Robert A. Morton

Meso-scale transfer of sand during and after storms: implications for prediction of shoreline movement

Abstract Monitoring beach volume changes of the Texas Coast following a major hurricane reveals the impact of storms on sand dispersal and shoreline movement at spatial and temporal scales encompassing tens of kilometers and decades. Beach volume histories at profile sites show the interdependence of sand exchange among adjacent sites and the spatial autocorrelation of sand movement. Beach volume histories also indicate periods when either longshore or cross-shore transport predominate and illustrate the long-term effects of coastal structures on beach mobility. This study confirms that net losses of sand from updrift barriers may not be directly linked with net gains of sand on adjacent downdrift barriers. Instead, sand dispersal within a coastal compartment may depend partly on the dynamics of shoals and temporary sand storage at the intervening tidal inlet. In our study, sand eroded from the updrift barrier (Galveston Island) is deposited in a terminal sand flat of the barrier, whereas sand accreted to the downdrift barrier (Follets Island) is derived from the intermediate ebb-tidal delta (San Luis Pass). Unlike continuous sand bypassing on some microtidal, wave-dominated coasts, sand bypassing at San Luis Pass is episodic, event driven, and inefficient, and sand is not transferred directly from one barrier to the next. p ]Because storms rapidly redistribute beach sediment, they can be the most important factor controlling short-term (

Recent Subsidence-Rate Reductions in the Mississippi Delta and Their Geological Implications

Abstract The Mississippi Delta has long been characterized as an area of rapid subsidence; however, recent subsidence rates are substantially lower than previously reported. Tide-gauge records indicate that rates of relative sea-level rise were slow from 1947 until the mid-1960s, relatively fast from the mid-1960s until the early 1990s, and then slow since the early 1990s. These trends and rates are independently verified by repeat benchmark surveys and height monitoring at continuously operating geographic positioning system stations. Subsidence rates for the slow periods were a few millimeters per year, comparable to rates averaged over geological time scales that are attributed to natural processes such as shallow sediment compaction and deep crustal loading. The decadal pattern of slow, then rapid, then slow subsidence may be caused by natural deep-basin processes (e.g., gravity gliding and salt migration), but it is more likely related to rates of hydrocarbon production that followed the same temporal trends. If accelerated subsidence was primarily induced by reservoir compaction and fault reactivation associated with fluid withdrawal that also accelerated in the 1960s and 1970s, then the recent reductions in subsidence rates likely reflect a balancing of subsurface stresses and a return to near preproduction conditions.

Sediment distribution and evolution of tidal deltas along a tide-dominated shoreline, Wachapreague, Virginia☆

Abstract Borings from the barrier island/lagoon system of the Eastern Shore of Virginia penetrated an unconformity which separates Pleistocene barrier island and offshore marine sediments from the overlying Holocene tidal delta and barrier island sediments. Offshore marine sediments and deposits within the flood-tidal delta (marsh, tidal flat-bay, inlet-mouth bar complex) are recognized on the basis of sediment color, composition, grain-size changes in the vertical sequence, presence of organic matter, and faunal suite. Subsurface data, historical records, and morphology of lateral accretion on barrier islands suggest that major inlets in the vicinity of Wachapreague have been relatively stable throughout Holocene time; they appear to be located where Pleistocene stream valleys previously existed. Holocene barrier islands apparently developed on drainage divide areas following post-Wisconsin transgression of the sea. The initial phase of tidal delta development was characterized by vertically accreting, fan-shaped, inlet-mouth bars; tidal channels stabilized after bar crests had shoaled sufficiently for marsh to form. With landward progradation across the lagoon, sand-rich deposits graded laterally away from the inlets and vertically into clayey sand and silty clay of the tidal flat-bay and marsh environments. Ebb inlet-mouth bars developed asymmetrically southward in response to littoral drift. Flood tidal deltas also built preferentially toward the south as indicated by: (1) sand distribution of the inlet-mouth bar complex; and (2) greater development of marsh south of the inlets.

Attributes and Origins of Ancient Submarine Slides and Filled Embayments: Examples from the Gulf Coast Basin

Large submarine slides and associated shelf-margin embayments represent an intermediate member in the continuum of unstable shelf-margin features. On seismic profiles, they may resemble submarine canyons, but are different in their size, morphology, origin, and hydrocarbon exploration potential. Two large Neogene submarine slides, located in the northwestern Gulf Coast Basin, formed on the upper slope and flanks of prominent shelf-margin deltas. The basal detachment surface of each slide is a structural discontinuity that may be misinterpreted as an erosional unconformity and, therefore, misidentified as a stratigraphic boundary separating depositional sequences. Regional stratigraphic correlations indicate that both slides were initiated after the continental platform wa flooded. The condensed sections deposited during the rise in relative sea level contain the basal detachment surfaces. Beyond these observations, the relationships between the slides and sea level fluctuations are uncertain. The shelf-margin embayments created by the slides apparently were partly excavated during periods of lowered relative sea level and were filled during the subsequent relative sea level rise and highstand. Eventually the preslide morphology of the shelf margin was restored by coalesced prograding deltas. Submarine slides exhibit landward-dipping, wavy, mounded, and chaotic seismic reflections that are manifestations of slump blocks and other mass transport material. Composition of these internally derived slide deposits depends on the composition of the preexisting shelf margin. Embayment fill above the slide consists mostly of externally derived mudstones and sandstones deposited by various disorganized slope processes, as well as more organized submarine channel-levee systems. Thickest slope sandstones, which are potential hydrocarbon reservoirs, commonly occur above the basal slide mudstones where seismic reflections change from chaotic patterns to overlying wavy or subhorizontal reflections.

Effects of Hurricane Eloise on beach and coastal structures, Florida Panhandle

Natural process-response changes in beach and dune configuration attendant with passage of Hurricane Eloise caused extensive structural damage to residential and commercial establishments in the vicinity of Panama City Beach, Florida. Damage was largely attributed to storm surge and wave set-up with subsequent beach scour; wind damage and flooding were minimal. Dune retreat and scour behind failed seawalls (from 12 to 18 m) resulted in many foundation failures and partial or complete collapse of the building superstructure. Because of the physiographic setting and storm characteristics, damage was primarily restricted to a narrow strip of shorefront property paralleling the Gulf of Mexico shoreline. Overwash was minimal owing to dune ridge and cliffed headland elevations. Sand removed from the beach was transported westward both alongshore and offshore. Much of the damage could have been prevented or minimized by proper location and design of buildings and an acute awareness of active coastal processes.

Subaerial storm deposits formed on barrier flats by wind-driven currents☆

Abstract Coastal storm deposits produced by moderate- to high-velocity wind-driven currents are documented on aerial photographs of Texas Gulf Coast barriers following Hurricanes Carla, Beulah, and Celia. Sedimentary features on back-barrier flats that serve as criteria for inferring wind-driven currents include: (1) counterclockwise curvature of washover channels, (2) flame-shaped fans emerging from deeply incised channels, (3) closely spaced, slightly curved striations, (4) transverse sand waves along bay shorelines, and in some cases, (5) rhomboid bed forms. Flame-shaped and striated fans composed of sand and shell fragments also form on higher ground between washover channels. These fans coalesce laterally to form thin sediment wedges with crenulated landward and seaward margins; sediment by-pass areas separate these wedges from storm berms. The preserved storm features were formed after storm landfall by undirectional flow. Surface drift velocities, which lack precision because of underlying assumptions, were estimated by (1) small-wave theory applied to rhomboid bed forms, (2) ratios of surface drift velocity to wind velocity (3–5%), and (3) calculations based on drag coefficients. These estimates suggest that average wind-driven currents were between 1.3 and 2.9 m/sec.

Hydrology, morphology, and sedimentology of the Guadalupe fluvial-deltaic system

Third-dimensional channel characteristics, bed forms, sediment load, and discharge data are described for two suspended-load streams (Guadalupe and San Antonio Rivers) and a mixed-load stream (Coleto Creek) that together make up the suspended-load fluvial system of the Guadalupe delta of Texas. Discharge of the Guadalupe River is highly variable and increases downstream, but (1) percentage of sand and mean grain size of bed material, (2) channel gradient, (3) valley gradient, (4) channel sinuosity, and (5) channel width/depth ratio all decrease downstream. The most marked decreases in these parameters occur between the alluvial plain and the delta plain. The relationships between percentage of silt-clay in channel perimeters, width/depth ratios, and sinuosities for these small coastal-plain rivers with single channels support the classification and empirical relationships established by Schumm for modern alluvial channels. One notable exception is that downstream increases in silt and clay from alluvial plains to delta plains do not necessarily yield higher sinuosity channels. The low channel sinuosities and lack of point-bar accretion along lower reaches of many suspended-load alluvial channels and delta distributaries are related to extremely low channel gradients and a natural gradient threshold below which meandering is minimal.

Field Evidence of Subsidence and Faulting Induced by Hydrocarbon Production in Coastal Southeast Texas

ABSTRACT Three large, mature hydrocarbon fields in coastal southeast Texas were examined to evaluate competing hypotheses of wetland losses and to characterize subaerial and submerged surfaces near reactivated faults and zones of subsidence. Detailed topographic and bathymetric profiles and shallow cores at the Port Neches, Clam Lake, and Caplen Fields provide a basis for distinguishing between (1) extensive land-surface subsidence without significant subaqueous erosion, and (2) localized minor subsidence near faults accompanied by extensive subaqueous erosion. Subaqueous erosion results from submergence of wetlands, current and wave excavation of surface sediments and organic detritus, and exportation of the eroded sediments through adjacent water bodies with swift currents such as navigation channels. Responses to induced subsidence and fault reactivation are different at each field site. Detailed stratigraphic correlations of sediment cores show that at Port Neches, subsidence of 35 to 90 cm and minor marsh erosion (20 to 35 cm) created more than 15 million m3 of accommodation space in a nearly circular pattern over the field. At Caplen the marsh surface subsided only about 4 cm, but the surface eroded 30 to 40 cm vertically, creating about 3.5 million m3 of accommodation space. The breakup of wetlands and their conversion to open water appears to be in an initial stage at the Clam Lake Field where marsh plants are being submerged along a fault. The different surficial responses and wetland losses at each field are related to the primary type of hydrocarbon produced and the rates of production. Although the absolute magnitude of induced subsidence may be less than 1 m, even a minor reduction in land elevation is sufficient to cause major wetland losses.

Test of Environmental Geologic Mapping, Southern Edwards Plateau, Southwest Texas

The southern Edwards Plateau, southwest Texas, is the recharge area for a thick limestone aquifer that supplies potable water to more than 850,000 people and irrigation water for more than 2,500 sq mi (6,500 sq km) of cropland. Because the increasing population effects a booming residential and recreational development in this region, information on the geological environment of the plateau must be made available to planners so that the aquifer can be safeguarded. A reconnaissance environmental mapping technique has been developed in a pilot study that is to precede a major regional mapping program. Constraints in developing the technique were plateau geology, land use, factors controlling recharge, and available materials, including aerial photography and maps. The basis for defining the environmental mapping units is dominantly geomorphic; lithologic differences are indicated where necessary. The units are: karstic tableland, karstic lowland, deeply dissected carbonate, moderately dissected carbonate, carbonate, shale, alluvial fan, terrace, flood plain, fan plain, and alluvial-colluvial material. Results of this mapping are directly applicable in land use planning.

Holocene Development of the Southeastern Texas Coast, Sabine Lake Area, from Foraminiferal Biofacies

ABSTRACT Foraminifera were analyzed in cores of Holocene sediments from the southeastern Texas coast to better understand the post-glacial evolution of the Texas chenier plain and incised valley of the Sabine and Neches Rivers. Foraminifera are rare to absent in highstand interfluve marsh and in incised-valley fluvial floodplain deposits, whereas nearshore marine, estuarine, and chenier plain deposits contain abundant foraminifera. Overall foraminiferal diversity is low, and sample assemblages are dominated by Ammonia parkinsoniana and Elphidium gunteri. These species comprise about 78% of the fauna; the remaining taxa include Elphidium spp., miliolids, Brizalina spp., Bucella hannai, and Palmerinella palmerae. Samples are characterized by mixtures of well-preserved and poorly-preserved tests. This, along with the presence of Cretaceous foraminifera, suggests that many of the microfossils are reworked. Small adult foraminifera are common in cores from the chenier plain and are generally associated with fine-grained sediments deposited as mudflats between the sand-rich beach-ridges. Agglutinated foraminifera are surprisingly rare probably because geochemical conditions do not favor the preservation of agglutinated tests below about 2 m. Foraminiferal biofacies include a very low diversity, brackish (marsh) assemblage, a moderate diversity beach assemblage, a moderate diversity shallow marine assemblage, and a relatively high diversity, slightly deeper (but still nearshore) marine assemblage. These biofacies are consistent with those described from the Louisiana chenier plain. The faunal associations, diversity, abundance, and preservation provide insight to the paleoenvironments and depositional processes of the chenier plain.