Placer deposit formation in marine transgressive environments: Nearshore Mississippi model.

摘要

The nearshore of Mississippi was evaluated as a geologic environmental system to clarify the potential for heavy mineral deposition and to determine the mechanisms of deposit development applicable to transgressive environments. The analysis of the nearshore geological system considers the influence of antecedent physiography, and the attributes and relations among geomorphic, sediment, tectonic and energy-related elements. Results revealed that the stable barrier island ridge controls the modern environment, which is characterized by a sand sediment deficit, storm-dominated conditions, and a slow rate of relative sea level rise. Under present environmental conditions, the barrier islands are deteriorating in-place, truncating the barrier island ridge foundation. Barrier island sediment are spreading leeward and parallel to the barrier ridge axis into the adjacent sound.; Seabed gradient and energy-related processes along the seaward shoreface of the barrier island ridge represent conditions requisite for development of a strandline heavy mineral placer occurrence. The spatial occurrences of two groups and seven species of heavy minerals were evaluated using spatial and multivariate statistical analyses methods. The results substantiate strandline deposition of titanium minerals along the seaward flanks of the barrier ridge. For example, ilmenite concentration along the ridge flanks is 35.3% by weight of the sand-size heavy minerals, nearly 1.6 times the regional mean. By contrast, alumino-silicate minerals are dispersed uniformly throughout the region. Zircon is generally confined to the eastern portion of the region, possibly reflecting the mineral suite of the eastern province of the northern Gulf of Mexico.; Significant occurrences of titanium minerals were identified leeward of Horn, Ship, and Petit Bois Islands associated with washover and flood tidal features that are indicative of barrier island evolution. The occurrences reveal shore-normal transport of seaward strandline titanium minerals to relatively protected, leeward depositional sites. With continued gradation of the barrier islands, the leeward titanium mineral placer occurrences will develop into a series of lens-shaped structures that parallel the truncated barrier ridge axis. As a consequence of the stability of the ridge foundation and continued slow relative sea level (RSL) rise, the irregular-shaped placer deposits have a positive preservation potential. Without the ridge stability, barrier island shoreward migration would destroy the placer structures and disperse the titanium minerals. Thus, a method of placer deposit development was elucidated, and associated with the evolution of a transgressive geological system of the nearshore of Mississippi.; A model was created that describes the placer deposit development within a barrier-controlled transgressive environment. The model identifies deposit development as stages related to the continuum of nearshore evolution: (1) formation--strandline deposition of titanium minerals along the seaward flank of a barrier ridge, (2) development--transport and accumulation of mineral-enriched sediment to leeward of the ridge by processes associated with barrier island gradation, and (3) preservation--burial of the placer structures with transgressive sediment under marine conditions.; Several placer occurrence sites are cited as analogue situations to illustrate the concept and application of the model. The utility of the descriptive model primarily is in its application as a reference for interpretation of placer occurrences and deposits associated with other transgressive nearshore environments.