Angela McDonnell

Large deep-water coral banks in the Porcupine Basin, southwest of Ireland.

The Porcupine Basin, southwest of Ireland, was one of the earliest sites from where the deep-water corals Lophelia sp. and Madrepora sp. were recovered. These deep-water corals have since been found all along the Atlantic margins of Europe, in water depths ranging from 50 to more than 2000 m. Recent geophysical studies have demonstrated the mound-building potential of deep-water corals. Available data indicate that three major provinces of coral bank occurrences can be identified in the Porcupine Basin: (1) high-relief surface mounds which have a dimension of 1 by 5 km and a height up to 200 m (‘Hovland’ mounds), flanked to the north by (2) a swarm of buried mounds, somewhat smaller (up to 90 m), and with more irregular shapes than those recognised in area 1 (‘Magellan’ mounds), and (3) outcropping or buried, conical mounds (single or in elongated clusters, up to 150 m high) occurring on the southeastern slope of the basin (‘Belgica’ mounds). As far as can be inferred from shallow cores, the surface lithology predominantly consists of an upper layer rich in foraminiferal sand and terrigenous silty clay with intercalations of biogenic rubble. The banks host a remarkable number of colonies of living and dead Lophelia pertusa and Madrepora oculata. The living and dead assemblages are underlain by a significant layer of coral debris in a muddy matrix. Deep-water coral debris together with a living association of the same species covers the surface of the ‘Belgica’ and ‘Hovland’ mounds, which may suggest that these corals have played a significant role in the development of the mound structures. The capacity for mound formation by scleractinian corals in the aphotic zone has been known for some time. Examples are found at different locations along the shelves and the continental margins of the North Atlantic. The role of the corals in these deep-water build-ups is still a point of debate. Though the genesis and initial control of mound settings in this basin might be related to hydrocarbon seeps, it appears that the major development of the Porcupine coral banks in recent geological times has most likely been controlled by oceanic circulation and dynamics in water masses and nutrient supply.

Quantifying the origin and geometry of circular sag structures in northern Fort Worth Basin, Texas: Paleocave collapse, pull-apart fault systems, or hydrothermal alteration?

Three-dimensional seismic data reveal numerous subcircular sag structures in the northern Fort Worth Basin. The structures are defined by concentric faults that extend vertically upward 760–1060 m (2500–3500 ft) from the Lower Ordovician Ellenburger Group. The largest structures remained active into the lower Desmoinesian Strawn Group. Criteria are outlined for defining seismically resolvable sag structures, and a detailed quantitative analysis of the geometries of these circular features was undertaken. Results are compared and contrasted, with reviews of subsurface collapse mechanisms and strike-slip processes that are known to produce subsurface circular to subcircular sag geometries in plan view. In this manner, we develop several constraints for differentiating collapse-related sag structures from strike-slip–related sag structures. Qualitative analyses indicate that the geometries observed are strongly analogous to subsurface collapse features where material is removed at depth to create a void, into which the overburden subsequently sags and collapses. Quantitative analyses support the formation of these features by incremental collapse and suprastratal deformation above a linked system of coalesced, collapsed paleocaves within the Ellenburger Group. Observations and criteria presented herein provide valuable information in defining seismically resolvable collapse features worldwide and help distinguish sag features of collapse affinity from those of other origins.

Paleocene to Eocene deep-water slope canyons, western Gulf of Mexico: Further insights for the provenance of deep-water offshore Wilcox Group plays

In the Gulf of Mexico (GOM), an extensive deep-water fan system of the lower Tertiary Wilcox Group forms a significant exploration target, yet connections to equivalent-aged onshore fluvial, deltaic, and shallow-marine reservoirs are poorly documented. Using a large, three-dimensional (3-D) seismic survey (3300 mi2, 8500 km2), we examined the lower Tertiary structure and stratigraphy of the underexplored Texas coastal zone, approximately 60 mi (96 km) downdip of the paleoshelf edge and 200 mi (322 km) updip of the deep-water discoveries. Canyons are identified throughout the inferred Paleocene to upper Eocene interval near Matagorda Bay, with this study focusing on the youngest. These are typically 2–2.5 mi (3–4 km) wide and 600–1600 ft (200–500 m) deep. Main incisional axes trend downslope, and some bifurcate. Early salt movement appears to have created an irregular paleoslope topography and an altered slope gradient, thereby influencing depositional flow pathways. In a paleogeographic context, this canyon complex is interpreted to lie in a middle- to lower-slope setting and sits directly downdip of the Wilcox Group shelf-margin canyon systems. Although not all of the canyons necessarily connect updip to the shelf-edge canyons, or downdip to fan systems, they represent part of a large-scale complex along this western GOM margin that acted as a conduit to the successfully drilled, deep-water, lower Tertiary Wilcox turbidite play. These results have importance not only for the prospectivity of the Texas shelf but also for the regional understanding of the Paleocene through lower Eocene paleogeography of the GOM.

Canyon Heads and Channel Architecture of the Gollum Channel, Porcupine Seabight

The Gollum channel is one of the few lengthy leveed channel systems on the European NE Atlantic margin (Fig. 1). GLORIA 6.5 kHz long-range sidescan sonar imaging (Kenyon et al. 1978) and single-channel high resolution seismics with air gun source (Gainanov and Volkonskaya 1998; McDonnell et al. 1998) mapped six canyon heads at the shelf break (ca. 300 m) on the western margin of the Celtic Sea (Fig. 2). Channel heads are asymmetric and progressively infilled from the south due to the influence of northerly contouritic sand transport along the upper slope in depths above ca. l,000 m (Fig. 3, Fig. 4). The channel heads feed into an anastamosing channel system that transports turbidity currents into the southern Porcupine Seabight (Wheeler et al. 1998). Dating of turbidites suggests activity was predominantly during glacial stages with very limited or no activity during the last ca.10 ka. (1995) propose that sediments are being transported in the channel by reversing currents causing scour in areas. Open image in new window Fig. 1A, B. Location map for the Gollum Channel showing A location of the Poreupine Seabight in the NE Atlantic, and B the location of survey data imaging of the channel (e.g. 2=Fig. 2)

Provenance, Sandstone Composition, and Seismic Facies of Paleocene Wilcox Sandstones, Gulf of Mexico Basin

Upper Paleocene sandstones of the lower Wilcox Group are deep (>4.5 km) to ultradeep (>6 km) exploration targets below the present-day shelf and in deep water of the Gulf of Mexico. These sandstones were sourced by continental-scale drainage systems that terminated in the Houston and Holly Springs deltas in Texas and Louisiana, respectively. We used seismic data to document the location of sandy fairways into the deep basin and petrographic data to document regional variation in detrital mineral composition of lower Wilcox sandstones. We identified three major sediment fairways that carried sandstone from the shelf into the deep basin in the northwestern Gulf by superimposing areas of moderate- to high-amplitude seismic reflections within the slope to basin-floor transition seismic facies onto the lower Wilcox isopach map. Sandstone composition was determined by point counts of 275 thin sections from 34 wells from onshore Texas and Louisiana. Provenance differences between the Houston and Holly Springs deltas resulted in differences in detrital mineral composition. Lower Wilcox sandstones in the Houston delta are mostly lithic arkoses, whereas those in the Holly Springs delta are feldspathic litharenites. Difference in feldspar content may distinguish reservoir sandstones in deep water derived from the two main deltaic sources.