Geology topics

Robert A Morton

Publications and source records attributed to Robert A Morton.

4 recordsLinked to original sources

USGS2010

First-order controls of extreme-storm impacts on the Mississippi-Alabama Barrier-Island chain

Predicting the morphological impacts and associated hazards of extreme storms on barrier islands is facilitated by examining historical poststorm images and identifying the predominant alongshore and cross-shore patterns of erosion and deposition for different island segments. Morphological changes on the Mississippi–Alabama barrier-island chain produced by 12 Category 3 and stronger hurricanes since 1852 were analyzed to investigate whether barrier-island responses to extreme storms are controlled primarily by local geomorphic conditions or primarily by storm characteristics. Results of those analyses demonstrate that (1) antecedent topography and geomorphic conditions (island width, land elevations, nearshore bathymetry, subaqueous-boundary conditions) tend to exert greater control on local barrier-island impacts than storm parameters (path, intensity, wind speeds, water levels, shelf duration), and (2) types and alongshore patterns of storm overwash and island breaching are commonly repeated for the same island segments. Even when impact patterns are identical, magnitudes of sequential impacts, such as the inland distance of sediment transport, are unequal and are controlled by storm parameters (water levels, wind speeds) that influence wave heights, overwash-flow depths, and current velocities.

Alabama, Mississippi↗

USGS2009

Simple methods for evaluating accommodation space formation in coastal wetlands

Land-surface subsidence and erosion are the principal processes that form accommodation space in interior coastal wetlands when they are converted to open water. The relative contribution of subsidence and erosion to wetland loss can be estimated by comparing elevations and vertical offsets of stratigraphic contacts that are correlated between adjacent sediment cores. Accommodation-space measurements assume that wetland-sediment thicknesses and the elevation of stratigraphic contacts were originally nearly uniform over short horizontal distances (tens to hundreds of meters). The accommodation space attributable to erosion equals the difference in wetland-sediment thickness between wetland cores and adjacent open-water cores taken at formerly emergent wetland sites. The accommodation space attributable to subsidence equals the elevation difference of a stratigraphic marker correlated between the two cores using the wetland core as the reference standard. Together, subsidence plus erosion at an open-water core location equals the accommodation space created by land loss, which is the difference between the adjacent emergent wetland elevation and the existing water depth.

Wetlands↗

USGS2009

Geological processes and sedimentation rates of wind-tidal flats, Laguna Madre, Texas

Coastal flats worldwide that are periodically exposed to arid climates and periodically flooded by marine waters are unique depositional environments because they receive sediments surficially and interstitially from both land and sea. The wind-tidal flats bordering Laguna Madre, Texas, which fit this unique category, are modified by eolian processes when subaerially exposed, and by wave and current processes when submerged. Floodwater is derived from the lagoon and driven onto the flats by strong and persistent winds during the passage of cold fronts and tropical cyclones. Low surface gradients of the flats prevent rapid drainage and promote seawater evaporation. The depositional products of these processes are interbedded and interlaminated sand, mud, marine shells, algal mats, and evaporites. This assemblage of sediments is geologically diagnostic evidence for intertidal marine deposition and the same assemblage of sediments have been reported for modern marginal-marine flats in the Middle East. The wind-tidal flat surface at Laguna Madre is constantly changing. However, the net effect of natural changes during the past century is either negligible or the changes occur at such a slow rate that they are almost imperceptible. Sediments are repeatedly added to and removed from the surface of the flats in minor increments and in different areas at different times. Preservation potential is enhanced at a particular site by the development of thick mats of blue-green algae. The 14 C ages of buried algal mats yield average long-term (centuries to millennia) sedimentation rates for the wind-tidal flats that range from 0.13 to 0.96 mm/yr and average 0.57 mm/yr. The 210 Pb profiles yield average short-term (150 yr) sedimentation rates that are an order of magnitude higher, ranging from 0.7 to 8.3 mm/yr and averaging 2.9 mm/yr. The minimum present rate of relative sea-level rise in Laguna Madre (3.4 mm/yr) exceeds the historical sedimentation rates for most of the flats. If future sea-level rise is faster than the rates of aggradation, then the wind-tidal flats will progressively become more frequently flooded and will eventually become permanently submerged.

Texas↗

USGS2008

Historical changes in the Mississippi-Alabama barrier-island chain and the roles of extreme storms, sea level, and human activities

Barrier-island chains worldwide are undergoing substantial changes, and their futures remain uncertain. An historical analysis of a barrier-island chain in the north-central Gulf of Mexico shows that the Mississippi barriers are undergoing rapid systematic land loss and translocation associated with: (1) unequal lateral transfer of sand related to greater updrift erosion compared to downdrift deposition; (2) barrier narrowing resulting from simultaneous erosion of shores along the Gulf and Mississippi Sound; and (3) barrier segmentation related to storm breaching. Dauphin Island, Alabama, is also losing land for some of the same reasons as it gradually migrates landward. The principal causes of land loss are frequent intense storms, a relative rise in sea level, and a sediment-budget deficit. Considering the predicted trends for storms and sea level related to global warming, it is certain that the Mississippi-Alabama (MS-AL) barrier islands will continue to lose land area at a rapid rate unless the trend of at least one causal factor reverses. Historical land-loss trends and engineering records show that progressive increases in land-loss rate correlate with nearly simultaneous deepening of channels dredged across the outer bars of the three tidal inlets maintained for deep-draft shipping. This correlation indicates that channel-maintenance activities along the MS-AL barriers have impacted the sediment budget by disrupting the alongshore sediment transport system and progressively reducing sand supply. Direct management of this causal factor can be accomplished by strategically placing dredged sediment where adjacent barrier-island shores will receive it for island nourishment and rebuilding.

Alabama, Mississippi↗