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Depositional sequences and correlation of middle (?) to late Miocene carbonate complexes, Las Negras and Nijar areas, south-eastern Spain
... Because clasts in the volcanic breccia may have been reworked from slightly older deposits, we interpret the age of the volcanic unit as 8.5 ± 0.1 Ma. These new dates are the first reliable age control obtained for the five carbonate depositional sequences in the Las Negras area, which has remained controversial up until now Mankiewicz, 1991, 1993;Martin and Braga, 1993). The dates on the interbedded volcanic unit now indicate clearly that DS1A deposition started well within the Tortonian, as opposed to latest Tortonian or Messinian time (Martin and Braga, 1993), which is generally consistent with biostratigraphic data in the area reported by Serrano (1992). ...
... These new dates are the first reliable age control obtained for the five carbonate depositional sequences in the Las Negras area, which has remained controversial up until now Mankiewicz, 1991, 1993;Martin and Braga, 1993). The dates on the interbedded volcanic unit now indicate clearly that DS1A deposition started well within the Tortonian, as opposed to latest Tortonian or Messinian time (Martin and Braga, 1993), which is generally consistent with biostratigraphic data in the area reported by Serrano (1992). ...
Carbonate platforms in the Las Negras area evolved from onlapping ramps to fringing reef complexes later draped by cyclic shallow marine strata. Although sea-level history and paleoclimate had an effect on platform evolution, substrate topography played a dominant role. The strata are divided into five depositional sequences of Tortonian and Messinian age. The model of platform evolution provided by the Las Negras area exposures likely has applications for other Miocene carbonate complexes in the Mediterranean and could apply to other carbonate complexes in the rock record.
... Polopos is situated in a corridor which joins the Sorbas and Carboneras basins and which divides the Sierra Alhamilla and the Sierra Cabrera ( Fig. 1). The conglomerate units at Polopos represent the basal part of the first depositional sequence (DS1) of Franseen and Mankiewicz (1991), considered to be of late Tortonian to Messinian age by Martin and Braga (1993). The conglomerates represent the advancing littoral zone of the basal Messinian transgressive episode, and are laterally equivalent to the calcarenites of the Azagador Member and the basinal marls of the Abad Member, which progressively offlap the basement. ...
This paper documents a Miocene littoral boulder mass at Polopos, in southern Spain. The boulder mass overlies Paleozoic‐Mesozoic schist and dolomite basement rocks, and is representative of a basin‐wide Messinian transgressive episode. The boulder mass consists predominantly of dolomite boulders which were colonized by rock‐borers. The rock‐boring community is limited to the ichnotaxa Entobia and Gastrochaenolites, the first colonizers of a mobile substrate. Two colonization intervals may be recognised in the boulder mass. The oldest of these is represented by a dolomite boulder conglomerate in which over 62 percent of the clasts have Gastrochaenolites and Entobia borings on their lower surface alone. The younger interval is recorded by large dolomite boulders, with mostly large diameter Gastrochaenolites borings on their upper surfaces. These borings provide an insight into the evolution of the Miocene transgressive episode in the Sorbas Basin, and into the ecology of a substrate under high ambient energy levels.
... For more details on the lithologies and sedimentology, the reader is referred to Dabrio et al. (1981), Serrano (1990) and Mankiewicz (1996). The stratigraphy of Níjar is complex due to its tectonic and morphological setting and has been described, interpreted and debated by previous researchers (Fig. 4 and see Franseen & Mankiewicz, 1991Martin & Braga, 1993). ...
This study investigates the controls on three-dimensional stratigraphic geometries and facies of shallow-water carbonate depositional sequences. A 15 km2 area of well-exposed Mid to Late Miocene carbonates on the margin of the Níjar Basin of SE Spain was mapped in detail. An attached carbonate platform and atoll developed from a steeply sloping basin margin over a basal topographic unconformity and an offshore dacite dome (Late Miocene). The older strata comprise prograding bioclastic (mollusc and coralline algae) dominated sediments and later Messinian Porites reefs form prograding and downstepping geometries (falling stage systems tract). Seven depositional sequences, their systems tracts and facies have been mapped and dated (using Sr isotopes) to define their morphology, stratigraphic geometries, and palaeo-environments. A relative sea-level curve and isochore maps were constructed for the three Messinian depositional sequences that precede the late Messinian evaporative drawdown of the Mediterranean. The main 3D controls on these depositional sequences are interpreted as being: (i) local, tectonically driven relative sea-level changes; (ii) the morphology of the underlying sequence boundary; (iii) the type of carbonate producers [bioclastic coralline algal and mollusc-dominated sequences accumulated in lows and on slopes of < 14° whereas the Porites reef-dominated sequence accumulated on steep slopes (up to 25°) and shallow-water highs]. Further controls were: (iv) the inherited palaeo-valleys and point-sourced clastics; (v) the amount of clastic sediments; and (vi) erosion during the following sequence boundary development. The stratigraphy is compared with that of adjacent Miocene basins in the western Mediterranean to differentiate local (tectonics, clastic supply, erosion history, carbonate-producing communities) versus regional (climatic, tectonic, palaeogeographic, sea-level) controls.
The Neogene-Quaternary Almeria-Nijar basin includes the Carboneras Fault, which constitutes a major left-lateral feature of the Betic Cordilleras. New gravity data help to determine the geometry of the sedimentary infill. The region underwent NE-SW extension during the Tortonian and local NW-SE compression during the first stages of Sierra Alhamilla uplift. During the Messinian, the sinistral strike-slip motion along the Carboneras Fault Zone, the dextral strike-slip motion along NW-SE-oriented faults, and the development of large folds such as the Sierra Alhamilla antiform, suggest clockwise rotation (towards the north) of the maximum stress axis (sigma(1)). During the Pliocene, a NNW-SSE-oriented compression also contributes to fold development. Finally, during the Quaternary, an ENE-WSW-directed extension controls the development of NW-SE-oriented normal oblique faults. The most recent local normal activity of the Carboneras Fault is related to this extension, whereas its behaviour as a left-lateral strike-slip fault may be a consequence of the accommodation of NW-SE normal fault displacements. Basic rock bodies, recognized by means of a detailed study of the magnetic anomalies, are related to the volcanic activity known to have occurred in the area in Late Miocene times.
Small and isolated Messinian patch reefs showing a typical internal succession and an oligospecific reef-building community are described from the western margin of the San Miguel de Salinas Basin (SE Spain). These thrived in a mixed carbonate -siliciclastic environment and are known from two different stratigraphic levels. The reef growth is characterized by a shallowing-upward sequence and can be subdivided into four divisions: reef debris zone; coralline algal zone; branching coral zone; and reef crest zone. This ecological succession. of Messinian reefs reflects an increasing water turbulence and the accommodation of the reef-building organisms to shallower-water settings, which is conditioned by its own upward growth. The start of reef growth is controlled by a reduced influx of terrigenous deposits, which derived from fan sediments reworked by waves. The reefs were killed by smothering with quartzose calcarenites. The low diversity of the reef-building community indicates an arrested ecological succession without climax stages, controlled by sedimentation rate and siliciclastic supply. A salinity increase can be excluded, because the reef-associated fauna indicates normal-marine conditions during reef growth. There is no evidence for subaerial exposure of the reefs during the formation of evaporites at the end of the Messinian.
Late Miocene platform carbonates from Nijar, Spain, have been extensively dolomitized. Limestones are present in the most landward parts of the platform, in stratigraphically lower units and topographically highest outcrops, suggesting that dolomitizing fluids were derived from the adjacent Nijar Basin. The dolomite crystals range from <10 to ≈100 μm existing as both replacements and cements. Na, Cl and SO 4 concentrations in the dolomites range from 200 to 1700 p.p.m., 250–650 p.p.m., and 600–7000 p.p.m., respectively, comparable with other Tertiary and modern brine dolomite values, and also overlapping values from mixing‐zone dolomites. Sr concentrations range between 50 and 300 p.p.m., and the molar Sr/Ca ratios of dolomitizing fluids are estimated to range between 7× seawater brine to freshwater ratios. The δ ¹⁸ O and δ ¹³ C of the dolomites range from −1·0 to +4·2‰ PDB, and −4·0 to +2·0‰ PDB, respectively. ⁸⁷ Sr/ ⁸⁶ Sr values (0·70899–0·70928) of the dolomites range from late Miocene seawater to values greater than modern seawater.
Mixtures of freshwater with seawater and evaporative brines probably precipitated the Nijar dolomites. Modelled covariations of molar Sr/Ca vs. δ ¹⁸ O and Na/Ca vs. δ ¹⁸ O from these mixtures are consistent with those of the proposed Nijar dolomitizing fluids. Complete or partial dolomite recrystallization is ruled out by well preserved CL zoning, nonstoichiometry and quantitative water–rock interaction modelling of covariations of Na vs. Sr and δ ¹⁸ O vs. δ ¹³ C. The possibility of multiple dolomitization events induced by evaporative brines, seawater and freshwater, respectively, is consistent with mineral‐mineral mixing modelling.
The basin‐derived dolomitizing brines probably mixed with freshwater in the Nijar Basin or mixed with fresh groundwater in the platform, and were genetically related either to deposition of the Yesares gypsum or the Feos gypsum. Dolomitization occurred during either the middle Messinian or the early upper Messinian. Nijar dolomitization models may be applicable to dolomitization of other late Miocene platform carbonates of the western Mediterranean. Moreover, the Nijar models may offer an analogue for more ancient evaporite‐absent platform carbonates fringing evaporite basins.
The Late Tortonian-Early Messinian Abad Member in the Sorbas Basin, south-east Spain contains abundant foraminifera. Planktonic foraminifera have been used to reconstruct the environmental conditions that developed within the basin. Benthonic foraminifera reveal a detailed picture of the palaeoenvironment. After an initial deepening episode, from upper epibathyal (c. 200 m) depths to lower epibathyal (c. 1000 m) depths, the basin progressively shallowed to shelf depths (c. 100 m); this palaeobathymetric reconstruction thus describes an initial rapid transgressive episode, a highstand and a regressive phase; the basin water mass became progressively warmer, and yet there was no significant increase in salinity or oxygen level in the water mass. The lack of any microfossil evidence to support an increase in the salinity levels towards the top of the Abad Member indicates a rapid environmental switch from normal marine conditions during the deposition of the Abad Member to hypersaline conditions during deposition of the overlying Yesares Member and precipitation of great thicknesses of gypsum. This research illustrates the necessity for high resolution sampling, approximate to the standard interval (10 feet: c. 3 metres) used in industrial boreholes, for Mediterranean Late Miocene stratigraphies, and indicates that all previous palaeoenvironmental studies on the Sorbas Basin have had inadequate sampling regimes which do not represent the evolving environmental conditions of the Abad Member.
Low-Mg calcite shells have been widely used to reconstruct the chemistry of ancient seawater. There is always a question:
are the shells chemically pristine? This paper presents the isotope and elemental geochemistry of low-Mg calcite bivalve shells
in late Miocene platform carbonates, SE Spain. The platform carbonates were extensively dolomitized, and limestone is restricted
to older stratal units, and to units mainly in topographically higher and more landward strata. Low-Mg calcite oyster shells
were completely dissolved out in the basinward dolomite, but are well preserved in the limestone. These shells appear to retain
the original growth microstructures, based on hand samples. Under the microscope, however, dissolution and recrystallization,
as well as pristine growth lines are all present. Sr isotopes in these shells range from that of normal Miocene seawater to
radiogenic values. δ18O and δ13C values, and Mg, Sr, and Na concentrations in these shells are rather variable. The high end members are consistent with
the typical values of modern/late Miocene normal-seawater low-Mg calcite shells, whereas the low end members are close to
those of diagenetic calcite cements, which have low δ18O, δ13C, Sr and Na values, and radiogenic Sr. The Nijar shells were altered physically and chemically to different degrees by diagenesis,
although these shells are consistent with some “criteria” of unalteration. The isotopic and trace-element data collected in
altered and pristine (or less altered) portions coexisting in the same shells are clearly differentiable. Quantitative simulation
of covariations of geochemical pairs indicates that solid mixing of unaltered and altered portions by sampling is consistent
with the variations in isotopic and elemental data recorded in the Nijar shells. The geological significance of this study
is that ancient fabric-retentive calcite shells may have been altered geochemically although they may appear pristine. Calcite
shells that underwent intensive diagenesis should be examined rigorously under the microscope, coupled with investigations
of multiple geochemical proxies to assess chemical alteration. Only the data of unaltered shells can be used to reconstruct
the chemistry of ancient seawater.
The Messinian (Late Miocene) marine stratigraphic record of the Sorbas Basin (S.E. Spain) is well preserved and can be considered as being representative of the entire western Mediterranean. It exhibits a series of features relating to: (1) the composition, characteristics and evolution of coral reefs; (2) changes between temperate and subtropical climates; and (3) the extensive development of microbial carbonates (stromatolites and thrombolites) at the end of the Messinian. Each of these features has global significance.
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