John W. Ladd

Seismic Evidence for Widespread Possible Gas Hydrate Horizons on Continental Slopes and Rises

Anomalous reflections in marine seismic reflection data from continental slopes are often correlated with the base of gas hydrated sedimentary rocks. Examination of University of Texas Marine Science Institute reflection data reveals the possible presence of such gas hydrates along the east coast of the United States, the western Gulf of Mexico, the coasts of northern Colombia and northern Panama, and along the Pacific side of Central America in areas extending from Panama to near Acapulco, Mexico. Suspected hydrates are present in water depths of 700 to 4,400 m and extend from 100 to 1,100 m subbottom. Geometric relations, reflection coefficients, reflection polarity, and pressure-temperature relations all support the identification of the anomalous reflections as the base of gas hydrated sediments. In most places, gas hydrate association is related to structural anomalies (anticlines, dipping strata), which may allow gas to concentrate and migrate updip into pressure and temperature conditions suitable for hydrate formation. The gas hydrate boundary can be used to estimate thermal gradients. In general, thermal gradients estimated from the gas hydrate phase boundary are higher than reported thermal gradients measured by conventional means.

Seismic Stratigraphy of the Bahamas

Seismic reflection profiles from the Straits of Florida, Northwest Providence Channel, Tongue of the Ocean, and Exuma Sound reveal a seismic stratigraphy characterized by a series of prograding Upper Cretaceous and Tertiary seismic sequences with seismic velocities generally less than 4 km/sec overlying a Lower Cretaceous section of low-amplitude reflections which are more nearly horizontal than the overlying prograding clinoforms and have seismic velocities greater than 5 km/sec. The prograding units are detrital shallow-water carbonates shed from nearby carbonate banks into deep intrabank basins that were established in the Late Cretaceous. The Lower Cretaceous units a e probably shallow-water carbonate banks that were drowned in the middle Cretaceous but which, during he Early Cretaceous, extended from Florida throughout the Bahamas region. The seismic reflection profiles reveal a sharp angular unconformity at 5-sec two-way traveltime in northwest Tongue of the Ocean, suggesting a rift-drift unconformity and deposition on thinned continental crust. No such unconformity is seen in central and southeast Tongue of the Ocean or in Exuma Sound, suggesting that these areas are built on oceanic crust.

Seismic Stratigraphy and Sedimentation of Magdalena Fan, Southern Colombian Basin, Caribbean Sea

Analysis of all available seismic data from the Magdalena Fan in the southern Colombian basin, Caribbean Sea, allows subdivision of the sedimentary section into six seismic sequences (units). Although sediments were deposited in the present-day Magdalena Fan region since about Late Cretaceous, terrigenous sedimentation became significant only in the late Cenozoic during deposition of the upper three sequences associated with the uplifts of the Andes. These upper three sequences comprise the Magdalena Fan proper. The uppermost sequence probably represents the last main phase of sedimentation subsequent to the major uplift of the Andes in the Pliocene. The morphologic and shallow acoustic (3.5 kHz) characteristics of this fan unit are: upper fan, 1:60-1:110 gradients, chann ls having well-developed levees, and several subbottom reflectors in all areas except in channels; middle fan, 1:110-1:200 gradients, numerous channels with very subdued levees, and several to few subbottom reflectors; lower fan, <1:250 gradients, small channels, relatively smooth sea floor, and few or no subbottom reflectors. The decrease in number of subbottom reflectors as well as in subbottom penetration downfan apparently results from increasing amounts of coarse-grained sediments. Features in the form of regular hyperbolic echoes and sediment waves are very common in the upper, middle, and to some extent the lower fan, although their heights gradually decrease downfan. On multichannel seismic (MCS) records, the upper fan exhibits conspicuous channel-levee development and coalesc ng wedge-shaped reflection patterns indicative of levee deposits. The middle fan is characterized by the presence of chaotic and discontinuous reflection patterns, which may have resulted from the presence of numerous channels, and hyperbolic features and sediment waves of the type recorded on 3.5 kHz records. The lower fan region has relatively flat, continuous reflections. Within the topmost seismic unit, several episodes of sedimentation can be inferred from MCS records. These episodes are probably related to uplifts in the source region, lowered sea levels, or shifting of the Magdalena River delta in space and time, subsequent to the major orogeny in middle Pliocene. A model of sedimentary processes proposed on the basis of the characteristics described visualizes a dominant role for channelized and overbank turbidity current deposition in the upper fan. The influence of these currents decreases and that of the unchannelized currents increases downfan. On the lower fan, deposition by unchannelized turbidity sheet flows forms the dominant mode of sedimentation. In addition, slumping on the back sides of oversteepened levees and along broad fronts of the continental slope, and other downslope mechanisms may also have influenced fan sedimentation significantly.

Deep seismic reflection results from the Gulf of Mexico

Deep sounding seismic reflection data show undeformed reflectors at depths down to 11 kilometers beneath the continental rise and abyssal plain and 7 kilometers in basins of the lower slope. Weak reflectors are visible beneath the salt of the Sigsbee Scarp and within salt ridges separating the lower slope basins.

Seismic stratigraphy of the western Venezuela Basin

Abstract Multi-channel seismic reflection records from the central Venezuela Basin reveal planar reflections within the upper half of the oceanic crust as well as within the overlying sediment layer. The high resolution of the reflection data allows a division of the sediment section into three seismic intervals bounded by strong laterally persistent amplitude peaks which can be traced throughout much of the basin. The underlying crustal reflection zone is divided into two seismic intervals based on an angular relationship between the reflections which constitute each zone. Deep-sea drilling permits local determination of age and lithology of the seismic intervals of the sediment section and the top of the uppermost crustal interval. The lateral continuity of high-amplitude peaks that separate the sedimentary seismic intervals suggests that the boundaries are for the most part time-synchronous throughout the Venezuela Basin. Thinning and local pinch-outs of the deeper sedimentary seismic intervals in several parts of the basin contrast with the continuity in thickness of the shallower sedimentary seismic intervals. This suggests that the deeper intervals were affected more than shallower intervals by bottom currents which scoured and redistributed sediment from Cretaceous to Eocene time, but which became much weaker in post-Eocene or Early Oligocene time. Perhaps the weakening of bottom currents in Late Eocene or Early Oligocene was related to the development of the Lesser Antilles island arc and the isthmus of Panama in Eocene time. Planar seismic reflections within the high-velocity crust of the Venezuela Basin extend 1.5 sec below the base of the sediment section. These planar reflections extend tens of kilometers laterally. The upper interval of this crustal section consists of fairly level internal reflections compared to the lower interval with more steeply dipping reflections. The boundary between these two seismic intervals is not a distinct reflection, but rather a zone of apparent merging of the two intervals. The reflections of the lowest seismic interval become lost in the noise at a depth approximately equivalent to the top of the region in the lower Venezuela Basin crust with refraction velocities of 7.2 km/sec.

Tectonic processes along the Middle America Trench inner slope

Summary The relationship between sedimentation and structural variations observed in geophysical data provides insight into tectonic processes on the convergent Middle America Trench margin. We observe that some of the hemipelagic and pelagic oceanic sediments seem to continue relatively undeformed beneath the lowermost slope. Trench sediment bodies restricted to ponds in the vicinity of submarine canyons are incorporated into the lower slope by folding and thrust faulting. This offscraping (shallow accretion) at the base of the Middle America Trench slope appears at least partly dependent on the presence of trench fill. The internal structure of the so-called accretionary zone, the top of which is usually identified on seismic reflection profiles by a prominent zone of diffractions, is rarely resolved in the seismic reflection data. The shallow part of the accretionary zone along much of the margin may consist of deformed slope sediments since down-section increase in both folding and other deformation are observed in the seismic reflection and drill data. Beneath the undeformed and deformed slope sediments, offscraped trench fill may occur if trench fill was present in the ancient trench. The deeper part of the accretionary zone representing the bulk of the volume is not resolved in the reflection data but may consist of other sediments or crustal rocks added by an underplating process at deeper structural levels.

Sedimentation in different tectonic environments of the Middle America Trench, southern Mexico and Guatemala

Summary Late Pleistocene to Holocene sediment facies and composition within and bordering the Middle America Trench offshore southern Mexico and Guatemala reflect two distinctly different tectonic provinces. The truncated Mexican margin with crystalline rocks onshore and a narrow shelf exhibits a locally thick sandy trench fill. Sand mineralogy of quartz, feldspar, and biotite matches the onshore source terrane. The Guatemalan margin with a volcanic terrane onshore and a wide forearc basin show smaller amounts of predominantly muddy trench fill. Sand composition of volcanic rock fragments, plagioclase, and heavy minerals accurately reflects the volcanic source. Trench fill in both areas corresponds to submarine canyon location, and extensive bypassing of the slope occurs. The outer shelf contributes little sediment to the Mexican slope and trench, the main source being in the littoral zone and inner shelf. Conversely, most Guatemalan slope and trench sand has come from the shelf. Trench fill correlates best with onshore geology, with similar slope sediments in both areas. The Mexican margin with locally thick trench fill displays clear evidence of accretion, the Guatemalan margin with meagre trench fill apparently has not accreted lower plate sediments recently. Possibly, larger volumes of trench fill encourage accretion.

Interpretation of multi-channel seismic reflection records from the Gulf of Mexico

Abstract New multi-channel seismic reflection data illuminate details of the structure, stratigraphy and geologic history beneath the abyssal plain of the Gulf of Mexico. These data show a thick sedimentary section lying on an irregular acoustic basement thought to be oceanic crust formed in early Mesozoic time. Six seismic units within the sedimentary section are defined on the basis of reflection characteristics and basin-wide continuity. One unit containing the salt (Jurassic?) that feeds the Campeche-Sigsbee Salt Dome Province can be traced northward toward the Sigsbee Escarpment but pinches out against the base of the Campeche Escarpment. The salt layer places limitations on the location or age of a plate boundary between North America and Africa-South America which has been suggested to have been active in the Gulf in Jurassic time. The four units lying above the salt reflect an extended period of pelagic sedimentation followed by mid-Tertiary-Pleistocene turbidite sedimentation.

Seismic Stratigraphy and Sedimentation, Magdalena Fan, Southern Caribbean Sea: ABSTRACT

Multi- and single-channel seismic records from the Magdalena Fan reveal six seismic sequences in the entire sediment column (with thickness from > 5.5 to 2.5 secs two-way traveltime). Although sediments were deposited in the Magdalena Fan since about the Late Cretaceous, terrigenous sedimentation became important only in later Cenozoic time during the deposition of the upper three units following the Andean uplifts. However, the uppermost seismic sequence is the fan unit, most influenced by influx of terrigenous sediments and deposited subsequent to the major uplift of Andes in Pliocene time. The morphologic and shallow acoustic (3.5 kHz) characteristics of this fan unit are: (1) upper fan, 1/50 to 1/100 gradients, with channels having well-developed levees and with se eral subbottom reflectors; (2) middle fan, 1/100 to 1/200 gradients, occurrence of numerous channels with very subdued levees and several subbottom reflectors; and (3) lower fan, < 1/200 gradients having small channels and relatively smooth sea floor with few or no subbottom reflectors. Large irregular to regular hyperbolic echoes and sediment waves are very common in the upper, middle, and to some extent the lower fan, and have resulted from slumping and other downslope mass movements. On multichannel seismic records, the upper fan exhibits conspicuous channel-levee migration and onlapping and coalescing wedge-shaped reflection patterns (from levee deposits). The middle fan is characterized by the presence of chaotic and discontinuous reflection patterns which resulted from the prese ce of numerous channels and the hyperbolae and sediment waves of the type recorded on 3.5 kHz records. The lower fan region has continuous and smooth reflection patterns. Within the topmost seismic unit, several episodes of increased terrigenous sediment influx have resulted in a seaward progradation of different fan regions in Pliocene-Pleistocene times. End_of_Article - Last_Page 589------------