Miles O. Hayes

General morphology and sediment patterns in tidal inlets

Abstract Tidal inlet sediments make up a significant portion of most barrier island complexes. Inlet-affiliated sedimentary units usually include an ebb-tidal delta (seaward shoal), a flood-tidal delta (landward shoal) and inlet-fill sequences created by inlet migration and recurved spit growth. The morphological components of ebb-tidal deltas include a main ebb channel flanked by linear bars on either side and a terminal sand lobe at the seaward end. This channel is bordered by a platform of sand dominated by swash bars which is separated from adjacent barrier beaches by marginal flood channels. The ebb-delta sand body is coarser-grained than other sedimentary units of the inlet and contains polymodal cross-bedding with a slight ebb dominance. Flood-tidal deltas consist of a flood ramp and bifurcating flood channels o the seaward side, which are dominated by flood currents and flood-oriented sand waves, and ebb shields, ebb spits and spillover lobes on the landward side, which contain an abundance of ebb-oriented bedforms. A proposed stratigraphic sequence for a typical flood-tidal delta contains bidirectional, large-scale crossbedded sand at the base, predominantly large-scale (flood-oriented) crossbedded sand in the middle, and finer-grained tidal flat and marsh sediment at the top. Inlets migrate at rates that vary from a few to several tens of meters per year, depending upon such variables as rate of longshore sediment transport and depth of the inlet. Inlet-fill sequences, which fine upward, contain coarse, bidirectional crossbedded sediments at the base, polydirectional crossbedded sands in the middle, and finer-grained aeolian sand at the top. Both tidal-delta morphology and relative size and abundance of ebb- and flood-tidal deltas are considerably different in different oceanographic settings. Microtidal (tidal range T.R. = 0–2 m) areas tend to have smaller ebb-tidal deltas and larger flood-tidal deltas; whereas, mesotidal (T.R. = 2–4 m) areas show just the opposite trend. Large waves tend to inhibit the development of ebb-tidal deltas and accentuate the growth of flood-tidal deltas.

Hydrocarbon source identification and weathering characterization of intertidal and subtidal sediments along the Saudi Arabian coast after the Gulf War oil spill

One year after the Gulf War oil spill, approximately 200 intertidal and subtidal (surface and subsurface) sediments were collected in the areas of Dawhat al Musallamiyah, Dawhat ad Dafi, and Ras Tanaqib along the Saudi Arabian coast as part of the Nearshore Geochemistry Processes Study (Leg II: 16 March–5 April 1992) of the Mt Mitchell Gulf survey. The sediment samples were analysed for total petroleum hydrocarbons and the environmentally important saturated hydrocarbon (SHC) and polycyclic aromatic hydrocarbon (PAH) components of the spilled oil. The target analytes included n-alkanes (C10–C34) and selected isoprenoids, individual and alkyl homologues of the two- through six-ringed PAH compounds and sulphur-containing heterocyclic compounds (38 individual and alkyl-PAHs), and selected cycloalkane biomarkers (steranes and triterpanes). These target analytes were also determined for reference crude oil samples. SHC, PAH alkyl homologue, and triterpane distributions in environmental samples were compared with reference crude oils—Kuwait crude oil, Light Arabian crude oil, and Iranian crude oil—to determine the source(s) and extent of weathering of the spilled oil. Diagnostic hydrocarbon parameters and ratios, especially the plots of the ratios of the C2- and C3-alkyl homologues of the dibenzothiophenes and phenanthrenes/anthracenes (i.e., C2D/C2P vs. C3D/C3P) and the same homologues of the dibenzothiophenes and chrysenes/benzanthracenes (i.e., C2D/C2C vs. C3D/C3C), and the ratio of the source indicator triterpane/trisnorhopanes, TM/TS were useful in differentiating hydrocarbon source(s) and characterizing the weathering of the spilled oil in the samples. From the chemical diagnostic parameters, the Kuwait reference crude oil was determined to be very similar to the Gulf War spilled oil collected during a shoreline survey of oil-impacted beaches of Saudi Arabia in June 1991, shortly after the Gulf War. Asphaltic pavement residual oil found at intertidal sites on Abu Ali and in isolated locations in Dawhat ad Dafi were chemically identified using chemical biomarkers as originating from a source other than oil spilled from the Gulf War. With the SHC and PAH data, a set of chemical characteristics was identified that corresponded to stages of spilled oil weathering. Sediments from exposed habitats generally exhibited an advanced weathering stage; whereas, the moderately exposed and sheltered habitat sediments were typically less weathered at a moderate weathering stage. The advanced stage of weathering was also apparent in the asphaltic pavement sediment. The hydrocarbon characteristics of the subtidal samples were usually dominated by background concentrations of hydrocarbons consisting of both petrogenic PAH and biogenic saturated hydrocarbon signatures.

Factors determining the long-term persistence of Exxon Valdez oil in gravel beaches

Abstract The largest amounts of, and the least weathered, oil found eight years after the Exxon Valdez oil spill occurred at depths of 25–50+ cm under the protective cover of a well-sorted cobble/boulder armor on intermittently exposed, coarse-grained gravel beaches within Prince William Sound, Alaska. In addition to the armoring, other factors enhancing the retention of the oil include flat slopes of the middle beach and a thick sediment veneer over a bedrock platform. Natural cleaning of the subsurface sediments was accomplished within three years on the finer-grained gravel beaches that have steeper slopes, a thin sediment veneer over the rock platform, and no surface armoring. Minor berm relocation was an effective technique for removing subsurface oil from the finer-grained gravel berms at the high-tide line. Extensive storm berm relocation caused disruptions to beach morphology and sediment distribution which lasted for up to six years.

Hydrocarbon characterization and weathering of oiled intertidal sediments along the Saudi Arabian Coast two years after the Gulf War oil spill

Abstract Two years after the 1991 Gulf War oil spill, shoreline areas of Dawhat al Musallamiyah and Dawhat ad Dafi along the Saudi Arabian Coast impacted by the largest oil spill in history were revisited to assess the changes, from weathering, in hydrocarbon composition of residual oil in intertidal sediments. This was done to determine the effect of different geomorphic habitats from exposed to sheltered on the extent of weathering of these oiled sediments. This 1993 study was a continuation of the nearshore geochemistry processes study conducted as part of the scientific expedition on the R/V Mt. Mitchell in the Arabian Sea in 1992. Over 70 surface and subsurface shoreline sediment samples were analyzed for saturated hydrocarbons (SHCs), defined as n-alkanes from C 10 to C 32 and selected isoprenoids, and polycyclic aromatic hydrocarbons (PAHs), defined as individual and alkyl homologues of the two- through six-ringed PAHs including the sulfur heterocyclic compounds. The extent of weathering of hydrocarbons over the two-year period was determined for sediments in each of the different shoreline habitats based on SHC and PAH distribution pattern, and diagnostic ratios, such as ratios of the C 2 - and C 3 -alkyl homologues of the distribution pattern, and chrysenes/benz(a)anthracenes. A classification depicting four stages in weathering (from 1 [fresh] to 4 [advanced]) was established for both SHCs and PAHs. Some sediments contained relatively fresh oil that showed no change in SHC or PAH distributions over the two years in the environment. Other sediments showed extensive weathering, involving multiple weathering stage changes. For SHCs, approximately 66% of the oiled sediment samples had progressed to the most advanced weathering stage (Stage IV) by the end of the second year. In contrast, only 25% of the oiled samples had PAHs at full Stage IV weathering. For the SHC hydrocarbons, changes in SHC distributions looked to be principally caused by selective microbial degradation of the n-alkanes. Weathering of spilled oil PAHs was a combination of losses of smaller PAH families (naphthalenes) and smaller carbon alkyl groups within a PAH family. The least weathered oiled sediments were predominantly from subsurface sediments in habitats with surface seals or in sheltered muddy areas which had reduced exposure from physical weathering processes involving air, sunlight, tidal flushing, and reworking by waves. Seventy percent of the subsurface samples contained residual oil that was relatively fresh (Stage I or II), whereas 20% of the surface samples had relatively fresh residual oil. The heavily oiled subsurface sediments, even in exposed habitat environments, showed the least weathered oil, particularly where liquid oil remained trapped under hardened near-surface oil residual (crust and pavement). Extremely weathered oiled sediments were found mostly in exposed or moderately exposed near-surface sediment habitats. PAHs in oiled sediments were examined for selective degradation of certain alkyl groups or isomers within an alkyl group by microbial action. PAH distributions of the residual oil indicated that selective microbial degradation of alkyl groups was neglibible and the loss of PAHs by any weathering mechanism (microbial, physical, chemical) looked to follow, at least in this two-year time period after the spill, the weathering pathway predicted by the physical processes (evaporation).

Weathering Patterns of Oil Residues Eight Years after the Exxon Valdez Oil Spill

Eight years after the Exxon Valdez spill, oil residues in Prince William Sound, Alaska ranged from moderately to extremely weathered. The least weathered residues were found in samples collected from gravel beaches with well-established armor. There had been little to no change in weathering stage for the oil from these sites since 1994. There was evidence of some physical erosion, but little chemical change for this deeply penetrated oil. In contrast, most other oil residues have increased in weathering, compared to 1994. Only one asphalt pavement was at a moderate weathering stage. All other samples contained hydrocarbons which were at advanced or extreme weathering stages.

Application of an Oil Spill Vulnerability Index to the Shoreline of Lower Cook Inlet, Alaska

Offshore tracts in Alaskas lower Cook Inlet are scheduled to be opened for exploratory petroleum drilling in the near future. Because of the potential for oil spills resulting from this activity, a field study of the coastal zone was conducted in June 1976. A total of 1216 km of shoreline was classified into 3 types: erosional (45 percent), neutral (38 percent), and depositional (17 percent). These were further subdivided into 16 subclasses on the basis of small scale morphological features. This classification was used in conjunction with a vulnerability index of potential oil spill damage, developed through study of three major oil spills, to predict the longevity of oil in the different coastal environments of the Inlet. On a scale from 1 to 10, 45 percent of the shoreline has low values of 1 to 4, which means that oil would be dispersed by natural processes within less than six months after a spill on these coasts. Values from 4 to 6 were assigned to 13.4 percent of the shoreline, where oil residence time may be up to one year. A 6 to 10 rating was assigned to 41.5 percent of the shoreline, where oil contamination may remain for periods of two to ten years, or possibly longer should no major clean-up procedures be initiated. We propose that the use of this type of vulnerability indexing, in conjunction with a biological susceptibility index and oil spill trajectory models, would provide a rational basis for decision making concerning the location of on- and off-shore oil facilities and the design of oil spill contingency plans.

Contamination of nearshore subtidal sediments of Saudi Arabia from the Gulf War oil spill

Detailed sedimentological and dynamic-process studies of the shallow, subtidal habitats along on the Saudi Arabian coast were carried out 1 year after the Gulf War oil spill, as part of the NOAA ship Mt Mitchell cruise. Satellite imagery and space shuttle photography were used to characterize the subtidal habitats and select sampling sites. A total of 197 bottom-observation dives were carried out, and 170 bottom sediment and sediment trap samples were collected for chemical analysis. The results show no evidence of large-scale sinking of oil from the spill. Detailed chemical results showed that surficial subtidal sediments (0–5 cm) have been contaminated at levels ranging from <5 to 900 μg g−1 total hydrocarbons (THC), with the highest contamination in sheltered, muddy basins. In these basins, total PAHs were in the range of 1–7 μg g−1. Little contamination was observed below 5 cm. The oil that had initially stranded in the intertidal zone was not accumulating in the nearshore subtidal region in significant quantities 1 year later.

Distribution and weathering of shoreline oil one year after the Gulf War oil spill

Detailed studies of the intertidal habitats along the Saudi Arabian coast conducted 1 year after the Gulf War oil spill as part of the Mt Mitchell cruise show that there is a striking correlation between the nearshore geomorphology and the persistence of intertidal oil. The most severely impacted areas studied were several halophyte marsh/algal mat complexes and mudflats at the heads of sheltered bays, where all the halophytes were dead and there was no sign of living epibiota in the mid to upper intertidal areas. The abundant burrows were heavily oiled, with some containing liquid black oil to depths of over 40 cm. The deep penetration of oil into the burrows and probable slow weathering rates of the oil could result in many years of pollution of these sheltered habitats. The presence of bubble sand, a sponge-like sand deposit with porosities probably as high at ±50%, resulted in penetration of the oil exceeding 40 cm. This deep oil will also remain in the sediment for many years, because of the slow erosion rates that occur in these sheltered environments.

Antidunes on Modern and Ancient Washover Fans

ABSTRACT Antidune deposits on a washover fan on Seabrook Island, South Carolina display a variety of bedding types. The predominant stratification form comprises nested, very thin lenses which are internally cross-laminated and which become thinner and more closely spaced downflow. In addition, cross laminae within the lenses tend toward backset forms in the bermward portions of the flow, and become more foreset-oriented in a downfan direction. These characteristics, together with the antidune fields relationship to other bed features on the fan, suggest that antidune formation is related to the occurrence of an undular hydraulic jump just downfan from the berm crest. Ancient deposits interpreted as washover sequences, including Ordovician and Pleistocene sandstones in South Africa and a Carboniferous sandstone in Kentucky, display sedimentary structures similar to those observed in the Seabrook antidune field. Because washed-out antidunes and their associated cross laminae are very subtle features, antidunes are possibly responsible for many of the gentle irregularities in otherwise flat beds and are therefore possibly far more common in the geologic record than previously recognized. Their sedimentary structures are distinct from degraded forms of ripple cross-lamination, and in many cases are identical to features usually described in the literature as low-angle truncation surfaces.

Stratigraphy and sediment characteristics of a mesotidal ebb-tidal delta, North Edisto Inlet, South Carolina

ABSTRACT Ebb-tidal deltas can form extensive strandline sand deposits on barrier-island shorelines. This paper describes the diagnostic textures, sedimentary structures, and geometry of an ebb-tidal delta in a mesotidal setting at North Edisto Inlet, South Carolina. Bedforms and surface textures at North Edisto Inlet reflect typical ebb-tidal delta circulation patterns. The main ebb channel is dominated by large ebb-oriented bedforms, and channel sediments become finer grained in the seaward direction. At the inlet throat, the coarsest available sediment forms a lag deposit. Marginal flood channels adjacent to the barrier islands are dominated by floodoriented bedforms composed of medium-grained sand. Channel-margin linear bars, flanked by bedforms with opposing orientations, develop where flood channels approach the main ebb channel. The extensive swash platform is dominated by wave energy, which effectively sorts swash platform sediments. Ebb-tidal delta stratigraphy is dominated by sequences deposited by shifting tidal channels and by the migration of swash bars on the swash platform. Proximally, the ebb-tidal delta sequence is dominated by relatively thick tidal-channel deposits which fine upward and have a sharp, disconformable lower contact. Main ebb-channel deposits up to 20 m thick flank the active channel. These deposits are composed primarily of well-sorted, planar-bedded, fine-grained sand. Flaser-bedded sands directly overlie medium to coarse-grained, seaward-directed, crossbedded sands at the base of the sequence. Adjacent to barrier islands, marginal flood-channel deposits dominate the ebb-tidal delta sequence. These are composed of moderately to intensely burrowed, wavy to flaser-bedded sand and mud. Barrier island accretion in the vicinity of inlets often involves flood-channel switching; marginal flood channel deposits could thus comprise a significant portion of the barrier island lithosome in the vicinity of inlets. Distally, wave-formed swash platform deposits dominate the ebb-tidal delta sequence. These are characterized by very well sorted, fine-grained sand; planar bedding; landward-oriented crossbeds; coarse shell-hash layers; and extensively burrowed zones. Swash platform deposits interfinger with shoreface deposits laterally and at depth. Distal ebb-tidal delta deposits are thus relatively thin and have an overall coarsening-upward grain-size trend.