Early Miocene Coral Research Articles

Early Miocene coral reef-associated bryozoans from Colombia. Part II: “Ascophora” Cheilostomatida

AbstractBryozoans are common and diverse in fossil and modern coral reefs. However, studies of bryozoans in ancient reefs are generally limited, and even less is known about fossil bryozoan faunas associated with coral reefs in the Caribbean region. This is the second contribution describing the bryozoan assemblage from the early Miocene (Aquitanian) reefs of the Siamaná Formation in the La Guajira Peninsula, southern Caribbean. Here, we describe and illustrate 17 species of ascophoran-grade cheilostomes, including one new genus and three new species,Ditaxiporina colombianan. sp.,Poricella paulaen. sp., andCycloavicularia parvan. gen. n. sp. Of the remaining fourteen taxa left in open nomenclature, one is considered confer and three affinis to species previously described, one is identified at family level, and nine at genus level. The Siamaná bryozoan fauna differs in species and colony-form composition from those associated with other paleoenvironments from Oligocene and Miocene localities of North America, the Caribbean, and Brazil.UUID:http://zoobank.org/043eadcf-0e39-4c1b-b207-f7628d6b5c84

Detecting Photosymbiosis in Fossil Scleractinian Corals

The evolutionary success of reef-building corals is often attributed to photosymbiosis, a mutualistic relationship scleractinian corals developed with zooxanthellae; however, because zooxanthellae are not fossilized, it is difficult (and contentious) to determine whether ancient corals harbored symbionts. In this study, we analyze the δ15N of skeletal organic matrix in a suite of modern and fossil scleractinian corals (zooxanthellate- and azooxanthellate-like) with varying levels of diagenetic alteration. Significantly, we report the first analyses that distinguish shallow-water zooxanthellate and deep-water azooxanthellate fossil corals. Early Miocene (18–20 Ma) corals exhibit the same nitrogen isotopic ratio offset identified in modern corals. These results suggest that the coral organic matrix δ15N proxy can successfully be used to detect photosymbiosis in the fossil record. This proxy will significantly improve our ability to effectively define the evolutionary relationship between photosymbiosis and reef-building through space and time. For example, Late Triassic corals have symbiotic values, which tie photosymbiosis to major coral reef expansion. Furthermore, the early Miocene corals from Indonesia have low δ15N values relative to modern corals, implying that the west Pacific was a nutrient-depleted environment and that oligotrophy may have facilitated the diversification of the reef builders in the Coral Triangle.

Paleoenvironmental conditions of Early Miocene corals, western Makran, Iran

Miocene coral-bearing limestones, distributed in the western Makran Basin, are attributed to Aquitanian and Burdigalian. This investigation is focused on three Early Miocene coral-bearing limestone sections in western Makran. Remarkably, well-preserved scleractinian corals and other components could be very definitive to reconstruct paleoenvironmental conditions. Lithologically, there are some differences between the studied sections. Furthermore, zooxanthellate corals are diverse and abundant in all sections. Based on different components and richness of zooxanthellate corals, it seems that the studied carbonate corals are precipitated in a subtropical condition, between 19 and 20 °C in all sections. In light of the dominancy of corals, the presence of symbiont-bearing larger benthic foraminifera (LBF), lithological features, bioeroding features, and encrusting organisms, the oligotrophic to slightly mesotrophic conditions are considered for Tejek section and the mesotrophic condition had been prevailed in Kermestan and Irer sections. According to light intensity in water column and coral morphotypes, Tejek section is considered to precipitate under euphotic to slightly mesophotic condition, while Kermestan and Irer sections were deposited under mesophotic to euphotic light conditions. In transparent water, photic zones continue to deeper depths, while in less transparent water, these zones are limited to shallower parts. A defined depth in the photic zone may represent euphotic, mesophotic, or oligophotic zone. Based on the water transparency, a taxon in a defined photic zone can occur in various depths. On the basis of trophic-light intensity-depth chart, the estimated depth ranges are 12–85 m for Tejek section and 5–62 m for Kermestan and Irer sections. Water energy as another important factor in environmental condition is acquired from coral morphotypes. Accordingly, Tejek section is precipitated under moderate-high energy and Kermestan and Irer sections are deposited in low to moderate-high energy. The presence of encrusting coralline algae, corals, and other constituents is indicative of different substrates in the studied areas. Corals favorably develop in normal salinity waters. The existence of colonial corals and occurrence of benthic foraminifera with hyaline wall indicate normal seawater conditions.

Evaluation of the effect of diagenetic cements on element/Ca ratios in aragonitic Early Miocene (~ 16 Ma) Caribbean corals: Implications for ‘deep-time’ palaeo-environmental reconstructions

Early Miocene corals (Siderastrea conferta and Montastraea limbata) from the Paraguaná Peninsula, Venezuela and a modern coral (S. radians) from Bermuda were examined in order to quantitatively assess the effect of skeletal preservation on element/Ca proxies used for palaeo-environmental reconstructions. The biostratigraphic age (Burdigalian) of the corals was confirmed via Sr isotope stratigraphy to 16.5±0.4Ma. Light and scanning electron microscopy revealed the presence of aragonite and calcite cements in both fossil and modern corals, but brucite cements were found only in the modern coral. Cement distribution is very heterogeneous, resulting in zones of excellent preservation juxtaposed with less well preserved regions in both modern and fossil coral. Oil-filled pore spaces were noted in the fossil specimens. Targeted LA–ICPMS depth profile analyses showed significant differences in Sr/Ca, Mg/Ca, B/Ca, Ba/Ca and U/Ca in aragonite and calcite cements compared to primary skeletal aragonite. Increased Sr/Ca and decreased Mg/Ca ratios were found in aragonite cements compared to skeletal aragonite, whereas in calcite cements these trends were reversed. B/Ca ratios were lower in both aragonite and calcite cements compared to primary aragonite. Estimates of the effect of 1% contamination by aragonite cements on coral Sr/Ca, Mg/Ca, B/Ca and U/Ca palaeo-sea-surface-temperature reconstructions for the fossil corals produced anomalies of −1.2°C, −0.2°C, +0.3°C and −0.1°C respectively. Similar percentage calcite cement contamination produced temperature anomalies of +1.7°C, +2.7°C, +0.3°C and −0.1°C. Because of both highly elevated and depleted Ba/Ca signatures in aragonite and calcite cements respectively, care has to be exercised when reconstructing past flood or upwelling events from fossil corals. When using targeted spatially-resolved analysis, well-preserved early Miocene corals may overall yield reliable ‘deep-time’ palaeo-proxy information in much the same way as commonly utilised Holo/Pleistocene corals.

THE EMPERADOR LIMESTONE REDISCOVERED: EARLY MIOCENE CORALS FROM THE CULEBRA FORMATION, PANAMA

Caribbean reefs underwent significant biotic change during the Late Oligocene and Early Miocene. This was a critical time in the evolution of the modern Caribbean fauna characterized by increasing endemism resulting from regional extinction of lineages that survive in the modern Indo-Pacific. An understanding of the dynamics and potential causes of the Oligocene/Miocene transition, however, is hampered by the relative lack of well-preserved Oligocene to early Miocene coral faunas in the Caribbean. Here we examine new exposures in the Culebra Formation of Panama that contain a well-preserved coral fauna of Early Miocene age. Taxonomic, stratigraphic, and paleoecologic study of the Culebra Formation exposed along the Gaillard Cut of the Panama Canal allows us to infer the paleoenvironments and reef coral communities from the Panama Canal Basin during this critical interval. The Culebra Formation consists of a deepening upward sequence with shallow-lagoon sediments at the base, overlain by fringing reef facies in the middle of the section, and open-shelf to bathyal facies at the top of the section. We recovered 31 species of reef corals from a combination of new and old collections. Comparison of our collections with other Late Oligocene to Middle Miocene reef coral assemblages confirms that there was a major faunal turnover after deposition of the Upper Oligocene Antigua Formation. This turnover consisted of a large number of extinctions followed by an increased rate of first occurrences so that regional diversity did not change appreciably. Improved stratigraphic resolution at this and other Caribbean localities is required to understand fully the dynamics of change during the Oligocene/ Miocene transition.

Oligocene reefal deposits in the Pisang Ridge and the origin of the Lucipara Block (Banda Sea, eastern Indonesia)

In 1998 we dredged early Oligocene/late Oligocene boundary, shallow-water reefal deposits and pelagic, early Pliocene muds in the submerged Pisang Ridge. The shallow-water deposits were identified as moderate- to low-energy coral-reef depositional environments, which confirms that the Pisang Ridge belonged to the continental to continental-arc Lucipara Block which also includes the Tukang Besi, Lucipara and Rama ridges. On the basis of these findings, it was possible to reconstruct parts of the general geological evolution of the Lucipara Block, which drifted away from northern Irian Jaya during mid-Miocene times and collided with the Kolonodale Block during the late Miocene. A late-early Oligocene calc-alkaline volcanic arc developed in the Weber Trough area, then uplifted to a shallow-water position at the early–late Oligocene boundary in the Pisang Ridge. Late Oligocene–early Miocene metamorphism subsequently developed, prior to the deposition of early Miocene coral reefs in the Rama Ridge. Locally, late Miocene metamorphism was identified in the Lucipara Ridge, prior to the latest Miocene–Pliocene general drowning and the splitting of the Lucipara Block into several small blocks throughout the Banda Sea region.

Sedimentary anatomies of Early Miocene coral reefs in South Corsica (France) and South Sardinia (Italy)

Sedimentary anatomies of Early Miocene coral reefs in South Corsica (France) and South Sardinia (Italy)

Oligocene and Early Miocene coral faunas from Iran: palaeoecology and palaeobiogeography

Oligocene and Early Miocene coral assemblages from three sections of central Iran are investigated with respect to their palaeoecological and palaeobiogeographic implications. These corals are compared with faunas from the Mediterranean Tethys and the Indopacific. Associated larger foraminifers are used for biostratigraphy and to support the palaeoecological interpretation. The studied sections are situated in the foreland basins of the Iranian Plate which is structured into a fore-arc and a back-arc basin separated by a volcanic arc. The coral assemblages from Abadeh indicate a shallowing-upward trend. Infrequently distributed solitary corals at the base of the section indicate a turbid environment. Above, a distinct horizon characterised by a Leptoseris-Stylophora assemblage associated with lepidocyclinids and planktonic foraminifers is interpreted as maximum flooding surface. Small patch reefs with a Porites-Faviidae assemblage are a common feature of Late Oligocene to Early Miocene coral occurrences and indicate water depths of less than 20 m. The diversity of the coral faunas shows marked differences. Oligocene corals from the Esfahan-Sirjan fore-arc basin comprise more than 45 species of 32 genera and occur in a wide range of environments. Early Miocene corals from the Qom back-arc basin are less frequent, show a lower diversity (13 genera with 15 species) and occur in single horizons or small patch reefs.

Carbonate deposition in accretionary prism settings: Early miocene coral limestones and corals of the Makran mountain range in Southern Iran

The regional mapping of the Makran mountain range on behalf of the Geological Survey of Iran represents a unique coverage: the entire area of the mountain range was compiled in a unified programme. During this mapping, Miocene limestones containing rich coral and foraminiferal faunas were recorded over a strike length of several hundred kilometres, as minor developments within thick neritic clastic sequences which in turn overlie great thicknesses of Eocene-Miocene flysch. These limestones include rigid bioconstructional frame-works, loosely compacted coralline assemblages and foraminiferal calcarenites: they includein situ recfal deposits and material redeposited quite close to their original site of deposition. Most are Burdigalian, as shown by the benthonic foraminifera, but some are Aquitanian. The geotectonic setting was an accretionary prism in a zone of plate convergence. The limestones and enclosing clastic sediments comprise an intensely folded, reverse-faulted and locally dislocated sequence, the duplex structure being the result of a major Late Miocene-Pliocene episode of regional deformation. This concentration of the intense tectonic deformation in a late major episode requires a different model for this zone of plate convergence to the model widely applied to such zones. The possible controls on limestone deposition are discussed-tectonic uplift and shallowing of the sea, climatic warming and eustatic factors. Depositional features of reefal formations in the late Jurassic of the Caucasus, the Pliocene-Recent of Halmahera, and the early Miocene of SE France are discussed in comparison with the Makran model.