Middle Miocene Sediments Research Articles

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.

The magnetostratigraphy of the Chitarwata and lower Vihowa formations of the Dera Ghazi Khan area, Pakistan

Three sections of the Chitarwata and lower Vihowa formations were sampled along the Dalana River on the southeastern flank of the Zinda Pir Dome in the southern Sulaiman Range near Dera Ghazi Khan, Pakistan. Together they provide nearly 800 m of a continuous sedimentary record of the Miocene derived from the uplifted Himalayan highlands. Previous studies have examined the Middle and Upper Miocene sediments, the Siwalik Group, of the Potwar Plateau to the north. However, detailed investigations of earlier periods are impossible in that area due to the absence of Oligocene and Lower Miocene sediments caused by continued overthrusting associated with the Himalayan orogeny. Fortunately, the Sulaiman basin to the south, which was further removed from the tectonic activity, provides a record of the Early Miocene in the form of the Chitarwata Formation. The Dalana A, B, and C sections (DGA, DGB, and DGC) were examined and sampled for this study. A magnetostratigraphic analysis was carried out to correlate and date the Chitarwata and Vihowa formations exposed in this area. The samples were subjected to step-wise demagnetization to resolve the primary and secondary remanent magnetization components. Thermal demagnetization trajectories indicate that the majority of samples have sufficient internal consistency that their calculated polarities are reliable. Likewise, the majority of the 126 measured sites showed statistically significant agreement between the three measured samples per site. However, the bimodal data set does not pass the reversal test and so is deemed unsuitable for tectonic interpretations. The individual sections were initially correlated using lithologic and stratigraphic methods, and the relationship was reinforced by the magnetostratigraphy. The composite DG section was tentatively correlated with the standard magnetic polarity time scale placing the disconformable base of the Chitarwata in the DGA section at just older than 22 Ma, the Chitarwata/Vihowa contact at 18.6 Ma, and the top of the Vihowa in the DGC section at about 16 Ma. The assignment of these dates to the Chitarwata will aid in future biostratigraphic and lithostratigraphic correlation of Early Miocene sediments, effectively extending the well-established Siwalik faunal sequence back by four million years.

Strontium isotope stratigraphy of Kita-daito-jima Atoll, North Philippine Sea: implications for Neogene sea-level change and tectonic history

A detailed chronology of a 432 m core from Kita-daito-jima, an atoll on the Philippine Sea plate, has been obtained by strontium isotope stratigraphy. The atoll carbonate extends to late Oligocene age. The top ∼ 100 m Plio-Pleistocene section of the core has been dolomitized, and dolomite also extends to middle Miocene sediments. Sr isotopes identify two dolomitization events, at ∼ 2 and ∼ 5 Ma. Atoll growth appears to have been continuous between 18.8 and 24.3 Ma and the fit of the growth history to a standard model provides a successful confirmation of Darwin's subsidence theory. The atoll growth appears to have commenced about 42 Ma ago (6 Ma after volcanism), producing a ∼ 2 km column of atoll carbonate, in agreement with bathymetry. The subsidence model defines an apparent loss of 360 m of the sedimentary record since the early-middle Miocene boundary. The amount in excess of the ∼ 100 m associated with sea-level fall is attributed to Plio-Pleistocene uplift at rates of ∼ 20–30 m/Ma associated with the forebulge of the Philippine Sea plate prior to its subduction. Uplift rates have also been estimated using (a) the heights of dated last-interglacial coral reefs on Kita-daito-jima and nearby Minami-daito-jima (∼ 17 m/Ma and ∼ 29 m/Ma), (b) Sr isotope ages of calcites from surface samples on Minami-daito-jima (∼ 21 m/Ma) and (c) contrasts in elevation with Okino-daito-jima 100 km away (∼ 22 m/Ma). The locations of hiatuses and ages of dolomitization indicate sea-level falls of ∼ 80 m at ∼ 17–16 Ma, ∼ 30 m at ∼ 16–15 Ma, ∼ 125 m at ∼ 11 Ma, and ∼ 90 m at ∼ 5 Ma and at ∼ 2 Ma. The data have been combined with those from Enewetak Atoll to produce a curve of sea-level change for the Neogene.

An unusual distribution of bicadinanes, tricadinanes and oligocadinanes in sediments from the Yacheng gasfield, China

The extracts of Middle Miocene sediment samples from the Yacheng gasfield offshore Hainan Island, China contain high amounts of compounds that originate from polycadinene. This polymer is present in the dammar resins produced by angiosperm trees such as the Dipterocarpaceae and disintegrates upon thermal stress. The fragments can react intramolecularly to form relatively stable compounds which are subsequently hydrogenated or dehydrogenated. Many saturated and aromatic compounds formed in this way are encountered in the sediment extracts. The presence of bi- and tricadinanes was established by gas chromatography-mass spectrometry. The occurrence of tetra- and possibly pentacadinanes was suggested by high temperature-gas chromatography.

Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbidite and contourite depositional systems

The construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to late Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the proto-Hatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fan-channel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.

Tectonic evolution of Sabah, Malaysia

Southeastward subduction of oceanic lithosphere in front of the rifted continental block of southern China under an emergent oceanic basement in the eastern part of Sabah controlled the development of an elongate basin trending NE-SW. This basin became the site for the deposition of Middle Eocene-Early Miocene sediments. Active opening of the South China Sea Basin in NW-SE and N-S directions during the Middle Oligocene-Middle Miocene caused further subduction and narrowing of the basin. As a result the Middle Eocene-Early Miocene sediments were compressed into a fold-thrust belt trending approximately NE-SW in the western part of Sabah and NW-SE in the northern and eastern part of Sabah. The subduction was accompanied by volcanic activity in the eastern part of Sabah during the Early-Middle Miocene. The deformed sedimentary pile and underlying oceanic basement were then subjected to a NW-SE extension related to the opening of the Sulu Sea Basin during the Early-Middle Miocene. This resulted in the development of extensive chaotic deposits in the eastern and central part of Sabah. This extension also controlled the development of several circular basins for the deposition of the thick, Early-Late Miocene sediments. Continued extension in this region resulted in further southeastward subduction in southeastern Sabah producing the Late Miocene-Quaternary volcanics.

A petromagnetic study of Miocene rocks bearing micro-organic material and the magnetic mineral greigite (Sokolov and Cheb basins, Czechoslovakia)

Abstract The Middle Miocene sediments of the Cypris series from the Sokolov and Cheb brown-coal basins (western Bohemia) were subjected to petromagnetic investigation. During the laboratory thermal treatment, at a temperature of 350–400°C, the samples rich in micro-organic material changed into carbonified samples which exhibited similar physical and technological properties to coal samples collected from the underlying coal seam. The experimental results may have a bearing on several problems concerning the origin of fossil fuels and highlight specific rock properties which may have potential for identifying the micro-organic content. A strongly magnetic mineral greigite Fe 3 S 4 was identified in magnetic zones above the coal seam. The authigenic mineral greigite occurs in strata of greater thickness and it was found that a reducing environment, generated by the presence of organic matter, was required for the formation of the greigite. Metastable properties and the low coercivity spectra of this mineral are adverse for palaeomagnetic investigations.

Miocene facies models, paleoenvironments and sedimentary cycles in the Nile Delta area, Egypt

Lateral and vertical distribution of lithofacies and paleoenvironments of the Miocene sequence in the Nile Delta and its eastern adjoining area enabled recognition of the depositional history and eustatic sea level changes. Three successive cycles of sedimentation are deduced. The first cycle took place by the end of Oligocene and during the Early Miocene. The sequence of this cycle started with inner neritic facies succeeded by open bathyal facies, implying a transgressive marine cycle. The second cycle indicates a progressive regression of the sea from bathyal to transitional and continental environments during the Middle and Late Miocene. The third cycle developed in the Late Miocene-Early Pliocene and is characterized by transgressive sequence of transitional facies succeeded by neritic and bathyal facies. The proposed Miocene eustacy in the Delta area is compared with the global eustatic sea level changes. It is interesting to note that the first and third transgressive cycles were controlled by global eustatic rises in the sea level and the second regressive event was related to local active tectonics. The effects of syndepositional fault activation are demonstrated in the middle and upper Miocene sediments.

Paleomagnetic results of Tertiary sediments from Corsica: evidence of post-Eocene rotation

Middle Eocene (Lutetian) and Lower to Middle Miocene (Burdigalian-Langhian) sediments from Corsica have been investigated in a paleomagnetic study to constrain the amount and timing of tectonic rotation of the Corsican block with respect to Sardinia and stable Eurasia. The sediments are generally characterized by weak and unstable magnetization, and only five Eocene and five Miocene sites out of 23 sites sampled provided interpretable data. Our best estimate for the Eocene paleomagnetic field for Corsica suggests ∼ 37° of counter-clockwise rotation, and no latitudinal change, with respect to stable Eurasia in post-Eocene time. This is similar to the amount of tectonic rotation documented from Oligo-Miocene volcanics from Sardinia, which is therefore consistent with Corsica and Sardinia being part of the same tectonic block since at least the early to middle Tertiary. Paleomagnetic results from the Miocene sites may represent remagnetizations but the presence of reversed polarity magnetization at three of the five accepted sites indicates acquisition before the late Pleistocene. The final rotation history of Corsica nevertheless is left unresolved.

Regional Biostratigraphy and Paleoenvironmental History of Miocene of Onshore and Offshore Alabama: ABSTRACT

ABSTRACT Subsurface Miocene sediments of coastal Alabama and the adjoining state and federal waters area consist of a clastic wedge varying in thickness from about 500 feet in south-central Baldwin County in southern Alabama to a maximum of about 6,000 feet in the northeastern portion of the Main Pass Area. Relatively deep-water and open-marine transgressive basal Miocene clays and shales unconformably overlie a gently southwestward dipping late Oligocene-earliest Miocene carbonate platform. Middle and late Miocene sediments consist of a regressive offlapping sequence of sand and shale deposited in varying neritic paleoenvironments. Analysis of planktonic and benthonic foraminifera has resulted in a refined biostratigraphic zonation of these sediments, permitting the recognition of several regional time-equivalent datum levels, or biohorizons. These biohorizons are shown on a series of subsurface cross-sections which indicate the dramatic southwestward thickening of middle and late Miocene sediments within the Cibicides carstensi and Discorbis 12 interval-zones. The paleoenvironmental history of the Miocene has been reconstructed on a series of paleobathymetric maps drawn for selected regional biohorizons. Among other features, these maps have proven the existence and outlined the margins of previously unrecognized shallow-neritic deltaic sediments in southeastern Mobile County and in the Chandeleur and Viosca Knoll (north) Areas. Analysis of sedimentation rates, which range from less than 25 to 1,370 feet per million years, further aids in understanding the coastal shelf, deltaic, and open-marine depositional history of the Miocene of Alabama and the adjoining state and federal waters areas.

Tectonic evolution of the northwestern Red Sea margin as related to basement structure

At the northwestern, Egyptian margin of the Red Sea a crystalline basement of Pan-African, Late Proterozoic age is exposed beneath a Phanerozoic sedimentary cover. This tectonic situation provided an opportunity to study the younger, Red Sea tectonic evolution in relation to features that are inherited from or reactivated in an underlying basement. Two structural maps compiled from published sources and our data, covering the eastern Egyptian basement from Hurghada to the south, show the most important structural elements: the major, central and southern parts of the basement in eastern Egypt (CED and SED) are dominated by low angle thrusts. Other fractures (N-S, E-W, NW-SE) are distinctly less frequent and of local importance. NW-SE oriented strike-slip faults (Najd System) overprinted the low-angle thrusts of the northern CED. These fault are related to NE-SW trending normal faults, which characterize the northern part of the basement (NED). Before discussing general relationships between basement structure and Red Sea evolution a detailed example of the post-Pan-African, Phanerozoic evolution of the low-angle thrust dominated domains of the CED and the SED from a traverse south of 25° N is presented. There, detailed structural mapping showed Phanerozoic normal faults close to the Red Sea (trending NNW-SSE) and at the margin of the Phanerozoic cover east of the Nile (trending NNW-SSE and NW-SE), leading to a horst-like position of the basement. Within this horst deformation is restricted to a local sinistral strike-slip fault trending N-S. The normal faults frequently used and reactivated fractures parallel or normal to the axial surface of earlier (Pan-African) folds. Igneous activity is confined to the western part of the basement, where mafic dykes, mafic to trachytic volcanics, the alkaline Gabal Abu Khrug ring complex and trachyte plugs (in that time sequence, from 104 to 80 Ma) are exposed. Mafic dykes are dependent on host rock properties, distributed mainly in relatively brittle granitoid rocks. Trachyte intrusions are generally not related to particular host rocks but may follow pre-existing structures. Crosscutting relationships with the igneous rocks imply a Late Cretaceous age for the faults in the west of the traverse. However, in the east, at the Red Sea coast, faults cut Middle Miocene sediments and are thus younger. Tectonic activity in the traverse area is therefore confined to two distinct episodes: a Late Cretaceous event of alkaline magmatism and normal faulting at the western margin of the basement and a Miocene or younger event of normal faulting at the present Red Sea coast. In contrast to earlier views neither transform faults nor large-scale Riedel shears related to Red Sea tectonics have been met in the area. This holds also true for the CED and SED as a whole, where only local faults remain for potential reactivation during Red Sea tectonics (except, perhaps, the Duwi Shear zone at Quseir). The prevailing N-S, E-W or NW-SE directions of these faults preclude an interpretation as possible transform faults normal to the spreading axis of the Red Sea. Therefore, it is no longer possible to assume a continuation of Red Sea transform faults into the continental crust.

Geology and fauna of the middle Miocene hominoid site at Muruyur, Baringo district, Kenya

The middle Miocene sediments assigned to the Muruyur Beds have yielded abundant faunal remains which indicate an age somewhere near the early part of the middle Miocene, perhaps being earlier in time than Fort Ternan but probably coeval or slightly later than Maboko. Available radioisotopic age determinations suggest that the beds are between 13.5 and 14 m.y. old, which seems to be too young when compared with the biostratigraphic estimate. The importance of Muruyur Beds lies in their rich fossil content which includes hominoids of an age which is in general poorly represented in East Africa’s fossil record. This article places the fossil discoveries on record, and discusses their geological context.

Structural Evolution and Hydrocarbon Trapping Mechanisms: Hankamer Field Area, Liberty and Chambers Counties, Texas: ABSTRACT

The Hankamer field area, located in the Houston salt dome embayment, is associated with a deep-seated salt structure, and related complex graben and radial fault systems. Palinspastic restorations, geochemical and paleontological studies, and paleobathymetric maps have been used to reveal the structural evolution of the upper Eocene through middle Miocene sediments. Data from 400 wells and 37 mi (60 km) of two-dimensional seismic data were integrated to generate structure, isopach, fault plane maps, and cross sections over a 74 mi/sup 2/ (192 km/sup 2/) area. The palinspastic restoration of a seismic profile was constrained by maintaining the bed length and the volume of salt in two dimensions, and accounting for the depositional environment and water depth, established from a paleontological study. The palinspastic restoration provided insight into the structural configuration of the salt dome during late Eocene through middle Miocene and the timing of various faulting episodes.

Origin of moldavites

Moldavites are supposed to be products of the Ries impact; but neither has the source material been identified in the Ries area nor is the mode of moldavite glass formation sufficiently clear. It is the purpose of this investigation to elucidate both problems by means of new chemical data. Fifty-nine moldavites from 20 localities in Bohemia and Moravia and 30 samples of Middle Miocene sediments from the Ries were analyzed for major oxides (X-ray fluorescence), 30 trace elements (neutron activation), water content (infrared spectroscopy) and 18 O 16 O - ratios . Microprobe analysis shows that moldavites are composed of thin laminae of optically and chemically somewhat different glasses, bent and curled in three dimensions, indicating the confluence of primary melt units under conditions of laminar flow. The moldavites cannot be derived from crystalline basement rocks and Mesozoic sediments involved in the Ries impact. The only lithology from which moldavites could have been formed is Middle Miocene sands, which formed a thin veneer in the Ries area at the time of the impact. Moldavites and these Middle Miocene sands are compositionally similar but not identical, which indicates that moldavite formation from sands involved chemical fractionation. Enrichments and depletions of individual elements in moldavites can not be explained by fractional vaporization. This was confirmed also by melting experiments with sand samples. Moreover, the negligible water content in moldavites and the scarcity of vesicles excludes the formation of moldavite melt by simple fusion of sands. For individual elements the distribution ratios q (concentration in moldavites: concentration in sands) and ionic radii R are significantly correlated: q = 2.21 R − 1.18. It is inferred that at the encounter of the impactor with the ground surficial sands were transformed by high pressures and temperatures into a plasma-like vapor. In the expanding and ascending vapor droplets of a liquid phase condensed, consisting of a framework of interconnected (Si, Al)O 4-tetrahedra which selectively incorporated cations to compensate its negative charge. The uptake of cations was controlled by the ionic radius because ions fit better into the interstices of the liquid framework the larger they are. The primary melt droplets separated from residual vapor and coalesced to moldavite bodies. The lower δO content of moldavites (11.5) as compared to dry sands (16) is explained by an admixture meteoric water ( δO = −10) which filled the pore space.

Occurrence and Distribution of Phosphatic Miocene Hawthorne Group on Georgia Inner Continental Shelf: ABSTRACT

Miocene-age strata contain economically significant quantities of phosphate in Florida and North Carolina. Previously, units of similar age and lithology offshore Georgia were poorly defined geographically and stratigraphically. Through interpretation of high-resolution seismic data and correlation with well logs, it is possible to delineate middle Miocene units on the Georgia shelf. The middle Miocene Coosawhatchie formation consists primarily of phosphatic, shallow marine, terrigenous sands and clays, with clays (Berryville clay member) dominating offshore and grading shoreward into the predominantly sandy Ebenezer member. The Tybee phosphoritic member, the basal unit of the middle Miocene in the coastal area, averages 20 ft thick in eastern Chatham County and is present in varying thickness along coastal Georgia. The Tybee phosphorite member is of interest as its phosphorite concentration is roughly comparable to the phosphorites currently being mined in Florida and North Carolina. It is not always possible to identify the Tybee phosphorite member as a unique reflector on seismic records. As the basal member of the Coosawhatchie formation, however, its geographic extent is approximated by the structure-contour of the base of the middle Miocene. The base of the middle Miocene deepens southward from about 82 ft below surface, adjacent to Tybee Island, to more » more than 395 ft, seaward of Cumberland Island. Middle Miocene sediments on the Georgia shelf range in thickness from 0 to greater than 175 ft, thickening to the east and south. « less

Karst landform development and related sediments in the Miocene rocks of the Red Sea coastal zone, Egypt

Limestones and evaporites are the main rock types constituting the exposed Middle Miocene sediments of the Red Sea coastal zone between Quseir and Mersa Alam. These rocks represent typical conekarst and karst ridge landforms with minor surface and subsurface solution features.

Diatom biostratigraphic and palaeoceanographic interpretations for the middle to high latitude North Atlantic Ocean

Summary Diatoms are useful for biostratigraphic correlation of Neogene sediment sections in the North Atlantic Ocean. The warm-temperate assemblage observed in cores from the middle and high latitude Atlantic enables partial application of the diatom zonations defined in the eastern equatorial Pacific. Sixteen of the equatorial Pacific diatom zones are recognized in cores from the middle latitudes of the North Atlantic. The diatom assemblage implies that surface waters were warm during the early Miocene in the Rockall Plateau region. A decrease in the quality of diatom preservation, and the occurrence of numerous hiatuses at approximately the early-middle Miocene boundary, suggest a major change in oceanic circulation. An increase in deep water overflow from the Norwegian Sea may have been a contributing factor. Although middle Miocene sediments at both middle and high latitudes are generally enriched with silica, a possible restriction of siliceous sedimentation towards the high latitudes is observed during this time. The interval representing the early late Miocene and earliest Pliocene is generally devoid of diatoms. Conditions favouring siliceous sedimentation continued at high latitudes and returned to middle latitudes of the North Atlantic during the late Pliocene, maybe in response to increased intensity of the Gulf Stream. The abundance and preservation of diatoms in high latitude Quaternary sediments is partially controlled by climatic fluctuations. The occurrence of the Denticulopsis seminae group suggest that cool, low saline waters may have occurred in the North Atlantic between 1.1 and about 0.70 Ma.

«Mud-mounds» en sedimentos lacustres someros del Mioceno medio de la cuenca de Madrid

Diversos edificios carbonáticos de carácter no tobáceo han sido reconocidos en sucesiones continentales correspondientes al Mioceno medio de la cuenca de Madrid. La asociación de facies en todos los casos observados conduce a caracterizar dichos edificios como propios de ambientes palustres o lacustres muy someros. Se describe en detalle la morfología y estructura interna de estos cuerpos cuyo núcleo aparece constituido por un espeso entramado de raíces con ciertas peculiaridades en sus rellenos. Distintas subfacies: núcleo, discontinuidades internas, corteza externa, brechas locales en los flancos , han sido distinguidas, sirviendo de base para la propuesta de un modelo de desarrollo en estadios sucesivos de los edificios, integrables bajo el término de "mud-mounds" en ambiente continental.

Volcanic geology of Carnley volcano, Auckland Islands

Abstract Camley Harbour, at the southem end of the Auckland Islands (l66°E, 50°51'S), is a dissected Miocene shield volcano formed on basement continental crust of the Campbell Plateau. Subaerial, marine, and glacial erosion excavated a section through ftows and pyroclastics of the volcanic pile to expose a swarm of dikes and sills which intersect the lowest lava flows, underlying Middle Miocene sediments and basement granite of Cretaceous age.A coarse-grained, crudely layered biotite gabbro intruding the volcanic succession is exposed at McClure Head and Circular Head and probably represents a partially unroofed magma chamber. Thick sequences of lava which vary from laterally extensive to thin discontinuous ftows are exposed on the slopes of Adams Island and on the F1eming Plateau. Basalt and hawaiite are predominant with minor evolved mugearites, benmoreites, trachyte, and rhyolites restricted to localised centres of eruption.All rocks are more or less porphyritic with phenocrysts of olivine, clinopyrox...

Regional Organic Geochemistry of Offshore Louisiana: ABSTRACT

Geochemical analyses of well cuttings from offshore Louisiana reveal trends inorganic matter characteristics that mimic thermal alteration effects, but are in fact related to depositional environments. Sediments deposited on the continental slope contain organic matter that is considerably higher in hydrogen content than organic matter from adjacent environments. These chemical variations are reflected in headspace composition of canned cuttings. Slope sediments, at or near the top of the oil window over a widespread area, are commonly encountered near the base of the penetrated section, leaving the greater part of the principal zone of oil generation unsampled. Active petroleum generation must occur deeper than 8,000-15,000 ft, depending on local geothermal gradient. Mod l calculations of thermal maturity, used in conjunction with burial history analysis, are consistent with our empirical observations of the maturation of organic matter based on vitrinite reflectance and geothermal gradient. In the absence of direct evidence from the source beds of Gulf of Mexico oils, detailed biomarker analysis can provide information on the nature and thermal history of the organic matter from which the oils were generated. Thermal maturity of the oils increases from west to east, indicating that source beds of eastern oils are older than those of western oils. Oils produced from the eastern offshore region were probably generated from early to middle Miocene sediments. The depocenter was located to the northwest, and deposition of oil-generating sediments enriched in marine organic matter occurred in deep water. Oils produced from the western offshore region were probably generated from Pliocene sediments. The depocenter was located close to where the oils are found today; hence, the ils are enriched in components derived from land plants deposited in oxygenated sediments. Oils produced from two locations close to the present-day shelf edge are significantly different from the other offshore oils. These were probably generated from middle to late Miocene sediments deposited in anoxic basins with high bacterial input. End_of_Article - Last_Page 1431------------