Palaeohydrology of the Mulhouse Basin: are fluid inclusions in halite tracers of past seawater composition?

Brine reactions processes were the most important factors controlling the major-ion evolution in the Oligocene, Mulhouse Basin (France) evaporite basin. The combined analysis of fluid inclusions in primary textures in halite by Cryo-SEM-EDS with sulfate-δ34S, δ18O and 87Sr/86Sr isotope ratios reveals hydrothermal inputs and recycling of Permian evaporites, particularly during advanced stages of evaporation in the Salt IV member which ended with sylvite formation. The lower part of the Salt IV evolved from an originally marine input. Sulfate-δ34S shows Oligocene marine-like signatures at the base of the member (Fig.1). However, enriched sulfate-δ18O reveals the importance of re-oxidation processes. As evaporation progressed other non-marine or marine-modified inputs from neighbouring basins became more important. This is demonstrated by an increase in K concentrations in brine inclusions, Br in halite and variations in sulfate isotopes trends and 87Sr/86Sr ratios. The recycling of previously precipitated evaporites was increasingly important with evaporation. Therefore, regardless of the apparent marine sequence (gypsum, halite, potassic salts), the existence of diverse inputs and the consequent chemical changes to the brine preclude the use of trapped brine inclusions in direct reconstruction of Oligocene seawater chemistry.European Association for Geochemistry; Geochemical Societ

Exploring the hydrochemical evolution of brines leading to sylvite precipitation in ancient evaporite basins.

Sylvite is a very common mineral in ancient evaporite deposits. Due to the absence of current deposits, the natural geochemical mechanism/s for synsedimentary sylvite precipitation and accumulation are not well understood. Numerous sylvite deposits or portions of them have been described as a result of diagenesis (i.e. Sergipe subbasin, Brasil). However, a number of deposits have been described as synsdimentary or being formed during primary evaporite deposition. It is the last group of deposits that can be studied to better understand the hydrochemical processes taking place in the brine at the onset of sylvite precipitation. The Salt IV sylvite beds from the Mulhouse potash basin, Alsace (France) have been described as synsedimentary in origin (LOWENSTEIN and SPENCER, 1990; CENDON et al., 2008). While sylvite in itself does not contain fluid inclusions viable for micro analysis, primary textures in neighboring halite are used as a proxy to understand brine evolution. Two halite-sylvite cycles from the B1 and B2 layers of the potash lower seam were selected. These exhibited clear primary halite crystal textures with sylvite adapting to an irregular halite sedimentary surface and finishing with a flat surface. The nine halite samples, selected at centimeter scale, provided close to 100 single fluid inclusion analyses, representing both the transition towards sylvite precipitation and the post sylvite precipitation. The fluid inclusion analyses revealed strong fluctuations in K concentration, well over the analytical error (<10%). These variations, in the same halite crystal, seem aligned in growth bands, with fluid inclusions within a certain growth band showing practically identical K concentrations, while neighboring bands exhibit a different concentration. Overall, the closer we are from a sylvite layer the higher K concentrations are. However, strong fluctuations continue when growth bands are compared. This pattern shows cycles of increasing K concentration along parallel growth bands with sharp falls followed by the initiation of a new increasing trend. The small “growth band” scale of the K concentration variations, suggests very sensitive processes within the brine with potential environmental changes (i.e. seasonal variations, day-night temperature fluctuations cycles) leading towards the final mass precipitation of a sylvite layer

Chemical and hydrological evolution of the Mulhouse potash basin (France): are "marine" ancient evaporites always representative of synchronous seawater chemistry?.

Brine reaction processes were the most important factors controlling the major-ion (Mg, Ca, Na, K, SO4, and Cl) evolution of brines in the Oligocene, Mulhouse basin (France) evaporite basin. The combined analysis of fluid inclusions in primary textures by Cryo-SEM-EDS with sulfate-delta S-34, delta O-18 and Sr-87/Sr-86 isotope ratios reveals hydrothermal inputs and recycling of Permian evaporites, particularly during advanced stages of evaporation in the Salt IV member. The lower part of the Salt IV evolved from an originally marine input. The basin was disconnected from direct marine inputs and a series of sub-basins formed in an active rift setting where tectonic variations influenced sub-basin interconnections and chemical signatures of input waters. Sulfate-delta S-34 shows Oligocene marine-like signatures at the base of the member. However, enriched sulfate-delta O-18 reveals the importance of synchronous re-oxidation processes. As evaporation progressed other non-marine and/or marine-modified inputs from neighbouring basins became more important. This is demonstrated by increases in K concentrations in brine inclusions and Br in halite, sulfate isotopes trends and Sr-87/Sr-86 ratios. The recycling of previously precipitated evaporites of Permian age was increasingly important with evaporation. This supports the connection of the Mulhouse basin to basins situated north of Mulhouse. The brine evolution eventually reached sylvite precipitation. The chemical signature of the resulting brines is not compatible with global seawater chemistry changes. The fast rate of intra and inter basin brine variations as well as the existence of contemporaneous brines with different chemical signatures, supports our interpretation. The existence of diverse non-marine inputs and associated internal chemical changes to the brine preclude the use of trapped-brine inclusions in reconstructing Oligocene seawater chemistry, without previously identifying all inputs. The general hydrological evolution of the Mulhouse basin is explained as a restricted sub-basin with a first marine stage. This gradually changed to a similar to 40% marine source at the beginning of evaporite precipitation, with the rest of inputs non-marine. The general proportion of solutes did not change greatly over evaporite precipitation. However, as the basin restriction increased the originally marine inputs changed to continental or marine-modified inputs from neighbouring basins north of Mulhouse basin. © 2008, Elsevier Ltd

Paleoecological constraints on reef-coral morphologies in the Tortonian–early Messinian of the Lorca Basin, SE Spain

Coral reefs represent one of the main carbonate factories that contributed to the control of the stratigraphic architecture of carbonate platforms, which had a widespread development during the late Miocene in the paleo-Mediterranean area. The late Miocene reef complexes of the Lorca Basin in southeastern Spain are composed of five mixed siliciclastic/carbonate units, middle Tortonian to early Messinian in age. The development of coral reefs probably ceased when the first evaporitic event occurred in the basin centre in the early Messinian. This study mainly focuses on the response of reef communities and the modifications of reef organisation to global and regional parameters. At the platform scale, the carbonates are intermixed with terrigenous deposits related to two main types of clastic systems: torrential fans and fluvial to deltaic systems. The amount of clastic input greatly affected reef growth and coral morphologies. Three different types of stratal geometries were delineated in the reef complex: sigmoids, bioherms, and patches and carpets. The reef frameworks are mainly constructed by a poorly diversified assemblage of corals composed of poritids, faviids, and mussids. Porites is the principal reef builder of the sigmoids and carpets where it is widely distributed. Tarbellastraea is common in bioherms and Acanthrastraea appears generally associated with Porites in patches. Five basic growth forms of Porites are observed: thin branching or “finger-shaped”, thick branching to columnar, domed to hemispheric, encrusting, and platy to dish. Differences in coral morphology are used to define a relative water depth zonation in monogeneric reefs. The distribution of these growth forms was principally controlled by water depth. The reef flat is dominated by small thin branching or finger-shaped corals that are replaced towards the reef front by domed to hemispheric corals commonly encrusted by coralline algae. Downslope, columnar morphologies grade into thin branching shapes. The reef morphologies are variable throughout the five mixed siliciclastic/carbonate units at the platform scale. The first and oldest unit is dominated by bioclasts, whereas units 2, 3, and 5 are Porites-dominated, sigmoid complexes. Unit 4 is a well-developed biohermal complex mainly composed of Tarbellastraea. These units started to develop as early as middle Tortonian and stopped as late as early Messinian, and show a progradational trend, where the two latest units are well developed. Thus, carbonate production changed from grain-producing biota in the basal unit to framework-producing biota in the overlying units, consistent with evolution from a distally steepened ramp to a reef-rimmed shelf. At the scale of individual reef units, the relative water depth zonation of the corals is controlled by ecological changes (substrate, nutrients, synecologic relations, and diversification of coral species). In the transects across the carbonate platform related to the different units, the coral zonation records changes in spatial distribution of corals in response to ecological stresses and changes in regional and global environments (tectonic, relative sea-level changes, and runoff)