Retrogradation

Abstract: Previous geologic research on Holocene Mississippi River deltaic deposits has verified that the present delta plain and associated nearshore barrier islands and submarine shoals are either direct or indirect products of cyclic delta-building events that have operated on a variety of temporal and spatial scales. A major depositional element of the modern delta plain is the delta complex, of which there are six: (1) Maringouin, (2) Teche, (3) St. Bernard, (4) Lafourche, (5) Balize, and (6) Atchafalaya. Major delta-building events have occurred at a frequency of one every 1- 2 kyr. Deposits associated with the six major delta complexes are fundamental constructional units of the delta plain, which collectively covers an area of - 30,000 km2. Sedimentary deposits associated with these delta-building events range in thickness from about 10 to 100 m. Their construction is modulated by stream capture, which develops a new delta complex by way of a new river course. Delta complexes may be comprised of one or more delta lobes. As a product of this delta switching, the depositional architecture of the delta plain consists of laterally offset and stacked delta lobes. Within delta lobes are subdeltas and even smaller crevasse splays. These smaller scale deltas sedimentologically and geomorphically mimic their larger delta lobe counterparts, but they are considerably thinner, cover less area, and have a shorter period of development an d abandonment. Subdeltas are usually < 10 m thick and may fill shallow bays that cover over 300 km2. They build and deteriorate on time-scales of 150-200 years. Crevasse-splays or overbank splays are < 5 m thick, cover only a few square kilometers, and are abandoned after several decades of active growth. Each delta evolves through a rapid regressional phase as water and sediment are capture d from an antecedent river course. If highstand conditions persist long enough, deltas may prograde to the outer shelf to form wedges of deltaic sediment much thicker than their inner shelf counterparts. The delta-building process starts with the filling of interior lakes (lacustrine deltas), which is followed by bayhead delta-building at the coast, and finally by progradation across the marine shelf (shelf delta). Delta complexes and delta lobes, as well as their smaller counterparts, experience three phases of growth and abandonment: (1) rapid growth with increasing-to-stable discharge, (2) relative stability during initial stages of waning discharge, when sediment input balances the collective effects of subsidence, and (3) abandonment, followed by rapid subsidence-driven subaerial delta deterioration. In the rapid growth stage, formerly eroding-subsiding coastal environments experience delta plain accretion and coastal progradation from renewed sediment input. On the abandonment side of the cycle, marine processes overwhelm fluvial processes and rework the delta perimeter. Forced by the combined processes of subsidence, the delta surface undergoes progressive submergence. Transgressive sand bodies created by wave reworking of the delta evolve from headland beaches and spits, to barrier islands, and finally to submarine shoals as the abandonment phase is completed.

Abstract: Over the last century, the river-dominated Mississippi delta has received increasing attention from geoscientists, biologists, engineers, and environmental planners because of the importance of the river and its deltaic environments to the economic well-being of the state of Louisiana and the nation. Population growth, subsurface re source extraction, and increased land-water use have placed demands on the delta's natural geologic, biologic, and chemical systems, therefore modifying the time and spatial scales of natural processes within the delta and its lower alluvial valley. As a result, the combined effects of natural and human- induced processes, such as subsidence, eustatic sea level rise, salt water intrusion, and wet land loss, have produced a dynamically changing landscape and socioeconomic framework for this complex delta. Under natural conditions, the fundament al changes that result in land-building and land loss in the Holocene Mississippi River delta plain are rooted in the systematic diversion of water and sediment associated with major shifts in the river 's course-the process of delta switching. Research over the last half century has shown that major relocations of the Mississippi's course have resulted in five Holocene delta complexes and a sixth one in an early stage of development as a product of the latest Atchafalaya River diversion. Collectively, these Holocene deltas have produced a delta plain that covers an area of ~30,000 km 2 and accounts for 41% of the coastal wetlands in the United States. After a river diversion takes place, the resulting delta evolves through a systematic and semipredictable set of stages generally characterized by: (a) rapid progradation with increasing-to-stable discharge, (b) relative stability during initial stages of waning discharge, (c) abandonment by the river in favor of a higher gradient course to the receiving basin , and (d) marine reworking of a sediment-starved delta as it under goes progressive submergence by the combined processes of subsidence. Delta switching has taken place every 1000 to 2000 years during Holocene times, and resulting deltas have an average thickness of approximately 35 m. Within a single delta there are subdeltas, bayfills, and crevasse-splays that have higher frequency delta cycles ranging from several hundred years to a few decades. These depositional features are usually less than 10 m thick, and some have produced marshland areas of over 300 km 2 . The net result of these delta-building events is a low-lying landscape with components that are changing (building and deteriorating) at different rates. Geologically, these depositional cycles produce a thick accumulation of coarsening, upward deltaic deposits that have various thicknesses in response to development on a variety of temporal and spatial scales. In this river-dominated delta system, distributaries can prograde seaward at rates of over 100 m/year. The cumulative effect of the Holocene depository has been to depress the underlying Pleistocene surface. In a local setting, e.g., the modern Balize Lobe, differential loading causes the vertical displacement of underlying clay-rich facies (shale diapirs-mudlumps). The delta front of this lobe, which has prograded into deep water of the outer continental she lf, is characterized by rapid deposition of silt - and clay- rich sediments and slope instability, which results in sea ward displacement of sediments by a variety of mass-movement processes. Superimposed on the natural processes and forms of the Mississippi deltaic plain and its associated estuarine environments, are human impacts, most of which have been imposed in this century. The most significant impacts have resulted from a decrease in sediment input to the river from its tributaries and the alteration of the river's natural sediment dispersal processes through the construction of levees. Measures are now being taken to reinstate some of the delta's natural processes, thereby mitigating landloss so that decline in animal and plant productivity can be mitigated.

Abstract: Using conceptual reasoning and results from physical models, we describe the mechanical and kinematic characteristics of sediment wedges spreading seaward above a viscous evaporitic layer. Spreading can occur if the distal sediment overburden is thin and weak, or if it comprises preexisting salt bodies. Spreading is accommodated by proximal extension and an associated rise of diapiric ridges, by midslope seaward translation, and by distal shortening. Rapid sediment progradation can lead to the reactivation of older distal folds by later extension. Either retrogradation or renewed sedimentation following a long depositional hiatus can reactivate older grabens and diapirs in shortening.