Moyra E.J. Wilson

Hopping Hotspots: Global Shifts in Marine Biodiversity

Hotspots of high species diversity are a prominent feature of modern global biodiversity patterns. Fossil and molecular evidence is starting to reveal the history of these hotspots. There have been at least three marine biodiversity hotspots during the past 50 million years. They have moved across almost half the globe, with their timing and locations coinciding with major tectonic events. The birth and death of successive hotspots highlights the link between environmental change and biodiversity patterns. The antiquity of the taxa in the modern Indo-Australian Archipelago hotspot emphasizes the role of pre-Pleistocene events in shaping modern diversity patterns.

The SE Asian gateway: history and tectonics of the Australia–Asia collision

Collision between Australia and SE Asia began in the Early Miocene and reduced the former wide ocean between them to a complex passage which connects the Pacific and Indian Oceans. Today, the Indonesian Throughflow passes through this gateway and plays an important role in global thermohaline flow. The surrounding region contains the maximum global diversity for many marine and terrestrial organisms. Reconstruction of this geologically complex region is essential for understanding its role in oceanic and atmospheric circulation, climate impacts, and the origin of its biodiversity. The papers in this volume discuss the Palaeozoic to Cenozoic geological background to Australia and SE Asia collision. They provide the background for accounts of the modern Indonesian Throughflow and oceanographic changes since the Neogene, and consider aspects of the region’s climate history.

The Tertiary evolution of South Sulawesi: a record in redeposited carbonates of the Tonasa Limestone Formation

Abstract South Sulawesi, situated at the junction of three major plates and with an almost complete Tertiary sequence, is an ideal location in which to study syntectonic sedimentation. Redeposited carbonate facies of the lower/middle Eocene to middle Miocene Tonasa Limestone Formation in the Barru area prove to be reliable indicators of tectonic activity. South of the Barru area contemporaneous carbonate sediments formed on a relatively stable shallow-water platform, known as the Tonasa Carbonate Platform. Redeposited carbonate facies and interbedded marls from the Barru area are described and interpreted in this study. The immaturity and provenance of clasts indicate that the redeposited facies were derived from the faulted northern margin of the Tonasa Carbonate Platform. A relay ramp between at least two major NW-SE trending faults is the inferred configuration of this margin. Three main phases of faulting are indicated by the redeposited facies: late Eocene to early Oligocene, middle Oligocene and early to middle Miocene. This is consistent with other outcrop and seismic data from the region and with the inferred plate tectonic situation during the Tertiary.

An experimental study of solid matrix weakening in water saturated Savonnières limestone

Petrophysical properties of carbonate reservoirs are less predictable than that of siliciclastic reservoirs. One of the main reasons for this is the physical and chemical interactions of carbonate rocks with pore fluids. Such interactions can significantly change the elastic properties of the rock matrix and grains, making the applicability of Gassmann’s fluid substitution procedure debatable. This study is an attempt to understand the mechanisms of fluid-rock interactions and the influence of these interactions on elastic parameters of carbonates. We performed precise indentation tests on Savonni`eres limestone at a microscale level under dry, distilled water, and n-Decane saturated conditions. Our experiments display softening of the rock matrix after water saturation. We have found that mainly the ooid cortices, peloid nuclei and prismatic intergranular cement are affected by water flooding. We also observed a shear modulus reduction in Savonni`eres limestone in an experiment performed at ultrasonic frequencies. One of the most important results obtained in our experimental study is that the Gassmann fluid substitution theory might not always be applicable to predict the elastic moduli of fluid-saturated limestones.

SE Asian carbonates: tools for evaluating environmental and climatic change in equatorial tropics over the last 50 million years

Abstract This study reviews how shallow water carbonates are revealing environmental and climatic changes on all scales through the last 50 million years in SE Asia. Marine biodiversity reaches a global maximum in the region, yet the environmental conditions are at odds with the traditional view of ‘blue-water’ reefal development. The region is characterized by complex tectonics, major volcanism, high terrestrial runoff, nutrient influx, everwet and monsoonal climates, low salinities, major currents and ENSO (El Niño Southern Oscillation) fluctuations. Terrestrial runoff, nutrient upwelling, tectonics, volcanism and recent human activities are major influences on the modern development of carbonate systems. Coral sclerochronology is revealing how these factors vary locally over annual and decadal scales. The strong impact of vertical tectonic movements and the interplay with eustasy is evaluated from Quaternary and Pleistocene coral reef terraces. Isotopic data from terrace deposits indicates that interglacials may have been up to 3–6 °C warmer than glacials, consistent with the regions record from terrestrial and deep marine deposits. Study of outcrop and subsurface carbonate deposits reveals the impact of tectonics, siliciclastic, nutrient influx, eustasy and oceanography on individual systems over millennial timescales. Major changes in oceanography, plate tectonics, climate change and perhaps fluctuating CO2 levels impacted Cenozoic regional carbonate development. Results of studies from terrestrial and deep marine realms are comparable with those from the carbonates, but have yielded higher resolution records of changing currents, precipitation and the monsoons. There is considerable scope for further research, however, SE Asian carbonates are powerful tools in evaluating past environmental change in the equatorial tropics.

Dolomites in SE Asia — varied origins and implications for hydrocarbon exploration

Abstract Carbonates in SE Asia range in age from Palaeozoic to Recent, but are most important as reservoirs in the Neogene where they comprise a major target for hydrocarbon exploration (e.g. Batu Raja Formation, South Sumatra, Sunda and Northwest Java basins). Carbonates of pre-Tertiary, Palaeogene and Neogene age all show a strong diagenetic overprint in which dolomite occurs as both cementing and replacive phases associated with variable reservoir quality. This paper reviews published data on the occurrence and types of dolomites in SE Asian carbonates, and considers the models that have been used to explain the distribution and origin of dolomite within these rocks. Pre-Tertiary carbonates form part of the economic basement, and are little studied and poorly understood. Although some, such as in the Manusela Formation of Seram, may form possible hydrocarbon reservoirs, most are not considered to form economic prospects. They are best known from the platform carbonates of the Ratburi and Saraburi groups. in Thailand, and the oolitic grainstones of the Manusela Formation of Seram. The Ratburi Group shows extensive dolomitization with dolomite developed as an early replacive phase and as a late-stage cement. Palaeogene carbonates are widely developed in the region and are most commonly developed as extensive foraminifera-dominated carbonate shelfal systems around the margins of Sundaland (e.g. Tampur Formation, North Sumatra Basin and Tonasa Formation, Sulawesi) and the northern margins of Australia and the Birds Head microcontinent (e.g. Faumai Formation, Salawati Basin). Locally, carbonates of this age may form hydrocarbon reservoirs. Dolomite is variably recorded in these carbonates and the Tampur Formation, for example, contains extensive xenotopic dolomite. Neogene carbonates (e.g. Peutu Formation, North Sumatra) are commonly areally restricted, reef-dominated and developed in mixed carbonate-siliciclastic systems. They most typically show a strong diagenetic overprint with leaching, recrystallization, cementation and dolomitization all widespread. Hydrocarbon reservoirs are highly productive and common in carbonates of this age. Dolomite is variably distributed and its occurrence has been related to facies, karstification, proximity to carbonate margins and faults. The distribution and origin of the dolomite has been attributed to mixing-zone dolomitization (commonly in association with karstic processes), sulphate reduction via organic matter oxidation, and dewatering from the marine mudstones that commonly envelop the carbonate build-up. Dolomite has a variable association with reservoir quality in the region, and when developed as a replacive phase tends to be associated with improved porosity and permeability characteristics. This is particularly the case where it is developed as an early fabric-retentive phase. Cementing dolomite is detrimental to reservoir quality, although the extent of this degradation generally reflects the abundance and distribution of this dolomite. Dolomitization is also inferred to have influenced the distribution of non-hydrocarbon gases. This is best documented in North Sumatra where carbon dioxide occurs in quantities ranging from 0 to 85%. There are a number of possible mechanisms for generating this CO2 (e.g. mantle degassing), although the most likely source is considered to be the widely dolomitized Eocene Tampur Formation that forms effective basement for much of the basin. High heat flows are suggested to have resulted in the thermogenic decomposition of dolomite with CO2 produced as a by-product.

OLIGO-MIOCENE VARIABILITY IN CARBONATE PRODUCERS AND PLATFORMS OF THE CORAL TRIANGLE BIODIVERSITY HOTSPOT: HABITAT MOSAICS AND MARINE BIODIVERSITY

ABSTRACT A holistic approach is adopted here to evaluate basin-wide trends in carbonate systems, their subenvironments and the potential marine biodiversity partitioning between different habitats within the Coral Triangle biodiversity hotspot in central Southeast Asia. The Central Indonesian Kutai Basin is typical of many Cenozoic Southeast Asian basins in having extensive and varied carbonate systems, and is one of the few regions in the Coral Triangle with detailed systematic multitaxon evaluation of Oligo-Miocene deposits. The Kutai Basin may therefore provide data, when compared with other modern and global datasets, to better understand marine biodiversity development within the global diversity foci. Carbonate systems in the Kutai Basin included small-scale ephemeral features, such as delta-associated patch reefs, mixed carbonate-clastic shelves, and large-scale (> 10 km) land-attached and isolated carbonate platforms affected by varied energy regimes. Twenty-five–plus subenvironments or habitats are spread across the different carbonate systems, with the large-scale platforms hosting over 12–15 different habitats. These subenvironments include clastic-influenced mesophotic coral reefs/carpets, nonclastic- and clastic-influenced sea-grass beds, downslope reworked coral rubble, and shallow low-energy inner platform areas, to name a few. Perhaps paradoxically, the systems that two decades ago were almost unstudied, the clastic-influenced coral reefs/carpets, are now the only ones in the region to have been systematically studied for their marine biota. Detailed biotic studies have therefore only investigated < 4 of the 25-plus potentially habitable carbonate subenvironments within the basin. This variability of carbonate systems and their subenvironments, but paucity of detailed biotic data is typical of many basins within equatorial Southeast Asia. It is recognized that detailed multitaxon biotic studies are in their infancy for Southeast Asia, and that there are challenges of taxonomic bias and/or preservation for many groups within the fossil record. Currently, however, we cannot evaluate marine biodiversity partitioning across the varied habitat mosaics of Southeast Asian carbonate systems with possible pathways discussed for furthering this field of research. It is likely that we are (?grossly) underestimating biodiversity on a basin-wide and probably a system-wide scale. It is hoped that studies of this type will contribute towards better understanding of equatorial marine carbonate systems, their biological inhabitants and producers, and the spatio-temporal development of global biodiversity hotspots.

An investigation of porosity–velocity relationships in faulted carbonates using outcrop analogues

Abstract Porosity and permeability are notoriously difficult to predict in carbonates, especially prior to drilling when there is a lack of direct petrophysical data. The aim of this paper is to document the initial results of an integrated outcrop and laboratory study designed to investigate the relationships between pore systems and acoustic velocities in faulted Oligo-Miocene carbonates on the Mediterranean islands of Malta and Gozo. Depositional facies is shown to have a significant effect, with velocities in grain-dominated carbonates up to 1000 m s−1 higher than those in micrite-dominated carbonates. Based on outcrop structural data, the fault zones can be separated into three architectural components: a fault core; an intensely damaged zone; and a weakly damaged zone, with the last passing into undamaged protolith. Our data suggest that only the fault core component can be identified using porosity–velocity data, with P-wave velocity (Vp) values of 5000–6500 m s−1 at helium porosities of less than 5%. Our study is novel in that the prediction of elastic properties and acoustic velocities across fault zones is anticipated by linking laboratory-scale measurements with seismic-scale predictions through quantitative rock physics modelling.

The SE Asian Gateway

Collision between Australia and SE Asia began in the Early Miocene and reduced the former wide ocean between them to a complex passage which connects the Pacific and Indian Oceans. Today, the Indonesian Throughflow passes through this gateway and plays an important role in global thermohaline flow. The surrounding region contains the maximum global diversity for many marine and terrestrial organisms. Reconstruction of this geologically complex region is essential for understanding its role in oceanic and atmospheric circulation, climate impacts, and the origin of its biodiversity. The papers in this volume discuss the Palaeozoic to Cenozoic geological background to Australia and SE Asia collision. They provide the background for accounts of the modern Indonesian Throughflow and oceanographic changes since the Neogene, and consider aspects of the region’s climate history.