiles of excess 234 Th. Importantly, these findings argue that massive depositional episodes on the shelf following floods of the Eel River have a negligible long-term impact on bioturbating macrofauna and the potential to affect geochemical properties of the sediments. Down-core profiles of reactive Fe 3+ and Py-Fe(II) for the modern deposits suggest that highly reactive Fe phases are sulfidized well within ∼ 500-2000 years of burial, thereby limiting later pyritization, which could occur through sulfidation of less reactive phases. This result explains the low (≤ 0.4) degree of pyritization (DOP) values exhibited by both modern and ancient deposits of the ERB and lends support to the notion that pyritization in aerobic continental-margin sediments is largely associated with highly reactive detrital Fe oxides. Comparable mean C/S weight ratios for modern sediments (5.4 ± 3.3, 1σ) and mudrocks (6.9 ± 4.5) of the ERB suggest that the upper Pleistocene strata reflect a geochemical environment analogous to that of the modern margin. Specifically, the C-S-Fe signatures shared by the modern and ancient deposits are a consequence of similar detrital Fe mineralogies, initial organic-matter content (C org ≤ 1%) and composition (C/N = 13 to 17, δ 13 C org = -22 to -25‰), burial rate, and importantly, bioturbation intensity. The findings of this study have important implications for the use of C-S-Fe signatures as indicators of diagenetic processes in dynamic, continental-margin environments.

California↗