Abstract This volume consists of papers presented at the M. T. Halbouty Continental Margins Conference, held at the Tremont House in Galveston, Texas, February 5-9, 1989. The conference was sponsored by the American Association of Petroleum Geologists and the Departments of Geophysics and Oceanography of Texas A&M University. Convenors were Joel Watkins, Gregory Mountain, and Feng Zhiqiang. We gratefully acknowledge the support of the following companies: Amoco Production Company, Chevron Oil Field Research Company, Enserch Exploration Inc., Exxon Company International, Howell Corporation, Maxus Energy Corporation, Mitchell Energy and Development Corporation, Mobil Oil Corporation, Pennzoil Exploration and Production Company, Phillips Petroleum Company, Primary Fuels Inc., Standard Oil Production Company, Shell Oil Company, Texaco Overseas Holdings Inc., and Unocal Corporation. Financial support provided by these companies underwrote the travel expenses of many foreign participants and U.S. students as well as some of the costs incurred in the production of this volume. Without this generous support, neither the conference nor this memoir would have been possible. Anita Fickey, Michele Beal, Debbie Waits, and Crissy Ponzio of the Texas A&M Geophysics Department staff made arrangements, retyped manuscripts, and performed a myriad of administrative chores necessary for organizing the conference and editing the manuscripts. Their always cheerful assistance smoothed the road to publication. One of the worst ice storms in the history of southeast Texas descended on the area immediately before the start of the conference. For a period of time, the only road to and from the mainland was closed because of the severeicing

Abstract The Chinese continental margin contains 13 sedimentary basins. These basins formed in diverse ways at different times. They include cratonic rift basins, continental margin rift basins, back-arc basins, oblique-rift basins, and fore-arc basins. This paper reviews the different basins, their architecture, and petroleum potential.

Abstract Two large Cenozoic sedimentary basins comprise the East China Sea: the East China Sea Shelf Basin (ECSSB) and the Okinawa Trough Basin (OTB). The ECSSB formed between the Late Cre-taceous or Paleocene to Quaternary. This basin has a sedimentary sequence more than 12 km thick. In contrast, the OTB formed by back-arc spreading since mid-Miocene. The general tectonic pattern of the East China Sea is characterized by east-west zonation and north-south differentiation in tectonic evolution, sedimentary facies distribution, and magmatism. Five regional unconformities can be observed on seismic sections corresponding to five major tectonic movements. The most significant of these unconformities are of Eocene-Oligocene, middle Miocene-Pliocene, and Pliocene-Pleistocene age. The first commercial discovery in the East China Sea was drilled in 1983. Since then a number of other wells have also been successful. Future prospects for oil and gas in the East China Sea appear good.

Abstract The South China Sea is unique among the marginal seas of the Western Pacific. The study of the geological and structural characteristics of this basin are important for understanding the movements of the Eurasian, Indochina, and Pacific plates and the history of interaction between them. The modern passive continental margin in the northern South China Sea has been developed by the overlapping of the massive basement structures of Mesozoic age with rifting valleys of late Mesozoic to early Cenozoic age. Superimposed over these older structures are younger structures related to the mid-Cenozoic sea floor spreading of this region. Since this basin underwent short-term spreading, many of the older structural features in this region are well preserved. In this sense, the South China Sea is different from typical passive continental margins in terms of structural framework, geothermal history, sedimentation, and other geologic characteristics. Because of these differences, the northern South China Sea Basin is called a para-passive continental margin. The unique history of this basin, in combination with fluvial sedimentation systems from the Chinese continent, has formed multilayer and multitype conditions favorable for generation and accumulation of hydrocarbons (e.g., Pearl River Mouth). The exploration results obtained in the past ten years have led to discoveries of three large oil and gas fields, some medium and small oil and gas fields, and many petroliferous structures. These discoveries prove that this offshore area has great hydrocarbon potential.

Abstract The Pearl River Mouth Basin, a Tertiary depocenter with a multiphase history, is composed of a series of subbasins. The westernmost of these is the Wenchang B Depression, currently being explored by a partnership of Esso China Limited and Shell Exploration (China) Ltd., in cooperation with Nanhai West Oil Corporation of the People's Republic of China. From extensive seismic and well data in the depression, four distinct stages of tectonic development can be recognized. The first stage is crustal accretion that provides a relatively young and weak substrate for later basin evolution. Prerift sag basin development in the early Tertiary represents the second stage. Distribution of fluvial and lacustrine sediments of this period appears to be largely independent of later graben formation. The third stage of development was the rifting that created the present basin form. Initial low-angle normal faults were quickly overprinted by widespread east-west, high-angle normal faults that show evidence of right-lateral motion. Marine influences first appeared in the deeper parts of the basin midway through the graben infill sequences and gradually became more widespread. The fourth and final stage began in the upper Oligocene, when active rifting ended and margin sag toward the South China Sea began. Weak tensional and wrench faulting, coupled with rare compressional events, continues to the present.

Abstract The Chatham Rise is a continental submarine plateau extending east from the South Island of New Zealand. The rise, along with the rest of the New Zealand plateau, was separated from the Gondwana continental margin by rifting and extension in the mid-to Late Cretaceous. The structure of the rise is dominated by large half-grabens formed mainly by south-dipping listric faults of several kilometers downthrow that parallel the strike of the rise. The rise subsided due to thermal relaxation in the Paleogene. Clastic sedimentation, which prevailed in the Cretaceous rift and early postrift phases, gave way almost everywhere in the Paleogene to authigenic limestone and greensand deposition. Sediment starvation and erosion in the Neogene have resulted in a thin, mainly authigenic and volcaniclastic sedimentary section on the rise crest. However, late Neogene uplift of the Southern Alps along the developing Indo-Australian/Pacific plate boundary brought renewed clastic sedimentation at the western end of the rise. As well as tectonic effects, effects of paleosea-level change can be found in seismic and sedimentological data from the region.

Abstract Seismic reflection profiling east of the North Island, New Zealand, has defined the northeastern margin of an area of elevated basement rocks referred to as the Hikurangi-Chatham Plateau. The northeastern boundary of the plateau extends from the southern Kermadec Trench to the eastern end of the Chatham Rise. The plateau is bounded to the south by the Chatham Rise and to the west by the Hikurangi Trough. Gravity modeling and inversion of earthquake travel-time data indicate that the crust of the plateau is 10-15 km thick. Numerous volcanic peaks are observed throughout the plateau. The age and type (oceanic versus continental) of the plateau crust is unknown. The plateau is being subducted beneath the North Island in the Hikurangi Trough. I suggest that the buoyancy of the plateau, compared with the oceanic crust to the north, is responsible for the contrasts in subducting plate dip, trench morphology, and accretionary prism elevation between the Hikurangi subduction system and the Kermadec subduction system to the north. Postulated Triassic-Jurassic subduction southward beneath the northern Chatham Rise probably ceased following arrival of the buoyant Hikurangi-Chatham Plateau at the margin.

Abstract The Taranaki Basin has undergone a complex sedimentary and tectonic history since the late Oligocene, reflecting the impin-gement of deformation associated with the evolving Pacific and Australian convergent plate boundary. At various times through the Neogene, elements of a passive continental margin, retro-arc foreland basin, foreland fold and thrust belt, inverted subbasin, back-arc rift and back-arc contractional basin have been variously displayed within the Taranaki Basin. Following its initial Cretaceous development within a synrift setting, the Taranaki Basin evolved as a passive margin. Postrift cooling and foundering of the New Zealand continental plateau effected a widespread transgression, and by early to mid-Oligocene times, the Taranaki Basin lay almost submerged. A dramatic increase in subsidence in the late Oligocene is interpreted as foreland basin-type deepening. A basement block bounded by the Taranaki fault began to progressively overthrust westward to ultimately form the present-day eastern margin of the Taranaki Basin. This initially transpressive tectonic regime was a manifestation of a protoplate boundary through western New Zealand. Sedimentary response to this change in tectonic style was at first muted. Carbonate deposition was enhanced as a result of the increased intrabasinal submergence. Thereafter, the supply of terrigenous sediment into the basin increased in the early Miocene coincident with main overthrusting of the Taranaki fault. In the south and southeast parts of the basin, a steady northwestward progradation of the shelf edge began in mid-Miocene times as sediment supply exceeded subsidence. Submarine fans were deposited on the lower slope of the migrating shelf. On the basin's eastern flank, compression along the outer edge of the foreland thrust belt continued until around 10 Ma. This was expressed as ongoing growth of thin-skinned overthrusts within the Tarata thrust zone. Elsewhere in the east, foreland subsidence and bathyal water depths prevailed; whereas in the north, late Miocene volcanism occurred. These high-potash andesites are presumed to have deep subduction origins. Around 10 Ma, the compressive focus in the Taranaki Basin shifted from its eastern to its south and southeastern margins where former rift half-grabens were inverted in the latest Miocene. This is related to a shift in the direction of convergence across and alignment of the plate boundary. At the start of the Pliocene, accelerated progradation of the shelf began in the Taranaki Basin, and giant clinoform-bounded sediment wedges were built out to the north and west. The progradation is attributed to an excess of sediment supply over accommodation, caused by a marked increase in hinterland uplift and erosion. Relative sea-level lowstands were a less important cause of sedimentary imbalance and shelf outbuilding. Pliocene rifting of the North Taranaki Graben and down warping of the South Taranaki Graben are back-arc effects of Pacific plate subduction.

Abstract The arcuate New Ireland Basin is 150 km wide and trends northwest for 600 km between Feni and Mussau islands in northern Papua New Guinea. Multichannel seismic reflection data combined with refraction data show that much of the basin is a simple structural downwarp filled with up to 7 km of strata. Island outcrops and offshore reflection data indicate that New Ireland Basin contains thick sequences of Eocene to earliest Miocene volcanic rocks, Miocene shallow marine volcaniclastic rocks, Miocene shelf limestones, and latest Miocene to Holocene pelagic carbonates and volcaniclastic turbidites. Exploration for hydrocarbons in New Ireland Basin has been limited to geophysical surveys and to a few dredge and core stations. Offshore surveys show depocenters that contain more than 1000 m of Miocene elastics, overlain by about 2000 m of Miocene shelf carbonates and 2000 m of younger volcaniclastic rocks and bathyal carbonates. Counterparts of these units are exposed on New Ireland, where outcrop geology suggests, if these units are deeply buried offshore, that they may have some potential as both source and reservoir rocks for oil and gas. Possible hydrocarbon traps offshore include reeflike buildups and fore-reef deposits in the Miocene limestones as well as anticlines, normal faults, and stratigraphic pinch-outs along the basin margins. One multichannel seismic reflection line revealed a flat high-amplitude reflection, or “bright spot,” within the core of an anticline some 20 km east of New Ireland. The bright spot is about 2 km wide and occurs 1.2 s (1700 to 1800 m) beneath the sea floor in water depths of 2500 to 2600 m. Quaternary calderas developed during late-stage volcanism in each of the four island groups along the Tabar-to-Feni islands. Recent exploration within one caldera on Lihir Island, Luise, has revealed a sizable gold discovery. The gold occurs within pyrite in volcanic rocks adjacent to a 350,000-year-old monzonite intrusion. Exploration by drilling in the caldera indicates about 18.4 million ounces of gold in 167 million tons of host rock, which averages 3.4 g/t and excludes any host rock with less than 1.5 g/t.

Abstract The summit region of the Tonga Ridge is a relatively shallow (<1000 m) submerged plateau or platform underlain by a little-deformed sedimentary sequence of Eocene and younger volcaniclastic and carbonate beds. The platform sequence is thickest, 4 to 5 km, where lower Tertiary deposits fill structural basins in an underlying basement of arc-igneous rocks, most likely of early middle or early Eocene age. From insular exposures and seismic reflection data four regional stratigraphic units that roughly comprise beds of mostly late Eocene, early to middle Miocene, late Miocene and early Pliocene, and late Pliocene and younger age can be distinguished. Their separating unconformities are associated with carbonate buildups, including reefal masses. The unconformities are thought to mainly record uplift and subsequent submergence of the summit platform linked to tectonic events of plate-boundary arc rifting and crustal collision. The most likely reservoirs for petroleum accumulations are buried reefs, in particular of Eocene and early Neogene age. The lower part of the platform sequence has been sufficiently warmed to generate petroleum, but exploratory drilling at Tongatapu found neither thermogenic hydrocarbons nor source beds. In addition, organic-rich deposits have not been found at either insular or submarine outcrops. Although seeps of mature crude are known, the limited exploratory drilling and sampling of subaerial and submerged outcrops to date have not located the source beds. The lack of encouraging information about suitably positioned source beds dampens a sanguine assessment of the resource potential of the otherwise prospective platform sequence.

Abstract The central Sumatra fore-arc basin is made up of two subbasins (Singkel and Pini basins) separated by a structural high. Our seismic sequence and facies analyses identify eleven sequences that define the Neogene subsidence history of the fore arc. A comparison of Neogene structure and seismic facies maps reveals that, during the Miocene and early Pliocene, the two subbasins subsided independently and the subsidence was controlled by local structure. In the late Pliocene, basin subsidence was largely controlled by regional tectonic factors. Initial subsidence of Singkel Basin resulted from the lateral translation of the structural block between the Batee and Singkel faults. Regional basin subsidence resulted from the deflection of the descending oceanic plate created when material was added to and/or redistributed in the accretionary wedge. The following structural influences on the fore-arc basin subsidence have been indentified: (1) the location of the continental margin; (2) the presence of strike-slip faults traversing the fore arc; and (3) local and regional deformation within the accretionary wedge.

Abstract Sea floor spreading between Africa and the Madagascar-India-Seychelles block began during the Jurassic Magnetic Quiet Zone and was preceded by earlier rifting of Gondwanaland and deposition of Karroo sediments. Basal clastic deposition was terminated in the Early Jurassic by a regional transgression, largely a result of continental separation, followed by a general depositional regression on shelves. A major transgression occurred over most of East Africa, from late Callovian to Oxfordian, which was related to the final breakup of this area and subsequent phase of regional subsidence. Two distinct deformational episodes, documented by erosional unconformities and siliciclastic sedimentation, occurred during the pre-Aptian and late Maastrichtian. The older event was probably the distal intraplate effect of the separation of South America and Africa; whereas the Maastrichtian tilting of northern Somalia was possibly related to a rebound effect when the Oman subduction failed at about 70 Ma. The Cretaceous-Tertiary history of the Indian Ocean continental margins is the result of a complex depositional regression that covered the underlying Early and Middle Jurassic rifted margin. To the north, Oligocene-Miocene sediments were deposited during the opening of the Gulf of Aden and accumulated in disconnected structural depressions formed by downfaulted rotating blocks bordering the rising Somali plateau.

ABSTRACT The continental margin of East Africa began forming in the Permo-Carboniferous with the development of rift basins, and extension occurred intermittently over 150 million years until the Late Jurassic initiation of sea floor spreading. The margin developed by a combination of extensional and transform tectonics that separated plates containing Africa, Madagascar-Greater India, South America, and Antarctica. A salient aspect of the pre-breakup stratigraphy of the rift basins is salt in isolated Tanzanian grabens, in the Somali Coastal Basin, and in offshore Madagascan basins. At the initiation of sea floor spreading, sedimentary facies changed throughout the rift and pull-apart basins from dominantly continental to marine. Volcanic activity and faulting occurred at the same time. Sea floor spreading ceased in the Western Somali Basin in the Early Cretaceous. Vigorous abyssal circulation along the East African margin probably commenced in the mid-Cretaceous, and widespread regional volcanism occurred in the Late Cretaceous. Middle Jurassic through Holocene sediment thicknesses exceed 8 km in places along the margin.

ABSTRACT Latest Triassic and Early Jurassic extension of continental crust capped by a shallow water carbonate platform in the Southern Alps (northern Italy) resulted in the episodic creation of an array of half-grabens with west-tilted floors, each bounded by an east-dipping master normal fault. Faulting, platform drowning, and basin formation progressed stepwise from west to east over a period of about 20 million years. The foundering of each successive half-graben resulted in formation of a sequence-bounding unconformity over the remainder of the surviving platform. Each half-graben consists of a western thickly sedimented, deep-water basinal part and an eastern more thinly sedimented ramp or plateau part. A west-dipping antithetic fault commonly divides the two parts. The consistent polarity of the tilted blocks suggests they may be the shallow, brittle-crust expression of lithospheric extension along an east-dipping simple shear.

ABSTRACT The results of extensive MCS and sonobuoy reflection and refraction data are analyzed in order to determine the oceanic crustal structure and evolution in the Jurassic Quiet Zone off northwest Africa between 30 and 35°N. Within a zone characterized by a broad bathymetric arch, a unit, which we estimate is Late Cretaceous in age, with diffracted upper surface and chaotic seismic facies (UCF) is observed between sedimentary units of continuous and parallel seismic character. Results from CDP velocity spectra analysis define a velocity inversion between the UCF (4.7 km/sec) and the underlying sediments (3.1 km/sec). A deep crustal layer with velocities of 7.1-7.4 km/sec rather than the typical layer 3 velocity of 6.8 km/sec is observed in the sonobuoy refraction measurements. A good spatial correlation exists between this zone of high crustal velocities and the extent of the UCF implying a common origin for these features. The movement of Africa relative to the Canary hot spot, which can explain these phenomena and is consistent with stratigraphically derived ages, allows us to interpret the UCF as volcanic in origin. Within the framework of the hot-spot model, the UCF is evidence for a volcanic overprint on the region. Data off the conjugate margin of Nova Scotia were examined and a similar overprint was not found. These observations indicate that the evolution of these two margins differed at some time in their history and is consistent with the concept of the passage of a hot spot that rejuvenated the ancient crust off Morocco commencing approximately 60 MYBP.

ABSTRACT A seismic stratigraphic and lithostratigraphic study based on seismic reflection and well log data of Paleogene strata from the southern Beaufort Sea margin of Alaska and Canada has identified major unconformities and depositional sequences. A comparison of the age of the unconformities to regional uplift events, dated with fission track analysis, shows that tectonic events were more important than eustatic changes in controlling the occurrence of regional unconformities. Paleogene sediments were deposited in a foreland basin located north of the Brooks Range. This basin trends offshore and forms part of the southern continental margin of the Beaufort Sea. Mid-Mesozoic thrusting established the basic structure of the foreland basin, which was later filled with sediments and deformed by northward-directed thrusting. Regional unconformities occur near the Cretaceous-Tertiary boundary, within the Paleocene, middle Eocene, upper Eocene, and lower Miocene. These unconformities separate depositional units consisting of northward-prograding deltaic, slope, and turbidite facies. Most Paleogene depositional patterns can be explained as follows: thrust loading with widespread shale deposition in distal parts of the basin and coarse elastics near the thrust front (Late Cretaceous and middle Eocene); then subsequent thrust-belt erosion and uplift with sand deposition across the basin (Paleocene and Oligocene). Latest Eocene to Oligocene subsidence of the Mackenzie delta resulted from local thrust and sediment loading.

ABSTRACT Japan has been part of an arc-trench system or systems since the Mesozoic. During this time, major sedimentary basins developed in response to variations in plate motion. Active basin infilling began 0.5 to a few m.y. after changes in plate motion. Tectonic movement and eustatic sea-level changes control development of transgressive and regressive cycles in sedimentary basins. Tectonic movement related to changes in plate motion activated tectonic subsidence in sedimentary basins that, in turn, resulted in transgressions. In Japan, the rates of transgressions and regressions are variable through the Mesozoic and Cenozoic. Different patterns of transgression and regression can best be interpreted in terms of the relative magnitude of interacting tectonic movement and eustatic sea-level changes. Rapid transgressions followed by slow regressions occur in Miocene-Pleistocene successions; slow transgressions and rapid regressions are found in the Paleogene-Cretaceous successions; and rapid transgressions and regressions are interrupted by long stillstands characteristic of the Jurassic-Triassic interval.

ABSTRACT Relative sea-level change had a major effect upon sedimentary depositional patterns in the outcropping arid shallow subsurface Upper Cretaceous stratigraphic section in the Gulf coastal plain of Alabama. In this paper, we examine the facies stratigraphy and stratigraphic breaks in the upper Santonian-Maastrichtian section in light of local biostratigraphy and biochronostratigraphic correlation. We examine the relationship of facies stratigraphy and stratigraphic breaks to relative sea-level changes, and we propose a preliminary depositional-sequence stratigraphy for the study area that is closely related to global patterns in relative coastal onlap.

ABSTRACT Deltaic complexes, shelf, slope, and Mississippi fan in the northern Gulf of Mexico show differences in lithologies, depositional environment, and stratigraphy. Each province reveals vertical sequences that can relate to fluctuations in relative sea level, especially for the late Neogene. Shelf and upper slope deposition during periods of relatively high sea level is characterized by thin, laterally continuous, condensed clay-rich sections, with thin calcareous layers, showing well-defined, strong, and laterally continuous seismic reflections. During periods of relatively low sea level sediments reveal expanded sections with variable thicknesses and lithologies, with well-defined depositional trends, displaying a wide range of acoustical responses. The Mississippi fan consists of successive channel-levee-overbank complexes, while the intraslope basins reveal a cyclicity in seismic and lithological characteristics.