Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations

Abstract Reconstructions of SE Asia at 5 Ma intervals for the past 50 Ma are presented. They are constrained by new data from the Philippine Sea plate, which forms the eastern boundary of the region, by recent interpretations of the South China Sea and Eurasian continental margin, forming the western boundary, and by the known motions of the Indian-Australian plate to the south. An attempt is made to satisfy geological and palaeomagnetic data from the region. The implications of these reconstructions for the Tertiary evolution of SE Asia are discussed in the light of other new data from the region. There are two regionally important periods of change during the past 50 Ma. Both appear to be the expression of arc-continent collision and resulted in major changes in the configuration of the region and in the character of plate boundaries. At c. 25 Ma the collision of the Australian continent with the Philippine Sea plate arc caused major effects which propagated westwards through the region. At c. 5 Ma collision of the Philippine arc and the Eurasian continental margin occurred in Taiwan. This appears to be a key to the recent tectonics of the region. Principal features of the model include the following interpretations. Middle Tertiary counter-clockwise rotation of Borneo closed a large proto-South China Sea and led to the development and destruction of marginal basins north of the Celebes Sea. The rotation implies that much of the north Borneo margin was not a subduction, but a strike-slip boundary for most of this period. It also suggests that the central West Philippine Sea, the Celebes Sea and the Makassar Strait formed part of a single marginal basin which opened between late Eocene and mid Oligocene, and narrowed westwards like the present South China Sea. Luzon is suggested to have formed in an arc on the north side of the Celebes Sea-West Philippine Basin, whereas most of the other Philippine islands probably formed part of an arc at the southern edge of the Philippine Sea Plate before the Early Miocene. Arc-continent collision in the early Miocene caused plate boundaries to change and initiated the clockwise rotation of the Philippine Sea plate. Since then the Philippine fragments have moved in a very narrow zone, mainly as part of the Philippine Sea plate, with significant strike-slip motion of fragments at the plate margin. Most subduction under the Philippines was oblique, mainly at the western edge, and north of Mindanao. The Molucca Sea was a very wide area which formed part of the Philippine Sea plate before c. 15 Ma and originated as trapped Indian ocean lithosphere. It has been eliminated by subduction on its east and west sides. The present-day double subduction system never extended north of the present Molucca Sea into the Philippines. The Sulawesi ophiolite has an Indian ocean origin and was emplaced on the west Sulawesi continental margin at the end of the Oligocene. The major change in plate boundaries at the beginning of the Miocene following arc-continent collision of the Australian margin with the Philippine Sea plate arc caused initiation of the Sorong Fault system and led to westward movement of continental fragments which were accreted to Sulawesi during the late Neogene. The Sula platform and Tukang Besi platform formed part of a single large microcontinent with the Bird’s Head before c. 15 Ma. They moved to their present positions after slicing of fragments from this microcontinent at different times and each was attached to the Philippine Sea plate for a few million years before collision. Most of the Banda Sea is interpreted to have an extensional origin and to have opened during the late Neogene. The reconstructions imply that there has been little convergence at the north Australian margin in Irian Jaya since the early Miocene and most convergence has occurred during the last c. 5 Ma. Movement of Philippine Sea arc fragments within the northern New Guinea margin along strikeslip zones probably accounts for the terrane character of this orogenic belt.

Reconstructing Cenozoic SE Asia

Tectonic evolution of the northern African margin in Tunisia from paleostress data and sedimentary record

Evolution of the northern Gulf of Mexico, with emphasis on Cenozoic growth faulting and the role of salt

Groundwater evolution in the Continental Intercalaire aquifer of southern Algeria and Tunisia: trace element and isotopic indicators

The southern part of the study area is part of the stable Saharan platform which, since the beginning of the Paleozoic, has been affected only by epeirogenic movements. Pre-Carboniferous Paleozoic strata are largely clastic, both continental and marine. Early invasions of the sea came from the west, but Carboniferous and younger transgressions came from a northerly (Tethyan) direction. Caledonian tectonic movements produced structural alignments and important regressions, but general uplift resulting from the Hercynian orogeny caused the major withdrawal of the sea. The dominant feature of post-Hercynian erosion in Algeria is a great T-shaped anticlinorium which projects into Tunisia. On the east, a projection of the Hercynian Nefusa uplift of Libya extends westward and c nnects with the anticlinorium. Just south of these positive areas is the Erg Oriental basin, which was connected during most of the Paleozoic with the Homra basin of western Libya. The former is bounded in the western part of the study area by the north-northeast-trending Hassi Touareg anticlinorium. Restriction of the western Tethys basin and post-Hercynian subsidence along the margin of the African landmass led to deposition of a thick series of Triassic and Liassic evaporites. Younger strata of the Saharan platform are relatively thin, and although continental sands were important during certain stages, lagoonal dolomite, evaporite, and shale are the dominant rock types. North of the platform was a subsident zone, the Jeffara flexure, where considerably thicker and more marine strata are characteristic. Associated with the flexure and extending beyond it is a complex series of faults. On the north and west, the principal later Mesozoic events consisted largely of epeirogenic uplifts with regression and local erosion, and transgressive downwarps. Frequent and more intense moveme ts characterized the early Cenozoic, and local deformation, marking the onset of Alpine orogeny, began in the Oligocene. Stronger orogenic pulses occurred at the end of the Miocene, but the last and most violent phase of the Tunisian Atlasic orogeny took place during the Pliocene-Pleistocene. In addition to the Saharan platform and Jeffara flexure, Mesozoic-Cenozoic tectonic features include stable shelves of the Pelagian craton in eastern and offshore Tunisia, and of Kasserine Island in central Tunisia. The latter was exposed during latest Cretaceous time and is bounded on the east by a series of north-south-trending faults and Jurassic horsts marking the eastern boundary of the Atlas zones. Between this axis and the Pelagian shelf and between the Saharan platform and Kasserine Island is a series of Cretaceous and Tertiary intracratonic basins, and in northern Tunisia are similar ones containing thick sections of Miocene-Pliocene clastic rocks. In northwestern Tunisia are many Triassic evaporite diapirs, and some thrust faults are present in a zone of very thick Oligocene flysch. Three hydrocarbon-productive trends are present in the study area: (1) Ypresian (early Eocene) carbonate rocks: facies change regionally southwestward from globigerinid mudstone (source beds) to dolomitized nummulitic calcarenite (reservoir) to evaporite (updip seal). Recent discoveries seem to be associated with small structures, but regional extent of the trend and attendant possibilities for stratigraphic entrapment suggest the potential for large reserves. (2) Paleozoic and Triassic sandstones: this trend contains the only proved, major reserves of Tunisia in the Triassic reservoir at El Borma field. Cambrian-Ordovician strata are productive at Rhourde El Baguel in Algeria, and sandstones of the Silurian-Lower Devonian Acacus Formation are capable of production in many Libyan well . Major fields are structurally controlled, but distribution of reservoir rocks is related to Hercynian uplift, which influenced erosion of Cambrian-Ordovician clastic strata and deposition of presalt Triassic detritus. Hydrocarbon source beds for both systems are believed to be Silurian euxinic shales. (3) Cretaceous shoreline: several small oil fields and one gas field currently are productive, mostly from bioclastic carbonate reservoirs of Aptian to early Senonian age. These accumulations are on surface anticlines in a foothills belt which is not deformed so severely as the main Atlas ranges.

Geology of Tunisia and Adjacent Parts of Algeria and Libya

The tectonic features of Tunisia are complex and include folds, all types of faults, evaporite diapirs, and the Saharan flexure, which separates a stable Paleozoic province on the south from a subsidence zone of Mesozoic and Cenozoic strata of the offshore Ashtart and Tripolitania basins. The remainder of the offshore region is mostly stable shelf of the Pelagian craton, which also extends onshore. The present study focused on this shelf, where two carbonate intervals contain proven hydrocarbon reservoirs: the Metlaoui Formation of earliest Eocene (Ypresian) age and the Zebbag Formation of Late Cretaceous (Turonian) age. Regionally, well-defined belts of Metlaoui carbonates trend northwest. On the northeast are open-marine deep-water micrites and marls with abundant planktonic foraminifers. Thick bars of nummulitid packstone/grainstone were deposited in shallow water at an angle to the paleoshelf. The reservoir is confined largely to the bars, and visible effective porosity is best developed in those areas among the foraminifers filled with sand-size nummulitid debris, where secondary solution enlargement has occurred. This lithology tested oil in two recent wildcat wells and is a commercial reservoir at Sidi El Itayem and Ashtart fields. Distribution of Zebbag carbonates is more complex. A northwest-trending platform was bounded on three sides by deep water, where shale and micrite with planktonic foraminifers were deposited. Predominately back-reef and lagoonal foraminifer/rudist wackestones and packstones occur in narrow belts, apparently controlled at least locally by block faulting. They tested oil in two recent discoveries. The most significant porosity is interparticle, generally enlarged by solution, in foraminifer packstones, but intraparticle voids in foraminifers and rudists commonly contribute to the porosity. Analyses of surface and subsurface samples identified the Bahloul (basal Turonian) and Bou Dabbous (Ypresian) formations as source rocks. Fluorescence spectra of several oils were compared with extracts from these samples and indicate the Bahloul to be the source of oils in recent Metlaoui and Zebbag discoveries.

Petroleum Geology of East-Central Tunisia

This study has led to a reconstruction of the environment of deposition of the upper Smackover section (Late Jurassic) at North Haynesville field, Claiborne Parish, Louisiana, and may aid in the prediction of other productive trends. The interval studied includes the oolitic and pelletoid "B" reservoir which contains about 16 million bbl of oil in place. Petrographic examination of etched core slabs resulted in a grouping of upper Smackover carbonate rocks into environmentally controlled facies, each of which has distinctive characteristics on the electric log. Because of probable hypersalinity in the Smackover sea, allochems are almost entirely nonskeletal. In order of decreasing importance, these include: pellets, oolites, lumps and composite grains, algal pisolites and oncolites, and quartz and anhydrite grains. The Smackover transition facies is gradational into the overlying Buckner shale and includes a shaly subfacies composed largely of pellets and recrystallized oolites in a matrix of shale and/or silt-size anhydrite or dolomite, and a sandy subfacies composed of pellets with abundant quartz sand grains and sparry calcite cement. Below the transition facies, in order of decreasing energy level, are: (1) the reservoir facies composed of oolites and superficially coated pellets with spar cement and primary intergranular porosity; (2) the mixed facies composed of subequal quantities of all grain types with spar cement and mud matrix; and (3) the pellet-mud facies composed of pellets in a mud matrix which commonly is dolomitized. The pellet-mud facies also contains minor amounts of very argi laceous dolomite and "disturbed mud." Deposition of nonskeletal particles in the late Smackover sea resembled in some aspects that of the present Bahama Banks. Pellet-mud was deposited in warm, quiet, shallow, probably hypersaline water. Subsequently slight differential uplift caused turbulence in local areas, where the mixed facies was deposited. Later regional uplift and regression brought about conditions favorable for extensive oolite accretion, resulting in reservoir-facies deposition. Continued uplift of land areas on the north supplied increasing amounts of terrigenous detritus to the transition facies, and carbonate deposition eventually ceased. Analysis of dry holes in North Haynesville field indicates that nondeposition or complete cementation of the reservoir facies is more important than present structure in limiting the productive area. However, only those wells favorably located with respect to present structure, as well as to structure during deposition and diagenesis, are productive. Additional porosity trends may be discovered by comparing field dry holes with those in other areas.

Petrology of Upper Smackover Limestone in North Haynesville Field, Claiborne Parish, Louisiana

Major reserves of oil exist in the Reforma area of Tabasco and Chiapas states and the Campeche Shelf of SE Mexico in high-energy, bank-edge, reef-derived or reef-associated carbonate rocks, ranging in age from Late Jurassic to earliest Late Cretaceous. It is the conclusion of this study that the Reforma reservoir facies does not extend into W Guatemala. However, there the potential for major reserves in bank and lagoonal carbonates of similar age is considered excellent. A variety of structures, mostly resulting from salt tectonics, is present. Known reservoir rocks include fractured carbonates with secondary porosity resulting from solution and dolomitization, and limestones with primary intergranular porosity.An indigenous source is likely for the large quantities of oil which have been tested at Rubelsanto. Seals, in the form of thick intervals of Cretaceous anhydrite and, in places, of Tertiary fine-grained clastics, are abundant.

Petroleum geology of northern central america

Offshore southern Kalimantan (Borneo), Indonesia, the Sunda Shelf is bounded on the south by the east-west-trending Java-Madura foreland basin and on the north by outcrops of the granitic core of Kalimantan. Major northeast-southwest-trending faults created a basin and ridge province which controlled sedimentation at least until early Miocene time. Compressional folds are restricted to two areas, and compaction folds over buried basement features are common elsewhere, as are horsts and tilted fault blocks, with contemporaneity of faulting and deposition. Rocks below the pre-Tertiary angular unconformity are considered to represent economic basement. The Tertiary intervals discussed are rock units, although certain ones may approximate time-stratigraphic units. Just above the unconformity, the oldest pre-"CD Limestone" clastic strata are fluviatile and lacustrine, the remainder consisting largely of shallow-marine, calcareous shale with interbeds of fine-grained, quartzose sandstone. Continued transgression led to deposition of the massive "CD Limestone," which typically is a nummulitic wackestone with zones of intraclastic and bioclastic packstone. These low-energy carbonate rocks were deposited on moderately shallow shoals or banks with restricted circulation. A flood of terrigenous detritus--Kudjung unit 3--resulted from post-"CD Limestone" uplift, and is more widely distributed. Unit 3 consists largely of fluviatile sandstone interbedded with shale and mudstone, grading upward to marine clastics with a few thin limestones near the top. Deepening waters of this transgression inundated most of the area, and because of the reduced size and relief of land areas, only fine detritus was supplied. The resulting Kudjing unit 2 is largely a shallow-basinal deposit, comprising thin, micritic limestones interbedded with calcareous shale and mudstone. Infilling of the basins was nearly complete by the end of Kudjung unit 1 deposition. Bank and basinal facies similar to those described constitute most of the unit and, on the south, patch reefs are abu dant in both. A barrier-reef facies was formed in an open-marine basin on the north and a buildup with reefal affinity along a shelf edge on the southeast. Eastern equivalents of Kudjung units 1 and 2 are known as the Berai limestone interval (comprising bank, reefal, basinal, and open-marine limestones, and marl); these units cannot be delimited from outcrop data. The Berai marl overlies these thick carbonate rocks and those of unit 1 and is thinner over positive areas, indicating incipient uplift although the principal deformation occurred later. Of the three oil fields in the area, two are shut in, but one has produced nearly 100 million bbl. Gas shows were recorded in most wells of the area, but the maximum flow was 1.8 MMcf methane/day, although larger flows with high percentages of carbon dioxide and nitrogen were reported. In the southern part, several oil shows were encountered, and more than 2,000 BOPD was tested from two thin unit 2 limestones. Fine-grained clastic strata of unit 3 are continuous with those farther south, where geochemical data indicate good source and hydrocarbon-generating potential. Sandstones with reservoir capability are present in the clastic intervals, and several carbonate facies have sporadically developed porosity. A variety of structural and stratigraphic traps is present.

William F. Bishop

Papers

Geology of Tunisia and Adjacent Parts of Algeria and Libya

Petroleum Geology of East-Central Tunisia

Petrology of Upper Smackover Limestone in North Haynesville Field, Claiborne Parish, Louisiana

Petroleum geology of northern central america

Structure, Stratigraphy, and Hydrocarbons Offshore Southern Kalimantan, Indonesia