Abstract At 138 stations in the eastern Mississippi Delta area measurements were made of phytoplankton production (C 14 O 2 uptake), chlorinity, temperature, suspended solids, Secchi disc depth, inorganic phosphate, soluble silica, and soluble Kjeldahl nitrogen. In addition, the phytoplankton in the water were identified and enumerated. These stations were generally located inshore of those taken by G. A. Riley (1937). The following results were obtained. 1. Surface phytoplankton production off the delta is equal to or greater than that of highly productive tropical or subtropical pelagic or neritic areas. 2. Surface production is quite variable; variation can be as much as seven-fold from one day to the next, at a given location. 3. Although seaward traverses made on single days showed that production generally increased at seaward locations, there were no over-all statistical differences between river, plume, and gulf areas at any given season. 4. During the period of high river discharge (May), surface production at the most seaward locations (gulf) was significantly greater than during the fall months. Phosphate and Kjeldahl nitrogen were also significantly greater in May than during the fall months in this area. 5. Integrated production in the water column was three to six times that occurring at the surface. Measured in situ water column production agreed within 20 per cent with that calculated by Ryther’s (1956) method in three out of six determinations. In the other determinations, the measured value was three to six times greater than the calculated value. 6. Rough calculations of the rate of sedimentation of organic carbon off the delta were compared with phytoplankton production. This comparison showed that it is unlikely that production contributes much organic carbon to pro-delta slope sediments. Production would have to proceed at the maximum rate throughout the year without losses of phytoplankton carbon to equal the rate at which carbon is deposited in these sediments. In the bottom-set beds farther seaward we cannot estimate from the present data the relative contribution of organic matter by phytoplankton to these sediments. 7. Theoretical calculations indicate that nitrogen is a more likely limiting nutrient in these waters than phosphate. Silicate may limit phytoplankton only in highly saline water, where no silicate could be detected. 8. Two hundred species of phytoplankton were identified from the delta, of which 86 per cent were diatoms. Two associations of primary species were recognized. One of these, consisting of species of Cyclotella, Melosira , and Navicula , was found in the river plume, and Blind Bay. The other consisting of species of Nitzschia, Thalassionema, Thalassiothrix, Skeletonema, Asterionella , and Chaetoceros , was found in the gulf, plume, and Breton Sound. During any given season the relative concentrations of phytoplankton do not differ significantly from area to area. Significantly fewer phytoplankton were found in the river and plume at times of low water, but no significant difference in phytoplankton concentration in the gulf was found between May and the fall months.

The East Tennessee seismic zone in the southern Appalachians is an ~75-km-wide, 350-km-long region of seismicity that extends from NE Alabama and NW Georgia to NE of Knoxville, Tennessee. It is the second most active seismic zone east of the U.S. Rocky Mountains. Although the East Tennessee seismic zone has not recorded historical earthquakes of M > 5, researchers have used hypothetical and theoretical relationships to suggest that it may be capable of generating an “infrequent” M ~7.5 quake. To help clarify the late Pleistocene earthquake history and the earthquake potential of the East Tennessee seismic zone, we conducted an 18-mo pilot study to seek evidence of paleoseismic activity and have made important discoveries. ENE of Knoxville, Tennessee, in late Pleistocene French Broad River alluvium, we discovered: (1) strike-slip, thrust, and normal faults involving bedrock and alluvium at three sites, and widespread bleached or clay-filled fractures; (2) paleoliquefaction; and (3) anomalous fractured and disrupted features at three sites attributable to liquefaction and forceful groundwater expulsion and fluidization during or immediately after two or more major late Quaternary earthquakes. All of these features were produced by seismic events with a probable minimum M ~6.5. Optically stimulated luminescence dates at four sites provide maximum ages of 73–112 ka for at least two events. Upward penetration of at least two generations of fractures, clastic-sediment intrusions, and faults into the Bt horizons of Ultisols at several sites implies that two strong shocks occurred sometime after ~73 ka, and possibly much later than 73 ka. Two exposures in terrace alluvium E and W of the Tennessee Highway 92 bridge S of Dandridge, Tennessee, were graded and geologically mapped at 1 in. = 5 ft. The site W of the bridge revealed at least three sets of crosscutting fractures that terminate upslope against the base of an overlying late Pleistocene colluvium. The E site revealed numerous fractures and a fault with ~20 cm of sinistral displacement. Moreover, several “fluidization boils” containing shale clasts from below are cut by younger, red, clay-filled fractures. Few of these fracture sets in the Quaternary sediments parallel those in bedrock of the Tennessee Valley and Ridge and Blue Ridge geologic provinces that host the East Tennessee seismic zone, and these fractures are poorly aligned with the present-day N70E maximum principal stress orientation. A third site, 5 km SW of Dandridge on the NW side of Douglas Reservoir, contains at least two NW-vergent thrust faults that transported weathered bedrock 25–50 cm over late Pleistocene alluvium. At the same site, a 12-m-long mode 1 branching fracture in Sevier Shale is filled with Quaternary sediment, and is truncated by the largest thrust fault at 1–2 m depth. This structure, including the Quaternary sediment it contains, is also displaced 10 cm along a NW-trending sinistral fault. The discovery of faults at the ground surface that displace both bedrock and terrace alluvium contrasts with the modern seismicity, which occurs at 5–26 km depth in rocks below the basal décollement of major Paleozoic thrust sheets. Collectively, these initial findings imply that the East Tennessee seismic zone has produced coseismic surface faulting and generated at least two strong (M > 6.5) earthquakes during the late Quaternary.

Paleoseismic investigations in fluvial deposits frequently use large-scale (many centimeters to decimeters wide) ground-failure features of liquefaction origin as indicators of larger earthquakes (i.e., exceeding M ~6). Such large features are not the only signature of seismicity, however. Seismic shaking often produces an abundance of small-scale features (millimeter to centimeter in size) such as sills, small clastic dikes, and ground fractures, which can vary widely in height and range from paper-thin to a few centimeters wide. These small-scale seismic signatures commonly form in field settings where large liquefaction features are absent, such as regions with a reduced susceptibility for liquefaction or sites far from earthquake meizoseismal regions where shaking levels were lower. Thus, these small signatures have the potential to significantly expand the geographic area useful for paleoseismic studies, yet they are not typically sought in most paleoseismic field studies because many can develop nonseismically, and interpreting their formative origin can be challenging. We examined small-scale features that occur in association with large liquefaction features at a variety of field sites across the United States. We present new criteria, with many photographic examples, to evaluate whether small-scale features and ground fractures were seismically generated. Although this research was done primarily in fluvial settings in the United States, these criteria should be applicable worldwide in many field settings with clastic sediments, potentially giving the study of small-scale seismic features and fractures a significant role in future paleoseismic investigations.

Abstract The lower Wabash and Ohio River valleys have experienced seismicity throughout geologic time. The rocks and sediments in southern Indiana, southern Illinois, and western Kentucky provide records of these past seismic events in the form of various types of filled fractures. In the field these features occur either as downward penetrating, surface-filled fractures created by tectonic deformation or seismicity, or as upward penetrating liquefaction features such as clastic dikes and sills created by strong earthquakes. The fractures are widespread and abundant in many places, and are usually seen in natural exposures such as stream banks and less commonly in man-made excavations. In contrast, their causative faults are rarely observed. Thus, compared to searching for faults, the study of filled fractures is a useful and relatively inexpensive technique for assessing the seismic history of a region. The fractures discussed are clearly of seismic origin on the basis of morphology, sediment characteristics, regional patterns, and proximity to known faults. Further research is needed to determine whether additional types of features, which we discuss and examine in the field, can also serve as paleoseismic indicators.

Abstract Laguna Madre is a linear coastal lagoon, developed on the pre-Holocene 3 3 Throughout the paper the age of transgressive de posits following the last glaciation is referred to as “Holocene,” in keeping with its generally accepted usage. erosion surface by the buildup of a barrier island, resulting from the transgression brought about during the postglacial rise in sea level. The Holocene deposits average 10-20 feet in thickness, although local depressions in the pre-Holocene surface may contain a considerably thicker section. The bulk of the Holocene deposits consists of subgraywacke and subarkosic sand. Local concentrations, however, include limestones, gravels, oolites, and relatively pure clays. Source of the clay is unknown, but heavy and light mineral analyses of the sand fraction indicate locally reworked earlier deposits. The northern Laguna sediments are derived from the pre-Holocene sediments off the Nueces delta province and the southern Laguna sediments are derived from the Holocene and pre-Holocene deposits of the Rio Grande distributary province. The immediate source of the lagoonal fill is the barrier island sand, which is believed to be derived from the earlier deposits of the nearshore gulf. During and after sea-level rise, the near-shore gulf floor was actively eroded by wave action to establish an equilibrium profile. The eroded material was sorted and the sand fraction was transported landward. Wind action piled the new beach sands into coastal dunes on the barrier. Where the earlier deposits were relatively un-consolidated, as offshore northern Padre Island, thick dune deposits could form because of large supply. Where the earlier deposits were more consolidated, and consequently less easily eroded, as in the vicinity of the Rio Grande delta, the dunes are lower and the barrier narrower. Subsequent to the formation of the lagoon about 5,000 years ago, partial filling was accomplished by washover storm waves, wind transport, and tidal activity.