Southeastern Gulf Of Mexico Research Articles

Tectonic implications of 40Ar/ 39Ar ages from a pre-Mesozoic metamorphic basement penetrated on Leg 77 of the deep sea drilling project in the southern Gulf of Mexico

Drilling penetrated pre-Mesozoic crystalline basement beneath abbreviated sedimentary sequences overlying fault blocks in the south-eastern Gulf of Mexico. At Site 538A located on Catoche Knoll, a foliated, regional metamorphic association of variably mylonitic felsic gneisses and interlayered amphibolite is intruded by post-tectonic diabase dikes. Hornblende from the amphibolite records 40Ar/ 39Ar plateau dates of c. 500 Ma, indicating early Paleozoic metamorphism. Biotite from the gneiss yields a plateau age of c. 350 Ma which likely dates a superposed late Paleozoic thermal event. A whole-rock diabase sample records a crystallization date of 190.4 ± 3.6 Ma. A phyllitic metasedimentary sequence was penetrated at Site 537 drilled 30 km northwest of Catoche Knoll. Whole-rock phyllite samples display variably discordant 240Ar/ 39Ar age spectra, but plateau segments clearly document an early Paleozoic metamorphism at c. 500 Ma. The age and lithologic character of the basement terrane penetrated at Sites 537 and 538A suggest that the drilled fault block are underlain by attenuated fragments of continental crust of “Pan-African” affinity. This support pre-Mesozoic tectonic reconstructions which locate Yucatan in the present Gulf recess during amalgamation of Pangea.

Early Late Cretaceous to Holocene Seismic Stratigraphy and Geologic History of Southeastern Gulf of Mexico

Multifold seismic reflection profiles were used in conjunction with results from Deep Sea Drilling Project (DSDP) Leg 77 to interpret the early Late Cretaceous to Holocene geologic history of the southeastern Gulf of Mexico. During the mid-Cenomanian(?) to early Paleocene(?), this region began to record the effects of a collision between a northward-migrating island arc (now part of Cuba) and a salient of the North American plate. More than 2 km (6,560 ft) of gravity-flow deposits accumulated in an elongate structural corridor or foredeep along the base of the modern Cuban slope, while the slope itself was the site of both folding and overthrusting. Clastics continued to dominate the depositional regime until the late Eocene, at which time the Cuban arc had been firmly we ded to North America. A late middle to early late Eocene hiatus in Site 540, which coincides with a prominent regional seismic unconformity, marks the transition from predominantly terrigenous input to pelagic/hemipelagic deposition. Since the late Eocene, the southeastern gulf has recorded multiple cycles of deposition and erosion. Unconformities displayed on seismic profiles are numerous. Erosional agents have included the Gulf Stream system, and turbidity currents and debris flows concentrated in the vicinity of submarine canyons. Continuing slope instability is indicated by slide/slump planes along canyon walls.

Comment and Reply on “Deep-sea erosional unconformity in the southeastern Gulf of Mexico”

Comment and Reply on “Deep-sea erosional unconformity in the southeastern Gulf of Mexico”

Amphipods of the Family Ampeliscidae (Gammaridea). III. Ampelisca parapacifica, a New Species of Amphipod from the Western North Atlantic with the Designation of a Substitute Name for A. eschrichtii pacifica Gurjanova, 1955

A new species of benthic amphipod, Ampelisca parapacifica, is described from the southeastern Gulf of Mexico and compared with the closely allied Ampelisca pacifica Holmes, 1908, from the eastern Pacific. The distinctive form of the third uropod separates this species pair from other members of the "macrocephala" subgroup of the genus Ampelisca Kroyer, 1842. Ampelisca parapacifica is distinguished from A. pacifica by antenna I peduncle segment 2 less t}an twice the length of article 1 and uropod I peduncle shorter than the rami. Previous Atlantic records of A. pacifica are assigned to the proposed new species. The Pacific populations of A. eschrichtii pacifica Gurjanova, 1955, were elevated to the species rank by Karaman, unaware that the name A. pacifica was preoccupied. A substitute name, A. karamani, nomen novum, is designated for the form described by Gurjanova.

Crustal Structure of South Florida Bank Derived from Ocean Bottom Seismometer Refraction Profiles: ABSTRACT

In March 1982, 6 seismic refraction lines, 70-90 km (43-56 mi) long, were shot in the southeastern Gulf of Mexico using the advanced University of Texas digital ocean bottom seismometers. Five lines were on the South Florida bank region in water depths of less than 1 km (3,300 ft) and one was in water depth of about 2.4 km (7,900 ft) off the northern coast of Cuba. After data reduction, first arrival picks were made and least squares lines were fitted to the picks to obtain the apparent velocities and intercept times for the layers. Using these values, flat layer crustal models have been initially computed. The 2 most dominant deep refractors have apparent velocities of 5.6-5.9 km/sec (3.5-3.7 mi/sec) and 6.2-6.6 km/sec (3.9-4.1 mi/sec). The top of these refractors varies in depth 2-6 km (6,600-13,000 ft) from the sea surface. They are interpreted to represent the crystalline basement. Basement rocks have been reached at a depth of 3.4 km (11,200 ft) in a well drilled in the Pinellas County arch. In the South Florida bank area, the deepest refractor observed has an apparent velocity of about 7.5 km/sec (4.6 mi/sec) at a depth of about 25 km (15 mi). Absence of any mantle velocity in these long profiles confirms the continental nature of this crust. The only possible mantle arrival (velocity = 8.4 km/sec, 5.2 mi/sec) was observed in the line off the northern coast of Cuba at a depth of about 26 km (16 mi). Similar crustal thickness has been observed in a refraction profile just northwest of this line. This deep crustal structure complements the shallow cru tal structures for this area. End_of_Article - Last_Page 473------------

Amphipods of the Family Ampeliscidae (Gammaridea). IV. Infraspecific Variation in Ampelisca agassizi

Considerable variation noted, in Ampelisca agassizi is herein described. The variations in the shape of the basis of pereopod 7 and the carina of urosomite I are most obyious. No relationship was found with the atypical specimens and their age, associated sediment or location. One of the most abundant and geographically widespread species of the family Ampeliscidae in the western Atlantic and eastern Pacific Ocean is Ampelisca agassizi (Judd 1896). In the western Atlantic this species is recorded from southern Nova Scotia to the Caribbean Sea in depths to 450 m (Barnard 1954b, Mills 1967, Bousfield 1973). Population densities of 15,000/m2 have reported by Dickinson et al. (1980) in the Middle Atlantic Bight region. In the eastern Pacific Ocean, representatives of this species have collected from Queen Charlotte Islands, British Columbia, to Ecuador in depths to 300 m (Dickinson 1982). Ampelisca agassizi was described from the waters off of the state of Rhode Island in the United States, from pelagic males gathered in a plankton tow and it was originally placed within the gerus Byblis. Since that work, this species has described under the names Ampelisca compressa Holmes, 1908, from females in the Atlantic and A. vera Barnard, 1954, in the Pacific. Mills (1967:645) first designated A. v era a jurior synonyn of A. agas sizi and noted that '?acific Coast specimens agree in remarkable detail with those from the Atlantic Coast as J. L. Barnard [1960] has stated and as I have able to confirm. Recent examinations of representatives of these disjunct populations by Dickinson (1982:6) led that author to write he has been unable to find a morphological basis for separating the populations on either side of the Isthmus of Panama. Maty Ampelisca Kroyer, 1842, species have reported with distributions. A detailed comparison of some of these nominal species has shown the Atlantic and Pacific populations to represent distinct taxa.Ampelisca bicarinata Goeke and Heard, 1983, has separated from the Pacific A. cistoides Barnard, 1954, (Bamard 1954a), and A. parapacifica Goeke and Heard, 1984, has shown to be discrete and is separated fuom A. pacifica Holmes, 1908. Atlantic populations of additional transpanamic species are yet to be elevated to the species rank (Goeke and Heard, unpublished data). Variation within the genus Ampelisca has considered a problem by workers in the past. Reid (1951:197) stated there appears to be a great range of real minor variations which is very strange considering the great constancy of characteristics in many species in other genera. However, Barnard (1960:6) states that he has identified nearly 10,000 specimens of lhe 23 species in southern California and is impressed with the ease with which they may be distinguished. He further notes that part of the confusion of workers in the past centered around gerontic males and females. The extreme sexual dimorphism of pelagic males may be disconcerting if not recognized. We agree with Barnard (1960) and note that the western Atlantic taxa are well defined. Much of the confusion arises from suites of closely related species, some of them common but as yet undescribed. The shape of the seventh leg and the dorsal carina of urosomite 1 are of paramount importance in the systematics of this genus,andminor variationis often attributable to the stage of development of the specimens (e.g., A. excavata, Gray and Barnard, 1970). Infraspecific differences noted in A. agassizi are not attributable to either developmental stage or location. It is not unusual to examine a group of specimens from a single collection and observe broad variation. For the purpose of this study, material is illustrated from the western, northwestern and southeastern Gulf of Mexico and from off the Atlantic coast of South Carolina. Pereopod 7 is normally diagnosed for A. agassizi as being broad distally. Specimens are often found where the basis is distally rounded (Figure 1K) and not the transversely rounded form figured by Mills (1967:Figure 3J) and Bousfield (1973:Figure 38). This atypical form of leg 7 often makes it difficult to identify the specimen with the use of currently available artificial taxonomic keys. Material which displayed this leg shape came from both the Gulf of Mexico and the East Coast. No relationship was noted with leg shape and developmental stage of the specimens or with sediment. The variation of the carina of urosomite 1 is not as critica1 as that noted for the seventh leg because it is often considered a secondary character. The dorsal elevation of the carina varies considerably in height and prominence. The degree of variation in the carina of urosomite 1 is well illustrated by a compilation of figures attributed to Ampelisca Manuscript received May 10, 1984; accepted July 9, 1984.

Comment and Reply on “Deep-sea erosional unconformity in the southeastern Gulf of Mexico”

Comment and Reply on “Deep-sea erosional unconformity in the southeastern Gulf of Mexico”

Seismic Stratigraphic Analysis of Lower Cretaceous Rocks, Deep Southeastern Gulf of Mexico: ABSTRACT

A dense grid of multifold seismic reflection lines was combined with results of DSDP Leg 77 to subdivide the Lower Cretaceous section in the deep southeastern Gulf of Mexico into four seismic sequences (I to IV). Age and gross lithologic assignments were made by correlation with DSDP holes 535 and 540. Isochron and seismic facies maps were constructed for each sequence. These data document the complex filling of the deep southeastern Gulf of Mexico by pelagics and fine-grained carbonate debris shed from the adjacent growing Early Cretaceous margins on the east and west. This sedimentation accompanied subsidence of a rifted continental (transitional) crust formed during the early rift history (Late Triassic-Jurassic) of the Gulf basin. The oldest unit (IV, Hauterivian-Berriasian) thins to the east, whereas unit III (Barremian-Aptian) thins to the west, suggesting a depocenter or source area shift to the east. A chaotic, hummocky, discontinuous unit (II, Aptian-mid-Albian) thinning to the south indicates deposition, in part, by northeast- or northwest-source debris flows from the flanking margins. Unit I (mid-Albian-Early Cenomanian) is a tabular parallel to subparallel sequence grading to more discontinuous facies at the base of the Florida escarpment, suggesting an eastern source. A prominent regional unconformity, characterized by erosional truncation, forms its cap. This corresponds to a 50-m thick pebbly limestone unit encountered in DSDP holes 97 and 540 that may span the entire Upper Cretaceous. End_of_Article - Last_Page 617------------

Sooty Tern behavior

A 4-year study of the breeding behavior of Sooty Terns (Sterna fuscata) was made at Bush Key, Dry Tortugas in the southeastern Gulf of Mexico. The results are compared with the behavior of other terns and the differences discussed, particularly in regard to the pelagic environment the Sooty Tern inhabits. Sooty Terns arrive at the Dry Tortugas some 2 months before eggs are first laid. At first they circle Bush Key at night only, but eventually land and extend the time they spend on the ground, arriving earlier in the evening and leaving later in the morning. After the first eggs are laid, flocking subsides. Aerial display by Sooty Terns consists mainly of the high fight in which two birds ascend, usually by jerk-fiying, circle, and then descend together in a coordinated glide. Sooty Terns may have a poorly developed low flight display. On the ground the major display is the parade, similar to that of other Sterna terns. Courtship feeding is rare. Terns often interfere with pairs attempting to copulate. The single egg is incubated about 29.5 days with the male caring for it somewhat more than the female. The birds are attentive over 95 per cent of the time, incubating when it is cool, shading when it is hot, and engaging in other activities such as preening, fighting, and loafing most commonly when they change between incubating and shading. Nest relief usually occurs in the evening after 24 or 48 hours of care. Dipping, in which adults dip their feet, bill and/or breast feathers in the ocean, is common during incubation and may cool the egg or provide it with needed moisture. Chicks are closely brooded for 4 or 5 days, after which adults seem to recognize them individually and spend progressively less time with them. By the end of the third week, chicks are alone much of the time except when fed. Adults feed the chick by regurgitation, spend about 3.5 hours per foraging trip, and often feed a chick several times after one trip. Males feed the chick somewhat more than females do. Vocal exchanges between parent and chick seem most important in individual recognition. Occasionally adults feed a chick other than their own. Chicks first fly when about 8 weeks old and leave the colony soon after that. The ground and aerial displays of Sooty Terns are similar to those of other Sterna terns, especially the Common Tem. The rarity of the low flight and courtship feeding, both of which are common in other terns, may be due to the different way in which Sooties carry food and the distance they travel to forage. Sooty Terns have a lower cutch size, longer period of development of the chick, and first breed when older than most other terns, many of which feed in marshes and coastal waters. These characteristics of Sooty Tern breeding biology are similar to those of many other pelagic birds. A distant fbod supply and high adult survivorship apparently have contributed to these differences from other terns.

Deep-Sea Chert in Gulf of Mexico: ABSTRACT

Chert from at least 9 different stratigraphic zones, ranging in age from Cenomanian (Late Cretaceous) through late Oligocene, was recovered at 4 sites in the southeastern Gulf of Mexico during Leg 10 of the Deep Sea Drilling Project. The sites are on the northern and eastern edges of the Campeche Scarp, in the Catoche Tongue area of the Yucatan Channel, and on the western approach to the Straits of Florida. Chert formation in all locations took place by late postdepositional replacement of deep-water, calcareous, foraminiferal nannoplankton oozes. At each site the degree of silicification of the carbonates increases with increasing age. Depth of burial and/or thickness of the overlying water mass appears to be independent of the degree of silicification. Preliminary petrographic analyses of the cherts reveal the presence of amorphous (opaline) silica; fibrous quartz (variety chalcedony); fibrous cristobalite; and anhedral, subequant, microcrystalline quartz. The type of silica present is dependent upon the stage of silicification and on the composition and texture of the components being replaced. Finely comminuted plant material and other forms of organic detritus are locally common but are unaltered by silicification. The exact source of silica for these cherts is not presently established. Radiolaria and other siliceous organisms are generally common in overlying sediments and may be important silica sources. Although drill-core data in the Gulf of Mexico are incomplete and far from conclusive, there is a suggested increase in the amount of silica both in the form of siliceous organisms and in chert--in the southeastern Gulf of Mexico. End_of_Article - Last_Page 348------------

Magnetic Anomalies in Straits of Florida

Seismic-reflection and magnetic profiles across three topographic highs in the western end of the Straits of Florida and southeastern Gulf of Mexico reveal the presence of diapiric structures which have magnetic anomalies greater than 100 ^ggr. These deep-water knolls may be related to the volcanic-anomaly belt of the Florida platform, the ultramafic rock outcrops of northwestern Cuba, or Cretaceous andesitic intrusive rocks of Yucatan and Florida.