DeSoto Canyon Research Articles

Prestack depth imaging in the eastern Gulf of Mexico

The Gulf of Mexico (GOM) shelf is one of the most thoroughly explored regions in the world. As downstream technology has advanced, exploration and production of hydrocarbons have progressively moved into waters greater than 3500 ft in the central GOM. On the other hand, the eastern GOM is largely underexplored. It is now attracting industry attention because the Minerals Management Service of the U.S. Deparment of the Interior has scheduled OCS Lease Sale 181 for December 2001. Previous investigations of the eastern GOM structural geology (e.g., Wu, 1990; DeBalko and Buffler, 1992; Dobson and Buffler, 1997) were based on 2-D time-processed seismic data with no tying well information. Approximately 3270 miles of 2-D seismic data were recently gathered over the Lloyd Ridge and DeSoto Canyon areas to evaluate the region's hydrocarbon potential for the upcoming lease sale (Figure 1). Most data were acquired in the deepwater Florida basin, but a small portion overlapped the basin/shelf boundary and the outermost portions of the Florida shelf. The region is geologically complex. It contains interbedded salt, a history of vertical and lateral salt movements, and the juxtaposition of dramatically varying lithology and velocity across the steep shelf slope feature known as the Florida Escarpment. Figure 1. Map of study area. Test line in red. The geologic and geophysical complexity of this area, limited previous exploration activity, and lack of well data present many challenges to developing an accurate image and understanding of the region. Due to the subsurface complexity, interpretation remains ambiguous, even when using these recent time data, because prestack depth migration (PSDM) has been shown to provide superior subsurface imaging in other parts of the GOM (Willacy, 1999). The authors elected to evaluate its effectiveness on these new data. If the test were to prove successful, depth imaging across the lease area might …

Ship and Satellite Studies of Mesoscale Circulation and Sperm Whale Habitats in the Northeast Gulf of Mexico During GulfCet II

Eighty-three encounters with sperm whales (Physeter macrocephalus) occurred on two cruises that made expendable bathythermograph + conductivity-temperature-depth surveys of cyclone-anticyclone eddy pairs in the Northeast Gulf of Mexico (NEGOM). In late summer 1996, 41 sightings of sperm whales were made and 10 acoustic contacts were registered. Of these 51 encounters, 90% were in a cyclonic area of lower than average dynamic height offshore that was surveyed from space by near-real-time altimetric sea surface height anomaly and then mapped in high resolution with shipboard measurements or within 100 km of SW Pass of the Mississippi River. In midsummer 1997, 23 sightings and nine acoustic contacts were made. Of these 32 encounters, 81% were in an offshore cyclonic area of lower than average dynamic height or within 100 km of the mouth of the Mississippi River. Time series animation of the 1996 and 1997 altimetric data indicated these cyclones are typically associated with Loop Current excursions into the NEGOM and that the two cyclones we surveyed had spun up 4-6 mo previous to our fieldwork. Although cyclones in the NEGOM are temporally persistent, their geographic location is spatially variable: the cyclone surveyed in 1996 was centered 150-200 km south and east of the Mississippi River delta in water 2-3 km deep, whereas that surveyed in 1997 was centered farther east in water 2-3 km deep over DeSoto Canyon. Sperm whales appear to have affinity for cyclonic eddies because the largest numbers of encounters with sperm whales also shifted east in 1997 compared with 1996.

ABSTRACT: Geologic Overview of the NE Mississippi Fan and Shelf to West Florida Terrace Region, Offshore Gulf of Mexico

Abstract This study is based on the joint venture (Schlumberger and TGS NOPEC) Phase 46, 16,000-line-mile 2D seismic and gravity/magnetic program that was acquired, processed, and interpreted from 1997-1999. It consists of a 2 2 mile grid (16,100 mi2), covering parts of Main Pass, Destin Dome, Viosca Knoll, Mississippi Canyon, Desoto Canyon, Atwater Valley, and Lloyd Ridge. Water depths range from 75 feet to over 10000 feet. Geological control from 44 biostratigraphy wells helped calibrate seven mapped horizons. In addition, a velocity cube was generated from this dataset. A Cretaceous carbonate shelf, a prominent escarpment to the southeast, dominates the east and northeast portions of the survey. It is overlain by primarily regressive Cenozoic clastics. West of the carbonate shelf is a Miocene to Plio-Pleistocene shelf /slope/fan complex. The shelf facies is developed in the northwest and progresses through slope to deepwater fan deposits toward the southeast. This complex contains numerous allochthonous salt bodies. In general, autochthonous salt structures are present east of the detached salt, underlying the slope fan complex. These continue eastward to the Cretaceous shelf edge. This region is becoming an increasingly attractive area to explore. For example, Crazy Horse in Mississippi Canyon (MC) Block 778 is a recent subsalt discovery within the study area.

Hydrographic Properties and Inferred Circulation Over the Northeastern Shelves of the Gulf of Mexico During Spring to Midsummer of 1998

A hydrographic cruise was conducted 5-16 May 1998 over the northeastern shelves of the Gulf of Mexico, Observed distributions of temperature, salinity, oxygen, and nutrients were consonant with prior occurrences of upwelling, particularly near the head of DeSoto Canyon. Shipboard, moored, and satellite observations indicated these upwelling events were related to the presence of an anticyclonic circulation feature over the canyon. In addition, several cool water events occurred during spring in the nearshore region west of Pensacola; these may be attributed to atmospheric effects. High river discharges from rivers west of the Apalachicola during winter and spring likely resulted in the extensive surface distributions of low-salinity water observed from Mississippi Sound to Cape San Bias during the cruise. The combination of cool bottom temperatures and relatively low surface salinities over the inshore shelf west of Cape San Bias, with the usual seasonal warming, resulted in enhanced vertical stability. This stability likely inhibited vertical mixing and contributed to the development of the relatively low concentrations of dissolved oxygen observed in the bottom waters.

Effects of Cold Air Outbreaks on Evaporation and Heat Loss from Three Regions in the Gulf of Mexico

Simultaneous hourly measurements of atmospheric pressure; wind speed; and air, sea-surface, and dew-point temperatures from three regions in the Gulf of Mexico in 1996 are incorporated in the analysis of sensible and latent heat fluxes and the evaporation rate from the Gulf to the atmosphere. The three regions included in the study are the deep western Gulf, the northern Gulf continental shelf break near De Soto Canyon, and the northern Gulf nearshore environment at Grand Isle, LA. After the case study of a severe cold air outbreak is presented, monthly variations of evaporation and heat fluxes are investigated. It is found that on an annual basis the sensible heat flux is nearly the same between the nearshore and the shelf break regions. For the latent heat flux, the northern shelf break and deep western Gulf are nearly equal and are higher than the northern nearshore region. Also, the evaporation rate and the rainfall amount are approximately in balance in the northern Gulf nearshore environment.

Sandstone Hardbottoms Along the Western Rim of DeSoto Canyon, Northeast Gulf of Mexico : ABSTRACT

ABSTRACT A prominent sandstone hardbottom, herein termed the DeSoto Canyon rim feature (DCRF), is present along the western rim of the DeSoto Canyon. The DCRF occurs as a northeast-southwest trending, isobath parallel ridge presently located in approximately 55 m of water. The ridge is over 12 km in length and ranges from 50 to 120 m in width. Relief varies from less than one to over seven meters. The DCRF varies along strike from a broad, low-relief mound to a prominent ridge with a steep, 7-meter high face on the seaward side. The feature displays strong orthogonal jointing. Erosion and undercutting has produced a prominent debris field along the seaward margin. The size of talus blocks indicates lithification extends several meters into the sediment. The DCRF is composed of fine to coarse grained, moderately to well-sorted sublitharenite. Quartz is the dominant framework component with lesser amounts of potassium feldspar and metamorphic and sedimentary rock fragments. Carbonate skeletal grains comprise from 2% to 15% of the lithologies. Terrigenous grains range from subangular to subrounded. The sand is lithified by Mg-calcite cement that makes up from 20% of the lithologies. Cement content is greatest toward the crest of the feature and on the seaward side. Fibrous, bladed, and drusy cements are all present and there is a marked substrate control on cement morphology. Fibrous cements occur as isopachous crusts on carbonate allochems, bladed cements occur as irregular crusts on quartz grains, and drusy cements are present as pore fills and irregular coatings on terrigenous grains. The cements have 18O values that range from +0.1 to +0.8 and 13 C values that range from +0.7 to +2.5. 14C dating of the cements provides average ages from 5,630 ± 70 to 10,560 ± 70 years B.P. Morphology, sedimentary structures, and sediment texture suggest the DCRF is composed of sediments transported by fluvial processes to near the shelf margin during a sea level lowstand and subsequently reworked during the Holocene transgression. Cement precipitation accompanied the Holocene transgression and may have occurred during periods of sea-level stillstand or short-term reversals in sea level.

An episodic chlorophyll plume on the West Florida Shelf

The episodic formation of an extensive pigment plume on the West Florida Shelf was detected using historical Coastal Zone Color Scanner (CZCS) data collected between 1979 and 1986. The phenomenon is confirmed by in situ observations made in March 1992. The plume occurs mainly during spring, when high pigment concentrations persist 1–6 weeks in a pattern which extends >250 km southward along the shelf. In general, the shelf and continental slope had low pigment concentrations during summer and high pigment concentrations during spring. The information currently available is insufficient to determine the cause of the plume with certainty. Plume formation may be associated with one or a combination of the following processes: (1) discharge from small, local rivers along the NW Florida coast; (2) seasonal changes in steric height differences between the shelf and deep Gulf of Mexico waters; (3) circulation of water associated with the Loop Current and upwelling in the DeSoto Canyon; and (4) discharge from the Mississippi and Mobile Rivers.

Syncoelicotyloides zaniophori n. sp. (Monogenea: Microcotylidae) from the Gills of Coryphaenoides zaniophorus (Macrouridae) from the Gulf of Mexico

Syncoelicotyloides zaniophori n. sp. (Monogenea: Microcotylidae) is described from the gills of the macrourid fish Coryphaenoides zaniophorus caught in the DeSoto Canyon area in the northeastern part of the Gulf of Mexico. This new species is differentiated from Syncoelicotyloides macruri Mamaev and Brashovjan, 1989 by its body size, number of testes, length of copulatory organ spines, morphology of immature portion of germarium, size and morphology of egg, and host.

Early Mesozoic rift stage half graben formation beneath the DeSoto Canyon salt basin, northeastern Gulf of Mexico

Multifold seismic reflection data provide the basis for recognition of an offshore Late Triassic‐Early Jurassic half graben complex beneath the DeSoto Canyon salt basin (DSCSB) along the northeastern Gulf of Mexico margin. The base of salt or equivalent (BSE) surface is a prominent unconformity recognized throughout the DSCSB that is mostly overlain and onlapped by extensive Middle Jurassic (Callovian age?) premarine evaporites (Louann salt and equivalents) and younger sedimentary rocks. Widespread dipping subsalt reflectors, truncated by the BSE, are the seismic expression of an interpreted thick section of synrift strata deposited within a half graben. The half graben probably overlies older prerift Paleozoic (and Precambrian?) sedimentary, igneous, and metamorphic rocks and widens basinward about a NE‐SW axis. Stratigraphic onlap relationships of subsalt reflectors close to the BSE surface suggest a lacustrine sequence may be present in the uppermost section of the rift fill; similar deposits occur within synrift strata below Aptian salt along the rifted West African margin. A NE‐ENE striking normal slip boundary fault beneath the Mississippi‐Alabama‐Rorida (MAFLA) shelf area and the inferred NW‐SE oriented Florida‐Bahamas transfer fault along the eastern margin of the DSCSB approximate the updip limit of rift fill. Trends of these structural lineaments and overall half graben morphology are similar to those of the South Georgia basin, a buried onshore early Mesozoic graben complex in northern Florida and southern Georgia, and the trend of exposed Triassic‐Jurassic continental rift basins along eastern North America. Structural architecture of the DSCSB half graben is consistent with NW‐SE rift phase extension during the early Mesozoic opening of the Gulf of Mexico.

PROBABLE CRETACEOUS-TO-RECENT RIFTING IN THE GULF OF MEXICO BASIN An answer to Callovian salt deformation and distribution problems? Part 1

Abundant evidence from the Northern Gulf of Mexico Basin suggests that late Early Cretaceous to Recent sea-floor spreading is the principal factor in the formation of the structural components of this basin. This would classify the Gulf of Mexico Basin as an active tectonic basin as opposed to the more accepted passive basin status. The following two-part paper presents a model for this lithospheric plate adjustment together with the geological evidence to support this movement. Part I discusses the rift model and supporting evidence. Some of this evidence is: a proposed triple junction rift zone radiating from a large dome centered in the DeSoto Canyon area; the division of the Gulf of Mexico Basin Jurassic salt province into three salt basins (the Interior Exterior and Challenger Basins); the linearity and structurally-positive attitude of the Cretaceous Shelf Edge; an excessive Exterior Basin salt mass that thickens towards the Sigsbee escarpment; large areas of non-diapiric salt found within the Exterior Basin; an extensional fault zone found underlying the Abyssal Plain; and the absence of a Late Mesosoic section from the Exterior Basin a section that would be needed to cover the salt until it was overlain by younger Tertiary sediments.

Basin Architecture, Salt Tectonics, and Upper Jurassic Structural Styles, Desoto Canyon Salt Basin, Northeastern Gulf of Mexico: ERRATUM

Basin Architecture, Salt Tectonics, and Upper Jurassic Structural Styles, Desoto Canyon Salt Basin, Northeastern Gulf of Mexico: ERRATUM

Early Mesozoic Rift-Stage Graben Formation, DeSoto Canyon Salt Basin: Implications for the Early Evolution of the Gulf of Mexico: ABSTRACT

Early Mesozoic Rift-Stage Graben Formation, DeSoto Canyon Salt Basin: Implications for the Early Evolution of the Gulf of Mexico: ABSTRACT

Buticulotrema stenauchenus n. gen., n. sp. (Digenea: Opecoelidae) from Malacocephalus occidentalis and Nezumia aequalis (Macrouridae) from the Gulf of Mexico

Buticulotrema stenauchenus n. gen., n. sp. (Opecoelidae: Opecoelinae) is described from the macrourid fishes Malacocephalus occidentalis Goode and Bean, 1885, and Nezumia aequalis Günther, 1878, collected in June 1971 from the DeSoto Canyon area of the Gulf of Mexico at a depth of 495 m. Eight of 25 (32%) M. occidentalis and 1 of 8 (13%) N. aequalis examined were infected with the new opecoelid. Specimens of Bathygadus macrops Goode and Bean, 1886, and Coelorhynchus coelorhynchus Risso, 1810, collected at the same time were not infected with B. stenauchenus. The new genus most closely resembles Genitocotyle Park, 1937, but differs in that it lacks an accessory sucker behind the genital pore, contains a blind rather than canalicular seminal receptacle, and has a slightly pedunculate acetabulum, a pharynx that is longer than wide, and a distinctive, long-necked, bottle-shaped body. Buticulotrema stenauchenus is also similar to Pseudopecoelus von Wicklen, 1946, but the latter lacks a blind seminal receptacle.

Neogene Planktonic Foraminiferal Biostratigraphy of the Northeastern Gulf of Mexico

We have evaluated the planktonic foraminiferal biostratigraphy of several Gulf of Mexico Eureka boreholes, emphasizing the Miocene depositional history of the northeastern region (De Soto Canyon). The E68-136 borehole (De Soto Canyon; 1826', 557m present depth) provides the most continuous Miocene record, although deposition was punctuated by several short (<2.0 m.y.) hiatuses. The E66-73 borehole (De Soto Canyon; 2802', 854m present depth) provides an uppermost lower to upper Miocene and upper Pleistocene record. E68-151 (north of the Florida Escarpment; 4340', 1323m present depth) provides a discontinuous record of lower, upper middle, and upper upper Miocene sediments. By integrating planktonic foraminiferal biostratigraphy with nannoplankton biostratigraphy and Sr-isotope stratigraphy, we were able to estimate the ages of the hiatuses associated with six distinct unconformities at E68-136 (23.7-23.2 Ma; 18.0-16.5 Ma; 16.3-15.2 Ma; 14.1-13.6 Ma; -10.4-9.5 Ma, 5.0-1.9 Ma), three at E66-73 (-11.5-10.8 Ma, 10.3-9.2 Ma, 6.5-1.7 Ma), and two at E68-151A (19.0-12.6 Ma, 12.5-6.9 Ma). We correlate these unconformities with previously reported unconformities from other deep-sea locations, although their causes (i.e., whether related to sea-level or deep-water changes) remain uncertain. We relate a distinct Pliocene unconformity to intensification of the Loop Current associated with the closure of the Isthmus of Panama.

Latest Oligocene to Earliest Pliocene Benthic Foraminiferal Biofacies of the Northeastern Gulf of Mexico

We examined uppermost Oligocene to lowermost Pliocene sections from four northeastern Gulf of Mexico boreholes for quantitative benthic foraminiferal faunal changes, stratigraphic ranges, paleobathymetry, organic carbon content, and planktonic foraminiferal relative abundances. The Eureka boreholes provide a depth transect in the De Soto Canyon area from the upper to lower bathyal zone: E68-136 (557m present depth, -600m paleodepth), E66-73 (857m present depth, 860-lOOOm paleodepth), E68-151A (1326m present depth, -1300m paleodepth), and E68-14 1A (1599m present depth, -1600m paleodepth). A number of taxa last appeared in the late Oligocene to early Miocene (Biochrons P22-N5) at E68-136; several of these disappearances constitute global last occurrences. A global benthic foraminiferal taxonomic turnover that began in the latest early Miocene in other parts of the ocean was restricted to the middle Miocene at E68-136 (Biochrons N9-N12), although faunal abundance changes began in late early Miocene Biochron N8. At middle bathyal borehole E66-73, ten taxa last occurred in Biochrons N8-N10, which is consistent with the timing of the taxonomic turnover in the Pacific and Atlantic. Depth-related faunal trends are examined and compared with previously published distributional data, resulting in revised paleobathymetric ranges of 12 taxa. Detailed age-paleodepth reconstructions reveal several stratigraphically and bathymetrically significant predominance biofacies in the northeast Gulf of Mexico: 1) Uvigerina pigmea dominated the middle-upper bathyal late Neogene; 2) Lenticulina spp. dominated the late Oligocenemiddle Miocene bathyal zone; 3) Oridorsalis spp., Gyroidinoides spp., and Globocassidulina subglobosa dominated the late Neogene lower bathyal zone; and 4) Uvigerina proboscidea was important in the late Neogene in the middle to upper bathyal zones. Four distinct bathymetric migrations are mapped, and 34 additional taxa are shown to have distinct paleobathymetric distributions. Planktonic foraminiferal biostratigraphic control allows us to evaluate the stratigraphic usefulness of benthic foraminiferal ranges. We revise the stratigraphic ranges of 12 bathyal benthic foraminiferal taxa, requiring re-correlation of the benthic foraminiferal zonal boundaries of Berggren and Miller (1989). INTRODUCTION Previous studies have shown that one of the largest benthic foraminiferal faunal changes of the Cenozoic occurred throughout the deep sea (>200m) in the late early to middle Miocene (Berggren 1972; Schnitker 1979, 1986; Woodruff and Douglas 1981; Boersma 1986; Thomas 1985, 1986a, 1886b, 1989, 1992; Woodruff 1985; Kurihara and Kennett 1986; Miller and Katz 1987; Thomas and Vincent 1987; Woodruff and Savin 1989; Miller et al. 1992; Thomas 1992; for an alternative view, see Boltovskoy and Boltovskoy 1988; Boltovskoy et al. 1992). These authors documented that major changes in taxonomic composition, percentages, and absolute abundances of benthic foraminifera began in the late early Miocene and culminated in the middle Miocene. Although this event has been well documented at open ocean locations, it remains poorly documented in marginal seas such as the Gulf of Mexico. The Eureka boreholes examined in this study (northeast Gulf of Mexico) yield faunal abundance changes and stratigraphic ranges that reflect this global middle Miocene benthic foraminiferal event. Benthic foraminiferal faunas have been recognized for their potential to assess paleobathymetry (e.g. Natland 1933; Bandy 1960). While Bandy (1960) promoted the concept that benthic foraminifera have distinct upper and lower depth limits, Streeter (1973) and Schnitker (1974) established that deep-water (>200m; bathyal-abyssal) benthic foraminifera are correlated to water mass properties that may vary independently of depth. Numerous subsequent studies have documented that deep-water benthic foraminiferal distributions are associated with physiochemical properties other than depth, and that depth alone does not control the vertical distribution of benthic foraminifera. For example, vertical distributions are correlated with water masses (e.g. Lohmann 1978; Corliss 1979; Schnitker 1979; Murray 1984; for Gulf of Mexico examples, see Pflum and 88? 86? 84? TEXT-FIGURE 1 Eureka bo ehole location map in the Gulf of Mexico. Contours are in meters. micropaleontology, vol. 39, no. 4, pp.367-403, plates 1-6, text-figures 1-32, tables 1-5, 1993 367 This content downloaded from 157.55.39.152 on Sat, 26 Nov 2016 04:11:28 UTC All use subject to http://about.jstor.org/terms M. E. Katz and K. G. Miller: Latest Oligocene to Earliest Pliocene benthicforaminiferal biofacies of the northeastern Gulf of Mexico TABLE 1 Age model parameters Datum Age (Ma) Depth (feet below sea level) E68-136 E66-73 E68-151 FO C. acutus & 5.

Neogene History of Deposition and Erosion in the De Soto Canyon Area, Northeastern Gulf Of Mexico.

Neogene History of Deposition and Erosion in the De Soto Canyon Area, Northeastern Gulf Of Mexico.

Ecostratigraphy of Plio-Pleistocene Benthic Foraminifers in ODP Hole 625B and Four Eureka Holes from the Gulf of Mexico

The benthic foraminiferal faunas from the Pliocene and Pleistocene sections of ODP Hole 625B (889m depth), drilled in the DeSoto Canyon area of the northeast Gulf of Mexico were examined from samples representing climatic maxima and minima according to the Joyce et al. (1990) 61 O record. The Pliocene and Pleistocene sections of four Eureka boreholes, two from the eastern Gulf of Mexico (E66-83A, 769m; E68-139, 1852m) and two from the western Gulf of Mexico (E66-40D, 776m; E67-116A, 1229m) were examined in less detail. The fossil faunas from these cores resemble closely those of the present-day Gulf of Mexico. The benthic faunas of the early Pliocene (about 5 Ma to 2.4 Ma) of all cores were very uniform. The respective fossil faunas are usually closer in composition to modem faunas that are deeper than the present coring sites. Faunal composition changed sharply at 2.4 Ma in the interglacial series of samples from Hole 625B, the time of the onset of Northern Hemisphere glaciations: several species that are indicative of very high biological productivity and/or moderate oxygen deficiency (e.g., Bulimina aculeata, Bulimina mexicana, Bulimina exilis) flooded the samples. This fauna occurs in three pulses during the late Pliocene and Pleistocene interglacial sample series: from 2.4 to l.9Ma, from 1.3 to 0.7 Ma, and from 0.4 Ma to the present. The intervening sections exhibit faunas that are essentially similar to the early Pliocene associations. An equivalent change in the faunas of the glacial series of samples from Hole 625B occurred 0.8 m.y. later, from about 1.6 Ma to 1.2 Ma in the early Pleistocene, persisting until about 0.4 Ma. The faunas of the Eureka boreholes closely follow the faunal evolution in Hole 625B. Discrepancies between faunal compositions of 625B and approximately coeval Eureka samples occur particularly in late Pliocene and Pleistocene intervals. These discrepancies originate from the chance recovery of Eureka samples they may represent intermediate climatic stages. Samples from Hole 625B were analyzed in three size fractions: 74 to 125pm, 125 to 150,um, and >150gm. The effect of different size fraction upon faunal composition is slight between the 125 to 150gm and the >150gm fraction. However, the contrast between the faunas of the fine fraction (74 to 125gm) and the coarse fraction (>125gm) is drastic. Exclusively small species belong primarily to the genera of Bolivina and Eponides, which are often indicators of very high biological productivity and/or oxygen deficiency. INTRODUCTION The microfaunas on the deep ocean floor live in an extremely stable environment, an environment that does not experience the drastic fluctuations that surface organisms have to endure. Deep sea benthic foraminifers show a remarkably slow rate of faunal turnover (Douglas and Woodruff 1981) that is inherent from the slow, random process of genetic mutation, which only occasionally receives direction from environmental change. One such environmentally forced event occurred during the late Eocene (Corliss et al. 1984), when the global deep ocean circulation switched from warm and salty low latitude source areas to cold high latitude areas. Boltovskoy and Boltovskoy (1988) argue that this was the one turnover during which the deep sea microfauna, that had its origin during the Mesozoic, became essentially moder. The difference between Oligocene and moder faunas, they argue, lies mostly in historically developed errors of taxonomic usage. However, many authors document another, perhaps lesser, faunal turnover during the late middle Miocene (e.g. Berggren 1972; Schnitker 1979b, 1986; Boersma 1986; Thomas 1986; Woodruff and Savin 1989; Miller et al. 1992). This Miocene faunal turnover is usually ascribed to the additional cooling of deep ocean water, coincident with the build-up of a permanent ice cap on Antarctica. However, Thomas and Vincent (1987) suggest that the faunal change predated the ice-cap formation and cooling. Very large climatic changes occurred during the last five million years that have also affected the deep ocean environment: the sudden beginning of intermittent ice sheet occurrences in the northern hemisphere at 2.4 Ma (Shackleton et al. 1984). It would be reasonable to expect significant faunal turnover at these climatic and oceanographic forcing points as well. Studies of late Quaternary piston cores (e.g., Streeter 1973; Schnitker 1974, 1979a) clearly demonstrated that the deep faunas rearranged themselves drastically in response to glacial/interglacial deep water changes. Several works on the PlioPleistocene sequences of DSDP and ODP holes (Schnitker 1984, 1986; Boersma 1985; Thomas 1986; Loubere and Banonis 1987; Boltovskoy and Boltovskoy 1988; Kurihara and Kennett 1988) also found a drastic change in the composition of the benthic faunas, but hardly any extinctions or occurrences of new species.

Basin Architecture, Salt Tectonics, and Upper Jurassic Structural Styles, Desoto Canyon Salt Basin, Northeastern Gulf of Mexico

Despite awareness of the importance of continental extension during rifting, there are few quantitative studies that show the influence of crustal extension on basin architecture, the distribution of salt, and Late Jurassic sedimentation in the DeSoto Canyon Salt basin, northeastern Gulf of Mexico. Application of simplified isostatic principles using a lithospheric buoyancy model allow quantification of total tectonic subsidence, crust thickness, crustal extension, and crust type. Interpretation of over 4800 km of migrated multifold seismic reflection profiles and well data, integrated with computed isostatic relations, provide the basis to characterize Middle Jurassic (Callovian-age) salt halokinetic processes and to describe the structural development of overlying Upper Jurassic strata. An average crustal thickness of 25 km and crustal extension s values between 1.4 and 1.8 suggest the sedimentary succession is underlain by moderately stretched and attenuated continental crust. The widespread distribution and geometry of dipping subsalt reflectors, particularly in the shelfal areas, provide evidence for a Late Triassic-Early Jurassic phase of rifting prior to deposition of Middle Jurassic salt. The distribution of autochthonous salt and the overlying Upper Jurassic sediments reflect the presalt structural imprint and suggest that the basic architecture of the basin was established by the Middle Jurassic following significant attenuation of the crust. Although deposition occurred in a slowly subsiding, stable marginal setting, salt movement and associated growth faulting are the most significant tectonic elements affecting the stratigraphic and structural development of the overlying strata. Faults related to growth of salt structures root at the base of salt in what appears to be a common detachment or decollement for salt movement. The original distribution of salt is widespread and sheetlike with an estimated minimum thickness of 760 m. Progressive basinward subsidence of the margin and differential sediment loading by the overburden are the primary mechanisms initiating mobilization of salt and subsequent deformation of sediments during the Late Jurassic and from the Late Cretaceous through the early Cenozoic.

Evolution of the Destin Dome, offshore Florida, north-eastern Gulf of Mexico

Structural and stratigraphic relationships indicated by seismic reflection data suggest that uplift of the Destin Dome anticline resulted mainly from salt movement during Late Cretaceous to Early Cenozoic time. A thick succession of Lower Cretaceous sediment was deposited in the central part of the basin seaward of the present day Destin Dome. Sediment loading squeezed the salt both updip onto the shelf and downdip into the basin. A step in the pre-salt basement limited movement of salt and controlled the location of the dome. Vertical movement of salt was accommodated by extensional faulting. During the Early Cenozoic, sediment touchdown onto the pre-salt basement formed a salt weld between the anticline and the De Soto Canyon diapir field, cut off the supply of salt and stopped growth of the structure. The minimum accumulation thickness of the ‘mother’ salt is estimated to be 760 m and the original volume of salt deposited at the present day location of the Destin Dome anticline on the Florida shelf is estimated to be 1000 km 3. The volume of salt in place today is in excess of 2300 km 3.

Chronostratigraphy of Plio-Pleistocene Sequences, Offshore Louisiana

Plio-Pleistocene cool stages correlate with sequence boundaries in the Gulf of Mexico, offshore Louisiana. The cool/warm stages are defined by planktonic foram assemblages in the Vema 16-205 core in the eastern tropical Atlantic. The mean duration of each climatic stage was 230,000 years. These climatic stages were incorporated into a composite standard section with oxygen isotope events, planktonic foraminiferal and nannofossil datums, and magnetostratigraphic events from oceanic core holes: DSDP 502 in the Columbia basin, Caribbean; DSDP 552 on the Rockall Plateau, northeast Atlantic; DSDP 572C in the equatorial Pacific; Vema 28 core 219 on the Solomon Rise, west Pacific; and the Eureka 57 core 135 in DeSoto Canyon, offshore Florida. The oxygen isotope events and the paleontologic datums were identified in two exploration wells in the Green Canyon block, offshore Louisiana, and correlated with seismic sequence boundaries. Iterative graphic correlation with the composite standard refines the positions of the tops of the isotope events where data noise makes the pick equivocal providing a precise, high-resolution correlation tool. Each oxygen isotope event represents approximately 31,000 years. The sequences in the Green Canyon area are from 300 m to 800 m thick, and were deposited at rates from 800 to 1500more » cm/ka. The duration of the hiatuses at the sequence boundaries range from 27,000 to 200,000 years. These seismic/depositional sequences were deposited during the alter parts of the cool stage and the warm climates, which indicate rising sea level to highstand. The ages of the sequence boundaries are approximately 500,000 years, 900,000 years, 1.5 Ma, 2.0 Ma, 2.7 Ma, 3.0 Ma, and 3.75 Ma.« less