Subsurface seepage of seawater across a barrier: A source of water and salt to peripheral salt basins

Abstract

Many of the great Phanerozoic salt basins share a common paleogeography, in which a deep lake or sea is separated from the ocean by a narrow barrier. Examples include the Aptian evaporites of the South Atlantic rift basins and the Messinian salt deposits of the Mediterranean Sea and the North Caspian depression. Marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins. We test an alternate hypothesis, that subsurface seepage of seawater through the barrier was a signifi cant source of water, infl uencing both water level and water composition. Our model permits fl ux of water and salt into and out of the basin via (1) seepage through the barrier; (2) evaporation from the water surface; (3) rainfall and/or rivers; and (4) incorporation into basin sediments. Our investigation focuses on three settings, a modern analog in the Gregory Rift of northern Kenya, the Messinian Mediterranean basin, and the Early Cretaceous Aptian salt basins of the South Atlantic. We test our model through a numerical simulation, applying reasonable values for the dimensions of the barrier, differences in water level, and hydraulic conductivities to calculate fl ow rates through the barrier. Modeling results indicate that fl ux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignifi cant, depending on the climate, size of the peripheral basin, and hydraulic conductivity of the barrier. The fl ux of salt is signifi cant over the full range of modeled hydraulic conductivities, producing brines in the peripheral lake and/ or sea within 0.5 m.y. in most cases. This demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions, and that marine transgressions across the barrier are not necessary to explain the apparent high salinities.

A new scheme for the opening of the South Atlantic Ocean and the dissection of an Aptian salt basin

“…On the other hand, Dupré et al (2007) found that subsidence trends derived from backstripping calculations from the South Gabon Basin could correlate with a syn-rift origin for the evaporites. Nunn & Harris (2007) also underlined that the thick underlying Barremian-lower Aptian section from both margins north of the Rio Grande Rise-Walvis Ridge contained an extensive record of deposition from saline lakes, with a complete absence of any marine signature from the Neocomian to the middle Aptian. These authors proposed a model of subsurface seepage of sea water across a barrier to explain this apparent geological dilemma, which also would constitute a possible additional mechanism for the syn-rift deposition of the great evaporite sequence.…”

“…The deposition of these units is characterized by a low tectonic activity without major block faulting (Marton et al 2000;Moulin et al 2005) and is related to the formation of offshore basins such as parts of the Gabon Basin (Tessereinc & Villemin 1990). The lacustrine sedimentation was followed by the deposition of a thin layer of marine sediments known as the Chela Layer (Moulin et al 2005;Nunn & Harris 2007). Most authors agree that the salt was deposited during Aptian time in about 5 Myr (Moulin et al 2005 and references therein).…”

Seismic stratigraphy and subsidence analysis of the southern Brazilian margin (Campos, Santos and Pelotas basins)

“…These features indicate that middleelate Aptian sag depocenters evolved to salt basins during the last synrift stage. Hypersaline conditions in deep lakes of the sag margin resulted from either seepage of seawater through the Walvis RidgeRio Grande Rise (Nunn and Harris, 2007), intermittent marine incursions over this structural barrier (Karner and Gambôa, 2007), or a combination of both processes. Based on the available seismic and well data it is not possible to determine the processes controlling salt precipitation.…”

Natural contamination by As and heavy metals in soil, their bio-accumulation and potential sources: the case of a travertine limestone quarry, Greece

“…Lakes do not commonly contain forams, although some species may survive if introduced into coastal lakes or basins with near sea water salinities. Marine transgressions over the barrier, or subsurface seepage of seawater through the barrier have been proposed as the origin for saline conditions in these basins, and therefore a significant source of water, influencing both water level and water composition [30]. In addition to the river discharge a contribution of seawater at a coastal zone may lead to deposition of dissolved manganese and iron carried Table 7; where symbol m is the metal content in plant and m is the metal content in soil.…”