Richard T. Buffler

Cenozoic depositional history of the Gulf of Mexico basin

A Geographic Information System (GIS) database incorporating information from 241 publications, theses, and dissertations; well logs and paleontologic reports; and interpreted University of Texas Institute for Geophysics (UTIG) deep-basin seismic lines was used to map and interpret 18 basinwide genetic stratigraphic sequences that form the Gulf of Mexico basin Cenozoic fill. Eight principal extrabasinal fluvial axes provided the bulk of the sediment infill in the basin. First-order temporal and spatial use of these axes reflects four continent-scale phases of crustal uplift. Abundant sediment supply has prograded the northern and northwestern basin margin 150 to 180 mi (240 to 290 km) from its inherited Cretaceous position. Margin outbuilding has been locally and briefly interrupted by hypersubsidence due to salt withdrawal and mass wasting. Three depositional systems tracts characterize Cenozoic genetic sequences: (1) fluvial --> delta --> delta-fed apron, (2) coastal plain --> shore zone --> shelf --> shelf-fed apron, and (3) delta flank --> submarine fan. One or more examples of the fluvial --> delta --> delta-fed apron systems tract occur in each of the major genetic sequences. Immense volumes of sand have bypassed the shelf margin to be deposited in slope and base-of-slope systems, primarily within fluvial --> delta --> delta-fed apron system tracts, during all major Paleogene and Neogene depositional episodes. Deposition and preservation of volumetrically significant coastal plain --> shore zone --> shelf --> shelf-fed apron tracts is typical of Paleogene through Miocene depositional episodes only. Fan system origin was commonly associated with major continental margin failures, but large submarine canyons occur mainly in Pleistocene sequences. Thick, potential reservoir sand bodies occur in offlapping delta-fed slope and subjacent basin floor aprons, in autochthonous slope aprons and related infills of slide scars and canyon cuts, and in submarine fans.

Jurassic Reconstruction of the Gulf of Mexico Basin

The Jurassic evolution of the Gulf of Mexico Basin is related intimately to the break-up of the late Paleozoic supercontinent Pangea and to the early evolution of the Atlantic/proto-Caribbean system. A new two-stage model presented here is constrained by a refined oceanic crust definition in the Gulf of Mexico and by the known kinematic framework of the large continental blocks (North America plate, Afro-South America plate). Oceanic crust definition in the Gulf of Mexico was obtained by combining the results of independent geophysical data sets, including: (a) refraction data, (b) magnetic data, (c) multichannel seismic data in the eastern Gulf of Mexico area, and (d) gravity data. Reconstruction of the basin was completed using the Plates 2.0 plate reconstruction software to visualize the movement of the Afro-South America plate and the Yucatan, Florida-Bahamas microplates during the Mesozoic break-up. During the Late Triassic(?) to late Middle Jurassic syn-rift stage, the relatively stable Yucatan bloc...

Paleogeographic Evolution of Early Deep-Water Gulf of Mexico and Margins, Jurassic to Middle Cretaceous (Comanchean)

The paleobathymetric configuration of the early Gulf of Mexico is inferred from (1) Cretaceous carbonate shelf margins interpreted from seismic profiles and other stratigraphic data; (2) distribution of Jurassic and Cretaceous platform and basinal facies; and (3) hindcasting of subsidence history in the central basin. Substantial paleogeographic ambiguity results from uncertainty about (1) kinematics and timing of Late Triassic to Jurassic extensional opening of the Gulf basin, which probably involved major strike-slip faulting, (2) the magnitude of subsequent compressive deformation on the western and southern basin margins, and (3) possible accretion of allochthonous terranes. The initial sub-sea level topographic depression created by crustal extension is manifested by the distribution of Callovian (?) pre-marine salt and Oxfordian basinal marine facies. Late Jurassic sea-floor spreading split the main salt body and created a central deep-water (> 1 km) trough. From the Oxfordian to early Neocomian, ramp-like platform margins generally underwent net retreat, except in the northern Gulf, where major clastic progradation took place. Carbonate platform margins with sufficient paleorelief for geometric expression on seismic profiles developed early in the Cretaceous. These Cretaceous carbonate margins exhibit a variety of architectural styles, including (1) in the northwestern Gulf, two cycles of progradation (Coahuilan and Comanchean) terminated by drowning events; (2) in the southwestern Gulf, a single cycle of progradation; (3) continuous aggradation along the Florida and Campeche Escarpments. Paleobathymetric relief across Cretaceous carbonate margins ranges from high and steep, to low-relief, to ramplike. The close-of-Comanchean drowning event greatly and permanently reduced the extent of circum-Gulf carbonate platforms and especially rimmed margins. Locations of Cretaceous carbonate margins were influenced by basement hinge zones and paleohighs developed on attenuated continental crust, by the distribution of salt, and by clastic progradation. Eustacy is commonly invoked to explain behavior and architecture of carbonate margins in the Gulf, but other paleoenvironmental variables such as nutrient abundance are probably important as well.

Stratigraphy, palaeoenvironments and model for the deposition of the Abdur Reef Limestone:: context for an important archaeological site from the last interglacial on the Red Sea coast of Eritrea

Stone tools discovered within uplifted marine terraces along the Red Sea coast of Eritrea at the Abdur Archaeological Site, dated to 125±7 ka (the last interglacial, marine isotope stage 5e), show that early humans occupied coastal areas by this time [Walter et al. (2000) Nature 405, 65–69]. In the present paper the stratigraphy, facies types and faunal composition from 25 measured sections of the tool-bearing Abdur Reef Limestone (ARL) are documented in detail and interpreted to provide a palaeoenvironmental context for the stone artefacts and a model for the deposition of the ARL. The ARL represents a complex marine terrace sequence. Erosional surfaces indicative of interrupted sedimentation are locally observed at two levels within the ARL. They subdivide the complex into three subunits, named 5e1, 5e2, and 5e3, representing different stages of the marine isotope stage 5e sea level highstand, comprising six depositional phases (I–VI) of the ARL. Subunit 5e1 begins with the initial transgression of the 5e sea level highstand leading to the deposition of widespread lag gravels on which rich oyster beds developed in shallow water (phase I). It further records rapid deepening accompanied by the deposition of low-energy carbonates with scarce corals (phase II), and later shoaling characterised by local development of a fringing reef tract in a sedimented environment (phase III). Subunit 5e1 is capped locally by a burrowed hardground that is laterally equivalent to depositional discontinuities, interpreted as caused by a globally recognised mid-5e sea level low stand (phase IV). Extensive reef build-up in response to sea level rise and improved conditions for coral growth characterises subunit 5e2 (phase V). A possible second sea level drop during the 5e highstand is inferred from the oyster-encrusted upper surface of subunit 5e2. Subunit 5e3 encompasses restricted coral patches that developed on the upper surface of the underlying subunit during the last stage of the 5e marine high stand (phase VI). Two different toolkits are found in the ARL. One consists of bifacial hand axes and cores of the Acheulian industry, typically associated with the oyster beds encrusted on the transgressive lag deposits. The other consists of Middle Stone Age (MSA)-type obsidian flakes and blades, mainly found in the nearshore and beach environments alongside debris from marine invertebrates and large land mammals. The distribution of these tools suggests that foraging activities of early humans varied with environmental setting. The Abdur Archaeological Site represents a late example of the Acheulian/MSA transition, seen as a benchmark for early modern human behaviour, and is, to date, the earliest well-dated example of early human adaptation to marine food resources.

Seismic stratigraphy and geologic history of Blake Plateau and adjacent western Atlantic continental margin

A multifold seismic reflection profile across the Atlantic continental margin from the shelf off Jacksonville, Florida, northeast across the Blake Plateau, northern Blake basin, Blake Outer Ridge, and lower continental rise shows in some detail the structure, stratigraphy, and geologic history of this passive margin since the Late Triassic(?)--Early Jurassic. A seismic stratigraphic framework divides the sedimentary section into seismic intervals, each representing depositional sequences and having distinct acoustic-stratigraphic characteristics. Interval velocities calculated from 64 reflection velocity analyses were used in the geologic interpretation and to construct a geologic depth section along the seismic profile. Dipping reflectors 7 to 10 km beneath the eastern margin of the Blake Plateau probably represent the eastern edge of the rifted North American continental basement. Up to 8 km of Jurassic(?) through Cretaceous sediments overlie the regional post-breakup unconformity beneath the Blake Plateau. Shallow-water carbonate sedimentation persisted through most of the Cretaceous. In the Late Cretaceous the shelf margin shifted from the Blake Escarpment landward to approximately its present location as a result of continued subsidence and a Late Cretaceous overall rise in sea level. An abbreviated section of deeper water carbonate sediments followed on the plateau during the Tertiary owing to many changes in sea level and currents sweeping the plateau. Subsidence of the plateau appears to be in response mainly to sediment loading. Up to 2 km of Neocomian and older sediments were deposited in the deep sea adjacent to the Blake Plateau. Widespread seafloor erosion and the localization of depositional regimes began in the middle Cretaceous. During the Tertiary enormous quantities of mainly terrigenous material were deposited along the deep-sea continental margin as a large wedge up to 4 km thick. Currents following the seafloor contours influenced sedimentation throughout the Tertiary.

Deep Sea Drilling Project, Leg 77, southeastern Gulf of Mexico

In January 1981, R/V Glomar Challenger drilled five holes in the southeastern Gulf of Mexico to provide ground data for extensive seismic surveys and to document the pre-Tertiary history of the Gulf. Holes 535 and 540 were drilled in a basinal terrane for maximum penetration of the Cretaceous-Tertiary sequence. Rhythmic alternations of light bioturbated and dark laminated carbonaceous limestone represent the Early Cretaceous interval. Some of the dark layers are rich but immature oil source rocks. The limestones resemble the Blake-Bahama Formation in the North Atlantic but their stratigraphic age overlaps in part with the Hatteras Shale. Late Cretaceous rocks are almost totally missing in the basin sites and the Cenozoic section consists of chalk and marly carbonate ooze. Holes 536,537, and 538A were drilled on high-standing fault blocks. Hole 537 recovered phyllite that records 40 Ar/ 39 Ar plateau ages of about 500 m.y. and is overlain by an Early Cretaceous deepening sequence of alluvial to littoral elastics and oolitic-oncolitic limestones, capped by a thin sequence of Cretaceous and Cenozoic pelagics. In Hole 538A, basement consists of mylonitic gneiss and amphibolite, intruded by several generations of diabase dikes (that is, “transitional” crust). 40 Ar/ 39 Ar dates of hornblendes and biotite from the regional metamorphic rocks suggest a 500-m.y. (“Pan-African”) age with mild late Paleozoic thermal overprint. 40 Ar/ 39 Ar whole-rock dates from the dikes suggest intrusions between 190 and 160 m.y. ago. Basement is covered by a thin layer of pelagic chalk, followed by Early Cretaceous skeletal-oolitic limestones and, finally, Cretaceous-Tertiary pelagics. The oolitic-oncolitic limestones at both sites represent either parts of a shallow-water carbonate platform or platform talus deposited in deep water. Hole 536 bottomed in shallow-water dolomite (Jurassic or Permian), overlain by middle Cretaceous skeletal limestones with shallow-water biota and intercalations of pelagic chalk, interpreted as Cretaceous talus at the foot of the Campeche Bank. Cretaceous-Tertiary chalk and carbonate ooze cap the sequence. Among the most significant results of the leg are: (1) recovery of “transitional” crust with early Paleozoic (Pan-African) metamorphic rocks, (2) recovery of Early Cretaceous deep-water limestones with immature petroleum source beds, (3) recovery of mid-Cretaceous platform talus resembling the reservoirs in the Poza Rica and probably some of the Reforma fields of Mexico, and (4) discovery of a Late Cretaceous hiatus of 30 m.y. that corresponds approximately to the “mid-Cretaceous unconformity” recognized widely on seismic records in the Gulf of Mexico.

Seismic Stratigraphic Framework of Deep Central Gulf of Mexico Basin

The deep Gulf of Mexico basin is underlain by up to 10 km (33,000 ft) of Jurassic(?) to Holocene layered sedimentary rocks. The multichannel reflection seismic record from the deep Gulf of Mexico was divided into six seismic stratigraphic units for study of the geologic history of this accumulation. The basal Challenger unit (Jurassic(?) to middle Cretaceous) is considered coeval with early basin formation. We interpret it as a deep marine sequence overlying oceanic crust in the central basin and as continental and shallow through deep marine rocks, including thick evaporites, over adjacent transitional crust. The next three units, Campeche, Lower Mexican Ridges and Upper Mexican Ridges, indicate that from the Late Cretaceous through middle Miocene the basin filled progre sively from the west and north, most probably with siliceous turbidites interlayered with pelagic deposits. By the late Tertiary, however, salt and shale deformation within thick sedimentary sections along the western and northern margins trapped much of the incoming sediment supply on the shelves and upper slopes. The late Miocene to Pliocene Cinco de Mayo unit, therefore, represents a relatively starved interval. In contrast, the uppermost, or Sigsbee unit, includes the Mississippi Fan, an accumulation up to 3 km (10,000 ft) thick of mainly mass-transported deposits that bypassed the shelf and slope and were deposited directly onto the abyssal plain. In the western and southwestern portions of the deep basin, beyond the fan pinch-out, the Pleistocene section is largely a continuation o the Pliocene suspension deposits.

Chapter 3 Jurassic—early cretaceous tectono-paleogeographic evolution of the southeastern gulf of Mexico basin

Abstract A new opening model for the Gulf of Mexico basin provides a framework in which the Jurassic-Early Cretaceous tectono-paleogeographic evolution of the southeastern Gulf of Mexico and surrounding regions can be discussed. A detailed analysis of available seismic data and the results of DSDP Leg 77 define four major tectono-stratigraphic sequences bounded by major unconformity surfaces: crystalline basement, Paleozoic(?) pre-rift rocks, a Late Jurassic syn-rift sequence, and an Early Cretaceous post-rift sequence. The pre-rift rocks are interpreted to represent a pre-Mesozoic (Late Paleozoic?) sedimentary cycle. The Late Jurassic syn-rift sequence in the central continental domain of the southeastern Gulf of Mexico occurs in grabens or half-grabens and is interpreted to consist of two units, a lower non-marine unit overlain by a marine carbonate unit consisting of carbonate buildups (platforms) and adjacent deeper marine sediments. Jurassic rocks are absent over high-standing blocks as well as the adjacent Yucatan and Florida blocks. The Lower Cretaceous post-rift sequence drapes the entire area and consists of deep-water carbonate sediments in the central basin flanked by shallow-water platforms on the adjacent Yucatan and Florida blocks. Six tectono-paleogeographic maps covering the eastern Gulf of Mexico and northwestern Cuba (palinspastically restored to the southeastern margin of Yucatan) document the evolution of the area. During the late Middle Jurassic (Callovian) the southeastern Gulf was a bridge between Yucatan and Florida, separating an area of widespread extension and salt deposition to the north in the Gulf of Mexico from another area of extension and clastic sedimentation to the south between Yucatan and northern South America. By Oxfordian time Yucatan had rotated 11° counter-clockwise and major continental rifting and non-marine sedimentation in rift basins had begun all along the southeastern Gulf. To the north salt deposition had ceased and a major marine transgression culminated in deposition of Smackover carbonates. South of Yucatan shallow-water carbonate sedimentation also prevailed. During Kimmeridgian, Tithonian and into earliest Cretaceous time, rifting continued in the southeastern Gulf as Yucatan continued to rotate counter-clockwise. As the basin subsided a marine seaway, characterized by shallow-water carbonate platforms on high-standing blocks, became established, connecting the Gulf of Mexico with the proto-Caribbean. In the northeastern Gulf the mixed clastic/carbonate Haynesville and Cotton Valley sequences were deposited, while to the south of Yucatan shallow-water carbonate sedimentation gave way to deeper water sedimentation as the margin subsided. In late Berriasian spreading ceased in the Gulf of Mexico, Yucatan reached its present-day position, and rifting stopped in the southeastern Gulf. Carbonate platforms atop rift blocks drowned as the basin subsided and sea level rose, and the southeastern Gulf became the deep-water seaway that it is today. The marine transgression reached the Yucatan and Florida blocks, where extensive carbonate platforms became established and flourished throughout the Early Cretaceous. To the south of Yucatan, deep-water pelagic sedimentation continued throughout the Early Cretaceous.

Laramide orogenic influence on late Mesozoic-Cenozoic subsidence history, western deep Gulf of Mexico basin

The deep part of the Gulf of Mexico basin, which is underlain by oceanic crust, has generally been regarded as a tectonically stable part of the basin since its formation by sea-floor spreading and thermal subsidence during the Late Jurassic-Early Cretaceous. Subsequent subsidence is believed to have been caused mainly by thermal cooling and sediment loading. A backstripping study, however, has revealed a tectonic event in the western deep basin with tectonic-loading effect today. One possible explanation is Laramide fold-thrust loading along the western flank of the basin during ∼66-40 Ma.

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.