Christian Betzler

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.

Non-tropical carbonates related to rocky submarine cliffs (Miocene, Almerı́a, southern Spain)

Abstract Upper Miocene deposits in the Cabo de Gata region (SE Spain) provide a unique opportunity to study cliff-related temperate carbonates, a poorly known type of fossil non-tropical carbonates. The studied submarine cliffs lie on the western flank of the Monte Ricardillo volcanic dome. Two main biocenoses colonised the cliff walls: vertical and subvertical walls were either overgrown by vermetid gastropods, forming a vermetid framestone, or colonised by robust branching bryozoans, which were reworked post-mortem and accumulated as aprons at the foot of the submarine cliffs. Coralline algae in the vermetid build-up indicate palaeodepths below 15–20xa0m. Depressions in front of the cliffs were occupied by nodular bryozoans and bivalves. Within these depressions, barnacles settled on secondary hard substrates. With progressive flooding of the depositional area, substrate relief was filled in and a carbonate ramp with facies belts following the palaeobathymetric gradient evolved. A delicate branching bryozoan facies occurs in the proximal part of the ramp and a nodular bryozoan–bivalve facies in its middle part. Distally, these deposits give way to a coralline algal facies. These results provide a first insight into temperate-water carbonates related to rocky submarine cliffs. This work also reveals a major control of substrate relief on temperate carbonate facies and biofacies.

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.

Quaternary bryozoan reef mounds in cool-water, upper slope environments: Great Australian Bight

Bryozoan reef mounds are common features in the geological record, occurring within mid-ramp, slope paleoenvironments, especially in Paleozoic carbonate successions, but until now have not been recorded from the modern ocean. Recent scientific drilling in the Great Australian Bight (Ocean Drilling Program Leg 182) has confirmed the existence of shallow subsurface bryozoan reef mounds in modern water depths of 200–350 m. These structures have as much as 65 m of synoptic relief, and occur both as single mounds and as mound complexes. They are unlithified, have a floatstone texture, and are rich in delicate branching, encrusting and/or nodular-arborescent, flat-robust branching, fenestrate, and articulated zooidal bryozoan growth forms. The muddy matrix is composed of foraminifers, serpulids, fecal pellets, irregular bioclasts, sponge spicules, and calcareous nannofossils. The 14C accelerator mass spectrometry dates of 26.6–35.1 ka indicate that the most recent mounds, the tops of which are 7–10 m below the modern seafloor, flourished during the last glacial lowstand but perished during transgressive sea-level rise. This history reflects changing oceanographic current patterns; strong upwelling during lowstands, and reduced upwelling and lowered trophic resources during highstands. Large specimens of benthic foraminifers restricted to the mounds confirm overall mesotrophic growth conditions. The mounds are similar in geometry, scale, general composition, and paleoenvironments to older structures, but lack obvious microbial influence and extensive synsedimentary cementation. Such differences reflect either short-term local conditions or long-term temporal changes in ocean chemistry and biology.

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.

Late Neogene-Recent uplift of the Cabo de Gata volcanic province, Almeria, SE Spain

Abstract Cabo de Gata is a Miocene volcanic province in the Betic Cordillera in southeastern Spain. The distribution of coastal deposits in the successive marine sedimentary units overlying the last volcanic episode (about 7.5 Ma old) has been used to reconstruct the post-volcanic palaeogeographic evolution of the region during the Late Neogene. The current elevation of well-dated shoreline marker rocks has been used to estimate uplift amount and rates. Since the late Tortonian, a N45°E-aligned (the strike of the Carboneras fault system) topographic relief was emergent in the Cabo de Gata region. The extension and height of this island increased throughout the late Neogene. Smaller, independent islands were emergent and finally became connected to the main island during the Messinian. The Carboneras and Agua Amarga Pliocene sub-basins were the last two marine basins prior to the final emergence of the region. Since the last volcanic eruption (ca. 7.5 Ma), maximum uplift of sedimentary rocks in Cabo de Gata has taken place on the western margin of the N45°E-aligned palaeorelief. The altitude of the shoreline marker rocks in the successive sedimentary units decreases eastwards to the present-day coast and northwards of the Rambla del Plomo. Uplift rates since deposition remain nearly constant for the successive Messinian rocks and decrease slightly for the lower Pliocene outcrops. Most of the uplift took place before the Pliocene while the main island enlarged. Uplift amounts and rates since deposition of the upper Neogene sedimentary units in the Cabo de Gata area are similar to the ones estimated for laterally equivalent units in the eastern Betic basins (approximately 50 m/Ma). Despite its volcanic nature and the occurrence of the Carboneras fault system, the Cabo de Gata probably became elevated as a consequence of regional uplift in connection with the rest of the Betic Cordillera.

Sedimentation cycles and their diagenesis on the slope of a Miocene carbonate ramp (Bahamas, ODP Leg 166)

Numerous small-scale depositional cycles are present in the Miocene sediments of seismic sequence m along the margin of Great Bahama Bank (ODP Leg 166). These cycles consist of decimetre- to metre-scale alternations between light-grey and dark-grey/black wackestones/packstones. The light-grey layers are well cemented and nearly uncompacted. They contain planktonic and benthic foraminifera, and bioclasts. Bioturbation in these layers is moderate. The dark-grey wackestones/packstones are uncemented, strongly compacted and normally strongly bioturbated. The main components are planktonic foraminifera and fine-grained bioclasts. The dark layers are rich in aragonite and organic carbon and contain around 80% carbonate. The light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97.5%. Light δ13C and slightly negative δ18O values were observed in the dark uncemented layers while the cemented intervals show heavy δ13C values and slightly more positive δ18O values. The carbon isotope signal between the dark and the light layers shows variations of up to 1.45‰. Both δ13C and δ18O co-vary with carbonate content. The Formation MicroScanner images reflect the changes in carbonate mineralogy. The Natural Gamma-Ray Tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy. The internal stacking pattern of the cycles is closely tied to sea level. The dark layers are deposited during rising sea level, while the light layers reflect sediment production and export during highstand and falling sea level. These cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat-topped platforms. Most measured parameters suggest that primary sediment composition played an important role in the cementation process. The primary differences in composition were enhanced during shallow burial diagenesis. The initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite-rich layers leading to the formation of the light beds. Dissolution of metastable high-Mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite. Diagenesis was enhanced by the coarse grain size of the primary aragonite-rich layers. In contrast the more pelagic, dark calcite-rich layers were only slightly affected by diagenesis and could preserve their aragonite content. The uncemented layers subsequently were subject to strong mechanical compaction, whereas the cemented beds stayed nearly uncompacted.