Lesli J. Wood

Mass-transport complexes and associated processes in the offshore area of Trinidad and Venezuela

Mass-transport complexes (MTCs) form a significant component of the stratigraphic record in ancient and modern deep-water basins worldwide. One such basin, the deep-marine margin of eastern offshore Trinidad, situated along the obliquely converging boundary of the Caribbean and South American plates and proximal to the mouth of the Orinoco River, is characterized by catastrophic shelf-margin processes, intrusive and extrusive mobile shales, active tectonics, and possible migration and sequestration of hydrocarbons. Major structural elements found in the deep-water slope regions include large transpressional fault zones (i.e., Darien Ridge, Central Range, Los Bajos), along which mobile shales extrude to form sea-floor ridges; fault-cored anticlinal structures overlain by extrusive sea-floor mud volcanoes; shallow-rooted sediment bypass grabens near the shelf break; and normal and counterregional faults. A total of 10,708 km2 (4134-mi2) of merged three-dimensional (3-D) seismic surveys enable sub-sea-floor interpretation of several erosional surfaces that form the boundaries of enormous mass-transport complexes. The data show numerous episodes of MTC developments, which are characterized by chaotic, mounded seismic facies and fanlike geometry. Their extent (up to 2017 km2 [778 mi2]) and thickness (up to 250 m [820 ft]) is strongly influenced by sea-floor topography. Mass-transport flows show runout distances from the source area of 60–140 km (37–86 mi). Depositional architecture identified with these units includes (1) large-magnitude lateral erosional edges, (2) linear basal scours, and (3) side-wall failures. Mud volcanoes act as barriers to cross-slope mass sediment movements and form zones of shadowing on their downslope side that protect those regions from erosion. The subsequent erosional shadow remnants (ESRs) comprise preserved regions of older levee-channel complex sediments and are considered for the first time in this study as potential stratigraphic traps in deep-water deposits. Active tectonism in the region, high sedimentation rates associated with the Orinoco delta system, and abundant potential unstable hydrate suggest the viable presence of several higher frequency mechanisms at work for MTC generation than sea level fluctuations alone.

Deep-water polygonal fault systems as terrestrial analogs for large-scale Martian polygonal terrains

Discovery of giant polygonal terrains on Mars has prompted a 30-year debate over how they formed. The prevailing hypothesis is that small-scale Martian polygons formed by thermal contraction, as in terrestrial permafrost environments. Large-scale (>1 km) Martian polygons in the northern plains are visible in THEMIS, MOLA, Viking, and Mariner data, but how they formed remains enigmatic. We suggest that terrestrial deep-water marine polygons are morphological and perhaps genetic analogs to largescale Martian polygonal features. The terrestrial, deep-water polygons are imaged in three-dimensional seismic-reflection data acquired by the oil and gas industry in offshore Norway and the Gulf of Mexico. How deep-water polygonal fault systems form is a debated topic beyond the scope of this work. However, similarities between terrestrial deep-water polygonal fault systems and large-scale Martian polygonal terrains suggest that the latter could have formed during deep-water marine deposition. Deep-water polygonal faults form within fine-grained sediment at shallow burial depths. Increases in slope angles can trigger downslope disaggregation of deep-water polygons and mass wasting (forming debris flows). Physical models indicate that multidirectional extension can cause polygonal features to break up on a slope over a mobile substrate. Some knobby terrains in the Vastitas Borealis Formation seem to originate from disaggregation of large-scale Martian polygonal terrains. These analogies suggest a possible deep-water subaqueous origin for large-scale Martian polygonal terrains and support the idea of a late Hesperian–early Amazonian ocean on the northern plains of Mars.

Chronostratigraphy and tectonostratigraphy of the Columbus Basin, eastern offshore Trinidad

The Columbus Basin, forming the easternmost part of the Eastern Venezuela Basin, is situated along the obliquely converging margins of the Caribbean and South American plates. The two primary structural elements that characterize the basin are (1) transpressional northeast-southwest-trending anticlines and (2) northwest-southeast-oriented, down-to-the-northeast, extension normal faults. The basin was filled throughout the Pliocene and Pleistocene by more than 40,000 ft (>12,200 m) of clastic sediment supplied primarily by the Paleo-Orinoco Delta system. The delta prograded eastward over a storm-influenced and current-influenced shelf during the Pliocene-Pleistocene, depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower Pliocene to pre-Pliocene mobile shale facies. Biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two-dimensional and three-dimensional seismic data to construct a chronostratigraphic framework for the basin. As a result, several observations were made regarding the basins geology that have a bearing on exploration risk and success: (1) megasequences wedge bidirectionally; (2) consideration of hydrocarbon-system risk across any area requires looking at these sequences as complete paleofeatures; (3) reservoir location is influenced by structural elements in the basin; (4) the lower limit of a good-quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location; (5) reservoir quality of deep-marine strata is strongly influenced by both the type of shelf system developed (bypass or aggradational) and the location of both subaerial and submarine highs; and (6) submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure, migration, and trapping of hydrocarbons and the distribution of hydrocarbon type. (Begin page 1906)

Seismic geomorphology - an overview

Abstract Seismic geomorphology, the extraction of geomorphic insights using predominantly three-dimensional seismic data, is a rapidly evolving discipline that facilitates the study of the subsurface using plan view images. A variety of analytical techniques is employed to image and visualize depositional elements and other geologically significant features. This volume presents key technical papers presented at a recent research conference - the Seismic Geomorphology Conference (10–11 February 2005), co-convened by the Society for Sedimentary Geology and The Geological Society (London). These papers cover a broad range of topics, from detailed depositional element analysis to big picture regional issues, from lithology prediction to diagenetic modification of the stratigraphic section. This discipline is only in its early stages of development and will henceforth expand rapidly in response to the growing availability to researchers of high-quality three-dimensional seismic data.

Seismic Geomorphology: Applications to Hydrocarbon Exploration and Production

We are poised to embark on a new era of discovery in the study of geomorphology. The discipline has a long and illustrious history, but in recent years an entirely new way of studying landscapes and seascapes has been developed. It involves the use of 3D seismic data. Just as CAT scans allow medical staff to view our anatomy in 3D, seismic data now allows Earth scientists to do what the early geomorphologists could only dream of - view tens and hundreds of square kilometres of the Earths subsurface in 3D and therefore see for the first time how landscapes have evolved through time. This volume demonstrates how earth scientists are starting to use this relatively new tool to study the dynamic of a range of sedimentary environments.

Quantitative geomorphology of the Mars Eberswalde delta

The Eberswalde delta is composed of six separate depositional lobes that have prograded some 17 km from their apex. Crosscutting distributary relationships and compensated depositional lobes are clearly visible in images acquired by the Mars Global Surveyor Mars Orbiter Camera. Here, several lobe systems have been examined for sinuosity, radius of channel curvature, meander-bend width, and channel width parameters. Channel sinuosities of between 1.2 and 1.8 defi ne low- to moderate-sinuosity systems typical of the type transporting bedload or mixed grain-size loads. Channel systems increase in sinuosity as they get older. However, some of the younger systems show specifi c reaches of increased sinuosity. These localized changes may be due to either abutment against resistant beds of older lobes or rise in base level at the channel system terminus. If the former, such an effect of older deposits on the morphology of the channels suggests that these older lobes were fairly well indurated prior to the deposition of the youngest progradational lobes. Eighty-six percent of distributaries in the Mars Eberswalde delta are 100‐240 m wide. Comparatively, 62% and 44% of distributaries in the Atchafalaya and Wax Lake deltas of Louisiana, respectively, are of similar size. Small distributaries may indicate lower average and shorter duration fl ows or coarser sediment in Mars distributaries than those typical of the Gulf Coast systems. The volume of the material in the Mars deposit suggests long periods of sediment deposition. Sinuosity indexes, meander-bend migration, and ridge-and-swale point-bar topography suggest periods of stable discharge on the delta surface.

Stratal slicing of Miocene-Pliocene sediments in Vermilion Block 50-Tiger Shoal Area, offshore Louisiana

Many people have viewed modern land surfaces from commercial airplanes and marveled at the form and geometry of geomorphic features such as river channels, deltas, barrier islands, and dune fields. These views represent complete images of the modern time surfaces. We can classify the depositional nature of features on these images by interpreting their planiform geometry and geographic context. In fact, modern 3-D seismic technology has made it possible for us to image similar, but much older, geomorphic or depositional features preserved in the rock record. Unfortunately, although many reservoir-scale (well-to-well scale) features can be detected in vertical seismic lines, few such features can be resolved and interpreted in the vertical perspective because of the datas limited bandwidth. Only in the horizontal perspective are such depositional features large enough to be resolvable when displayed in map view on geologic time surfaces.

Quantitative seismic geomorphology of a Quaternary leveed-channel system, offshore eastern Trinidad and Tobago, northeastern South America

This article documents the application of techniques in quantitative seismic geomorphology in quantifying the morphometrics and architecture of deep-marine leveed-channel systems within an about 10,000-km2 (3861-mi2) study area offshore eastern Trinidad, West Indies. The principal goal of this study is to assess the relationship, if any, between sea-floor morphology and channel and levee architecture and morphology toward the development of predictive models of reservoir distribution and channel-system morphology that might be applicable to the interpretation of these types of deposits in similar settings around the world. Seven channel systems, classed into three types, within a 200-ms interval of data immediately below the modern sea floor provided the data for analysis. Results suggest that local structural features and sea-floor slopes exert more influence on channel morphology and occurrence than do eustatic sea level factors. Sinuosities, channel widths, meander-belt widths (MBWs), and meander-arc height (MAH) all increase as the channel systems age. Slope and sinuosity are directly related to one another, with sinuosity increasing as slope increases. Levee heights and widths increase downslope in areas of lower slope gradients. Channel sinuosity, MAH, and MBW increase immediately downslope from localized diapirs, and channels appear to migrate updip over time because of regional inflation of distal arc prism areas. Diapirs and uplifts cause overbank splays to become more confined and cause levees to shorten and taper rapidly. Regional tilt to the south appears to affect accommodation, creating a sink for sediments near the toe of slope. Regional tilt also affects flow processes in the channels, causing an increased overbanking of flows toward the south, away from the plate margin, resulting in higher, wider levees on the south sides of the channels.

Seismic geomorphology of offshore Morocco's east margin, Safi Haute Mer area

The lower continental slope of Moroccos west coast consists of Triassic-age salt manifested in the form of diapirs, tongues, sheets, canopies, and toe thrusts. Active salt diapirism and regional tectonics greatly influence the morphology of the modern sea floor, forming a severely rugose expression with ongoing minibasin development and episodic submarine failure. Detailed mapping of a 1064-km2 (411-mi2) seismic survey acquired in the Safi Haute Mer area revealed that Jurassic to Holocene salt mobilization continually affected distribution of sediment, causing a range of depositional flow styles, from slumps to sheet slides and mass-transport complexes (MTCs). Large sediment waves (20 km [12 mi] long, 1.5-km [0.9-mi] wavelength) were also documented at the end of the Aptian. An east-west–trending structural anticline downdip of the salt activated during initiation of the Atlas uplift in the latest Cretaceous to earliest Tertiary and shaped much of the lower slope into the Tertiary with a persistent canyon system and slope channels. The largest of the debris flows is a Cretaceous-age MTC, a 500-m (1640-ft)-thick flow that spans an area of up to 20,000 km2 (7722 mi2). Composing the MTC are (1) chaotic, mounded seismic facies; (2) internal syndepositional thrusts; and (3) transported megablocks (3.3 km2 [1.3 mi2]) with preserved internal stratigraphy. The MTC originated from an upslope collapse of a narrow shelf during the earliest phases of the Alpine orogeny.

Morphometry of mass-transport deposits as a predictive tool

Mass-transport deposits (MTDs) are gravity-induced units that represent an important component of modern and ancient deep-water stratigraphic successions. MTDs have been widely documented in the literature, but a comprehensive compilation of quantitative morphometric parameters associated with their external architecture is still lacking. This work presents a morphometric database that contains 332 data points that document the length, area, volume, and thickness of MTDs from different geologic ages and a variety of continental margins around the world. The compilation contains data collected from interpretations done by the authors in eastern offshore Trinidad and the Gulf of Mexico as well as from data mining from the peer-reviewed literature. Preliminary results indicate that there is a good correlation between a series of parameters that include the area, length, and volume of MTDs. On the other hand, the correlation between thickness and volume seems to be harder to document mainly due to lateral variations in thickness that are typical within MTDs. Data analysis suggests that previous qualitative classification of attached and detached MTDs can be validated by using a quantitative approach. This validation suggests that morphometric parameters associated with the architecture of MTDs can be used as a hint to link geologic setting, deposit geometry, and potential causal mechanisms. In addition, the defined morphometric relationships that were encountered between the different morphometric parameters (e.g., length and area) are useful to predict MTD dimensions in areas of the subsurface where data are limited and/or data quality is low. Likewise, these morphometric relationships can be used in outcrop studies where exposure of the MTD units is also limited.