José M. Martín

Patterns and average rates of late Neogene-Recent uplift of the Betic Cordillera, SE Spain

Abstract The facies distribution in the sedimentary units infilling a series of Neogene basins has been used to reconstruct the relief generation and uplift across the Internal Zone of the Betic Cordillera in southern Spain. Uplift amounts and average rates can be estimated using the current elevation of the outcrops of well-dated deposits indicative of ancient sea-level positions. Coral reefs and coastal conglomerates record the initial development of emergent Betic relief during the Langhian. Continental and marginal marine deposits indicate the existence of a large island centred on the present Sierra Nevada–Sierra de los Filabres chain by the end of the Middle Miocene. The precursor of the Sierra Nevada–Sierra de los Filabres chain, originally part of this large island, remained emerged whilst the surrounding areas were re-invaded by the sea during the early Tortonian. At the end of the Tortonian the inland basins (Granada and Guadix basins) became continental, while the Sierras de la Contraviesa, Sierra de Gador and Sierra Alhamilla emerged, separating the Alboran Basin from the Alpujarra, Tabernas and Sorbas basins, which became narrow passages of the Mediterranean Sea. In contrast, the Sierra Cabrera emerged during the late Messinian, suggesting a progressive uplift from west to east of the sierras south of the Sierra Nevada–Sierra de los Filabres chain. During the Pliocene, only the low areas closest to the present-day coast remained as marine basins and progressively emerged throughout this stage. The highest average uplift rate recorded is 280 m/Ma for the Sierra de Gador, although the average uplift rates of upper-Neogene coastal marine rocks since depositon have maximum values of approximately 200 m/Ma. Most of the uplift of the Betic mountains took place before the early Pliocene. The recorded uplift of Neogene rocks was highest at the margins of western Sierra Nevada, where peaks higher than 3000 m occur. The average rates of uplift were lower to the east of this major relief. The main sierras and depressions in the present-day landscape correspond respectively to the emergent land, in which uplift was concentrated, and to the marine basins that existed before the final emergence of the region. The altitude of the sierras reflects the time at which they became emergent, the highest mountains being the first to rise above sea level.

Messinian events in the Sorbas Basin in southeastern Spain and their implications in the recent history of the Mediterranean

Abstract 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. Porites , which is the major and almost only coral component in reefs, is heavily encrusted with stromatolites. These reefs grew at the edge of the subtropical belt and were totally eliminated at the end of the Messinian because of global cooling. Lowermost-Messinian carbonate sediments in the Sorbas Basin reflect a temperate climate, whereas those immediately above, which contain bioherms and coastal reefs, are subtropical. The shift from temperate to subtropical conditions during the early Messinian was accompanied by an important change in water circulation within the western Mediterranean. Temperate times were marked by cold surface Atlantic waters entering the Mediterranean, whereas subtropical times coincided with warm surface waters entering the western Mediterranean from the east. The subtropical waters were thermally stratified, which favoured the deposition of euxinic marls and diatomites at the centre of the basin. The upwelling of nutrient-rich water promoted stromatolite development within reefs and Halimeda growth on adjacent slopes. Lastly, microbial carbonates (stromatolites and thrombolites) attained giant dimensions during the late Messinian, which can be regarded as a measure of their success in occupying a variety of ecological niches. This abundance of available habitats is believed to have resulted from the Messinian “salinity crisis”, which was followed by a re-colonization of the western Mediterranean. In this context stromatolite proliferation was due to opportunism of microbial communities in colonizing the new environments, rather than to a complete absence of other competitive biota. We do not believe that hypersaline conditions were a causal factor in stromatolite development because of the normal-marine biota associated with them.

Coral reefs in coarse-terrigenous sedimentary environments (Upper Tortonian, Granada Basin, southern Spain)

Abstract During the Upper Tortonian coral reefs developed on fan deltas and braid deltas along the active eastern edge of the Granada Basin. The corals grew in environments characterized by deposition of coarse sands and conglomerates. Frequency of sedimentation together with sea level rises controlled reef development and determined the architecture and composition of the carbonate deposits. In direct contrast to these reefs, others flourished at the same time along stable coastal margins of the basin. Their structure was influenced mainly by periodic rises in sea level. The mapping of all these reefs serves as an accurate guide to the Upper Tortonian geography of the Granada Basin.

Neogene coralline-algal growth-forms and their palaeoenvironments in the Almanzora river valley (Almeria, S.E. Spain)

Abstract The Almanzora river valley is an elongate E-W depression containing mainly Neogene sediments. During the Late Tortonian several horizons of crustose coralline algae developed in three distinct sedimentary contexts: (a) on coastal platforms and talus slopes, (b) on fan deltas and (c) associated with corals in patch-reefs growing at the furthermost extremes of coastal fan deltas. In the former two cases the predominant growths are as rhodoliths associated with mobile substrates. In the coastal systems an increase in depth results in a change in both the morphology and the composition of the rhodoliths. With depth there is a significant increase in the diversity of algal species and a concomitant increase in the size of the algal covering compared to the nucleus and thus the morphology of the rhodolith depends less and less on the shape of the nucleus itself. The massive crusts and columns, which mark the limit of development of the shallow-water rhodoliths, in the deeper examples are covered by branching and thin, leafy (laminar) growths. In the fan deltas the development of the rhodoliths is similar to that of the deeper coastal zones. In the patch-reefs the specific composition of the algal associations changes somewhat and the growths, developed on a stable substrate under low-energy conditions, are mainly vertical branches arising from thin crusts.

Nearshore, temperate, carbonate depositional systems (lower Tortonian, Agua Amarga Basin, southern Spain): implications for carbonate sequence stratigraphy

Abstract The bryozoan-rich lower Tortonian carbonates of the Agua Amarga Basin in southern Spain (Province of Almeria) provide an example of sediments formed in a nearshore, non-tropical depositional setting. Based on data derived from logging of sections and from field mapping, these lower Tortonian carbonates form a depositional sequence, which is subdivided into several depositional systems. A lowstand systems tract, which consists of volcaniclastic fan deltas and washover deposits, formed on the leeward side of a basement shoal which delimited the basin towards the south. A transgressive systems tract, which is characterised by a landward encroachment of deposits, is represented by submarine bars and dune deposits. A highstand systems tract consists of progradational beach deposits. Owing to strong reworking and re-distribution of the particles by currents and waves, microfacies differentiation of the deposits in poor. These Tortonian non-tropical carbonates display a style of deposition similar to siliciclastics suggesting that sequence stratigraphic concepts derived from tropical carbonates do not apply to non-tropical carbonates.

Internal structure of segment reefs: Halimeda algal mounds in the Mediterranean Miocene

Halimeda reefs in the upper Miocene strata (∼6 Ma) of the Sorbas basin, southeastern Spain, shed light on the internal structure of more extensive but less accessible Holocene counterparts, and challenge conventional reef concepts. Coarse discoid segments, released by Halimeda during life or immediately after death, dominate the lenslike mounds. Their chaotic, loose appearance disguises the reefal nature of the mounds. Segments, accumulating at or very close to sites of growth, were quickly stabilized by microbial and cement crusts that bound them into distinctive rigid gravel fabrics. This early lithification generated relief but inhibited off-mound export of sediment, although large blocks detached locally and moved downslope. Encrustation of parautochthonous Halimeda gravel created a unique reef type: segment reefs.

Oolite stromatolites and thrombolites, Miocene, Spain: analogues of Recent giant Bahamian examples

Abstract At El Joyazo in Almeria, southeast Spain, closely packed thrombolite and stromatolite domes of Messinian age form a bioherm 12 m thick surrounded by oolite. Individual domes, up to 1.5 m high and 4 m across, are typically composite, with distinct stromatolitic and thrombolitic parts which are sharply juxtaposed. Both parts are composed of ooid and peloid fine-medium sand. Stromatolitic parts show good lamination and often contain small finger-like columns. Thrombolite is more abundant than stromatolite and has a distinct clotted fabric produced by irregular fenestrae. In size, general shape, internal structure, depositional environment (ooid shoal), grain size (fine-medium sand), and grain composition (ooids, peloids) the stromatolitic parts of the domes are closely comparable with Recent “giant” subtidal columnar stromatolites at Lee Stocking Island, Bahamas, and we regard them as analogues of these Recent examples. At present no modern analogues are known for the thrombolites. Previously, large Messinian stromatolites elsewhere in southeast Spain have been attributed to hypersalinity caused by the Mediterranean “salinity crisis”. The El Joyazo examples formed in current-swept ooid shoal environment, probably of normal marine salinity. We suggest that mobile sands, rather than hypersaline conditions, provided the stress to inhibit predators and competitors. These “giant” Messinian coarse-grained domes, like the Lee Stocking Island stromatolites, may have formed because of the presence of algae, such as chlorophytes and diatoms, capable of trapping large grains.

Record of climatic change in neritic carbonates: turnover in biogenic associations and depositional modes (Late Miocene, southern Spain)

In order to evaluate the geological record of climatic change in neritic carbonates, we studied Late Miocene rock outcrops in southern Spain. Six episodes of reef growth are documented (Burdigalian to Messinian) in Neogene basins of the Betic Cordillera, which were located close to the margin of the global reef belt. The reefs are characterized by various zooxanthellate corals which decrease in diversity with time, andHalimeda; the youngest reefs of the latest Messinian are characterized by the dominance of the genusPorites. Late Miocene coral reefs and reef-rimmed platforms alternate over time with non-reefal carbonate ramps characterized by skeletal calcirudites or with gypsum such as that formed during the Messinian salinity crisis. The calcirudites lack reef corals, calcified green algae and extensive marine cement, but exhibit skeletal components described from both modern and fossil nontropical carbonates. These include bryozoans, mollusks, foraminifers, echinoderms and minor balanids, as well as coralline algae of a bryomol association. The presence of some larger foraminifers indicates high temperatures, close to the lower temperature threshold of the reef assemblage. Sea level lowstands and highstands are documented by wedges of bryomol carbonate and chlorozoan patch reefs or prograding platforms. Thus, temperate climate depositional modes correspond to relatively low sea levels, and warm-water modes to high sea levels. The Neogene infill of the Agua Amarga and Sorbas basins documents two of these cycles. Other climate/sea-level cycles (including Messinian gypsum in the cool water depositional mode) are well established in adjacent Neogene basins in southern Spain. This type of composite sequence seems to occur only along the margin of the global reef belt and indicates an oscillatory latitudinal movement of the margin, which is associated with global climatic change. The analysis of turnover in neritic depositional carbonate systems may therefore be considered a sensitive tool for reconstructing climatic change from the fossil record. However, warm-water modes and temperate-water modes of carbonate sedimentation and diagenesis differ significantly. For this reason the interpretation of composite system sequences by sequence stratigraphy requires an extended concept. The particular type of mixed bryomolchlorozoan depositional sequence also bears some potential for drowning, because sea level rise may be faster than the net production rate of temperate carbonate systems.

Genesis and evolution of strontium deposits of the granada basin (Southeastern Spain): Evidence of diagenetic replacement of a stromatolite belt

Abstract There are important strontium deposits in the Granada Basin, the most noteable of which (Montevives) has an annual production of approximately 50,000 tons of almost pure celestite. Two types of mineralization can be recognized: a primary variety consisting of stromatolitic carbonate that has been partially replaced by celestite, and a secondary variety consisting of celestite-pebble karst deposits. Both are included in an evaporitic Messinian succession. The primary variety of mineralization is within carbonates that interfinger with, and prograde across, gypsum deposits. The development of these deposits can be interpreted in the context of the general evolution of the Granada Basin during Late Tortonian and Messinian times. Open marine conditions prevailed during the late Tortonian. During the transition from Tortonian to Messinian a restriction of the basin resulted in evaporite sedimentation, with stromatolites thriving at the basins margin. The stromatolites were distributed along a coastal belt that was limited on the east by the tectonically active Sierra Nevada with its local alluvial fans. Runoff from the Sierra Nevada produced a freshwater lens and surface salinities that permitted the development of stromatolites, rather than the accumulation of gypsum. The replacement of stromatolitic carbonate by celestite occurred within the mixing zone of the coastal aquifers during sedimentation and/or early diagenesis. An essentially marine origin is considered for the strontium. Supplementary influxes from continental weathering are also thought to have been produced. Further restriction of the Granada Basin led to complete desiccation and the deposition of a 20 m thick halite layer. Later, gypsum deposits were exposed, and resulting cavities (“dolinas”) were filled with celestite pebbles. The return of sediment accumulation within lakes buried and preserved these deposits.

Contrasting models of temperate carbonate sedimentation in a small Mediterranean embayment: the Pliocene Carboneras Basin, SE Spain

Temperate carbonates consisting of bioclastic (bryozoan–bivalve–coralline algal) packstones to rudstones formed in the Carboneras Basin, a small embayment of the Mediterranean Sea in SE Spain, during the early Pliocene. Transgressive systems tract carbonate deposits exhibit three distinct sedimentary styles, with contrasting lithofacies patterns and stratal-geometry arrangements. Palaeotopography and local hydrodynamic conditions were the major factors controlling sedimentation. On the steep northern margin, affected by southeasterly wind-driven storms, a prograding platform (distally steepened ramp) with well-marked, platform-slope clinoforms developed. Bivalves extensively colonized the outer platform, and rhodolith pavements covered its edge. Coarse-grained, bioclastic sediments were frequently removed from the platform by storms and redeposited on the platform slope. On the southern margin of the basin, longshore currents driven by southeasterly winds hydrodynamically accumulated carbonate particles to form a spit-platform, on top of which some shoals developed. These shoals were dismantled from time to time by the northern storms, and the sediment was redeposited on the lee-side slope of the spit-platform. On the gentle and protected western margin, oyster banks and rhodolith pavements colonized the inner-ramp environment whereas coralline algal–bryozoan–bivalve bioconstructions formed on the outer ramp. This example shows that several sedimentary styles of temperate carbonate deposition can coexist in a single basin.