George F. Hart

Origin and classification of organic matter in clastic systems

Abstract A classification of organic particles based upon their degrada‐tional state and biological origin is outlined for use in palynologi‐cal analysis. The classification has been used in various forms since 1969 and has proved useful in analyzing organic matter from clastic rocks ranging from the Devonian to the Modern. The organic particles are referred to as macerals and are divided into phytoclasts (plant derived), zooclasts (animal derived), protistoclasts (protistan derived), and scleratoclasts (fungal derived). In addition, the term receptorclasts is used to include precipitated organic matter. Phytoclasts are subdivided into the following categories based upon their level of decay: well preserved, no evidence of biodegradation; poorly preserved, minor biodegradation; infested, highly disrupted cell walls; amorphous structured, remnant cell structure only; amorphous non‐structured, blocky mass. In addition to the amorphous non‐structured phytoclasts there are the amorphous non‐structured protist...

Towards a predictive model for the palynological response to sea-level changes

A model of the predicted palynologic response to sea-level changes was formulated and tested in 3 different depositional areas. Both the subsurface Mancos Shale-Tocito Sandstone interval of New Mexico and the St. Stephens Quarry section in Alabama show a correspondence between systems tracts and the composition of the palynoflora. These intervals are not directly associated with deltaic sedimentation. The third area is a subsurface Wilcox section studied in southwestern Louisiana that lay on the distal edge of a major deltaic system

Organic matter variations in transgressive and regressive shales

Abstract Organic matter in the Upper Cretaceous Mancos Shale adjacent to the Tocito Sandstone in the San Juan Basin of New Mexico was characterized using organic petrology and organic geochemistry. Differences in the organic matter found in these regressive and transgressive offshore marine sediments have been documented and assessed within a sequence stratigraphic framework. The regressive Lower Mancos Shale below the Tocito Sandstone contains abundant well preserved phytoclasts and correspondingly low hydrogen indices. Total organic carbon values for the regressive shale are low. Sediments from the transgressive systems tract (Tocito Sandstone and overlying Upper Mancos Shale) contain less terrestrially derived organic matter, more amorphous non-structured protistoclasts, higher hydrogen indices and more total organic carbon. Advanced stages of degradation are characteristic of the phytoclasts found in the transgressive shale. Amorphous material in the transgressive shale fluoresces strongly while that found in the regressive shale is typically non-fluorescent. Data from pyrolysis-gas chromatography confirm these observations. These differences are apparently related to the contrasting depositional styles that were active on the shelf during regression and subsequent transgression. It is suggested that data from organic petrology and organic geochemistry provide greater resolution in sedimentologic and stratigraphic interpretations, particularly when working with basinward, fine-grained sediments. Petroleum source potential for the regressive Lower Mancos Shale below the Tocito Sandstone is poor. Based on abundant fluorescent amorphous material, high hydrogen indices, and high total organic carbon, the transgressive Upper Mancos Shale above the Tocito Sandstone possesses excellent source potential. This suggests that appreciable source potential can be found in offshore, fine-grained sediments of the transgressive systems tract below the condensed section and associated downlap surface. Organic petrology can be used to accurately predict petroleum source potential. The addition of simple fluorescence microscopy greatly enhances this predictive ability because non-generative amorphous material is generally non-fluorescent. Organic petrology must also be used to properly evaluate the utility of T max from programmed pyrolysis as a thermal maturity indicator. Organic matter dominated by autochthonous amorphous protistoclasts exhibits lower T max values than that which is composed of mostly phytoclasts.

Environmental Determinations Using Absolute Miospore Frequency, Mississippi River Delta

Samples of modern sediments were collected from the main depositional environments of the Mississippi River delta according to a predetermined sampling pattern designed to determine if: (a) differences could be detected among environments and (b) which environments showed differences. The data were analyzed using a nested analysis of variance test and orthogonal comparisons. Significant differences do exist but are not large enough to be clearly definitive of a particular environment.

Fluorescent microscopy of particulate organic matter: Sparta Formation and Wilcox Group, South Central Louisiana

Abstract Particulate organic matter (POM) isolated from core samples of mudstones in the Mid-Eocene Sparta Formation and the Late Paleocene-Early Eocene Wilcox Group in Louisiana was characterized using transmitted and fluorescent light microscopy and Rock-Eval pyrolysis. Observations of maceral fluorescence combined with programmed pyrolysis results better define source potential than visual examination using transmitted (white) light alone. Transmitted light microscopic characterization of POM indicates that amorphous macerals of mixed terrestrial and marine origin dominate samples from both the lagoonal and shelf depositional environments. Microscopic examination under fluorescent light indicates that some of the amorphous matter, including terrestrially derived material, fluoresces to some degree. Hydrogen Index values show a positive correlation with the total number of fluorescing particles in the isolate, as well as with the relative fluorescence levels. Comparison of Sparta and Wilcox samples indicates that significant changes occur in the assemblages at levels of thermal maturation associated with the onset of petroleum generation. Thermally mature samples contain a higher percentage of moderately fluorescent particles and lesser percentages of highly and weakly fluorescent particles. This phenomenon is readily apparent in specific particle types, namely amorphous nonstructured protistoclasts, miospores and dinoflagellate cysts.

Palynofacies of some Miocene sands from the Gulf of Mexico, offshore Louisiana, U.S.A.

Abstract Diverse palynomorph assemblages of late early Miocene age were obtained from 19 conventional core samples taken from the Robulus “L”; #2 and #5 sands of Texaco Well #6, located offshore Vermilion Parish, Louisiana. Approximately 60 dinocyst, 100 miospore and 17 fungal spore taxa were recovered. Seven palynofacies were defined in the sands. These palynofacies are characterized sedimentologically by lithofacies distribution and palynologically by dinocyst distribution. The depositional sequences of the Robulus “L” sands were the result of cyclic deposition influenced by high energy events.

Subdivision of Wilcox Group (Sabinian) argillaceous sediments using particulate organic matter

Abstract Maceral analysis of a thick, argillaceous dominated‐section from the Wilcox Group of western Louisiana revealed that these litho‐logically homogeneous sediments can be differentiated based on maceral spectra. Analysis of the data using quantitative univari‐ate and multivariate statistical techniques resulted in the definition of eight maceral facies. Comparison of the maceral facies with those from modern environments, coupled with a lithostratigraphic model, resulted in a refined paleoenvironmental interpretation: the sediments in the studied section were deposited on the Sabinian continental shelf to the upper continental slope environments and show evidence of deposition influenced by a fluvial plume associated with the western lobe of the Holly Springs delta complex and correlative Upper Wilcox delta complexes. Based on maceral facies, the stratigraphic section can be subdivided into an upper heterogeneous unit and a lower homogeneous unit. The upper unit reflects shallower water depths and m...

The biostratigraphy of Permian palynofloras

Abstract Classical paleobotanical theory separates the floras of the Permian Period into four distinctive provinces: (1) the Gondwanian; (2) the Angaran; (3) the Ca‐thaysian; and (4) the Euramerican. These floras are based on the stem and leaf remains which are the chief preservable macroscopic parts of plants. However, microscopic plant parts (miospores, megaspores, cuticles) can aid greatly in studying the development and differentiation of the Permian floras. Ideally, every type of plant remain should be considered when synthesizing an ancient flora. Palynology can aid materially in the study of the Permian paleobotanical provinces. Not only may it provide a good stratigraphic zonation at both the local and interregional level but it also allows the floral characteristics to be determined in sediments which contain no macroscopic plants. Already it is apparent from palynologic analysis that the Laurasian and Gondwanian land‐masses can be separated one from the other and that India is paleobotanically p...

MINERALOGY OF EGYPTIAN BENTONITIC CLAYS II: GEOLOGIC ORIGIN

Reconstructing the origin of bentonitic clays is often a challenging and rather complicated undertaking, but the analysis of certain predictor clay minerals is proving to be an excellent method to simplify this process. The goal of the present investigation was to use abundance changes of five X-ray diffraction (XRD) predictor minerals to determine the relative contributions of weathering and parent-rock changes to the origin of clay minerals in Egyptian bentonitic clays as the test case. The XRD predictor minerals, selected in an earlier discriminant function analysis of quantitative abundances of 14 minerals, provided a simpler approach to the interpretation of clay-mineral origins because they are the minerals that were most responsible for statistically significant differences among the samples. Changes in mineral composition were basically a function of parent-rock lithology, drainage, and climate interactions. A Paleo-Climate Index (CI; the ratio of coarsely crystalline kaolinite to Fe-rich smectite), and a Parent-Rock Index (PI; the ratio of the illitic phases and quartz abundances to pure smectite) were established to track the paleo-climate and parent-rock changes, respectively. Low CI values indicated that a long, seasonally dry climate prevailed during the Middle Eocene, uppermost Eocene, Lower Miocene, and Upper Pliocene bentonitic clay deposition. Lowermost Upper Eocene and the Middle Miocene bentonitic clays were produced when a wet climate prevailed throughout the year. Moderate to high PI values suggested derivation of the clays from the acidic basement crystalline rocks at Uweinat-Bir Safsaf uplift and Lower Paleogene shales during the Middle Eocene and lowermost Upper Eocene. The youngest Upper Eocene and Lower Miocene materials contained abundant Fe-smectite and low PIs indicating derivation from tholeiitic basalts. Diagenetic and sedimentary segregation modifications were not apparent. Direct evidence for in situ derivation from volcanic precursor materials was lacking in general, but volcanic eruptions were common in the region. The minerals in the Egyptian bentonitic clays formed as weathering products on land and have been transported by north-flowing streams and rivers to the sites of accumulation.

MINERALOGY OF EGYPTIAN BENTONITIC CLAYS I: DISCRIMINANT FUNCTION ANALYSIS

The purpose of the present investigation was to apply a discriminant function analysis (DFA) to quantitative mineralogical data from 124 Paleogene and Neogene bentonitic clays from the northern Western Desert of Egypt in order to establish an objective procedure for grouping the samples at three distinctly recognizable, but partially overlapping, levels of classification. These levels were province or geographic region, geologic age, and quarry. Quantitative mineralogical data were obtained by means of X-ray diffraction procedures employing least-squares fitting of simulated and standard mineral patterns with those from the laboratory. All data were transformed by a log-ratio procedure prior to the DFA. Fe-rich smectite (Feoct-1.4 a.p.f.u.), coarsely crystalline kaolinite, Fe-poor I-S (random with 60% S layers), quartz, and illite were the most important discriminator minerals. S-moderate I-S (random with 70% S), S-rich I-S (random with 80% S), two varieties of finely crystalline kaolinite, feldspar, and amorphous matter were also present. Calcite and gypsum were present in some samples. The median wt.% values for Fe-rich smectite, coarsely crystalline kaolinite, Fe-poor I-S, quartz, and illite in all samples were 16.6, 16.0, 15.2, 4.2, and 3.7, respectively. Abundances of quartz and feldspar have a good positive correlation, and finely crystalline kaolinite and Fe-rich smectite are negatively correlated. Other specific mineral associations are difficult to interpret visually because of the numbers of classes and variables employed in the investigation; however, DFA was successful in identifying statistically significant differences amongst the groups.At the province level, the back-classification of the samples was successful 92% of the time at the highest probability level, or 100% if the first plus second probability results were utilized. For samples of the same age, 80% of the first-choice assignments were correct and >90% were correct when the second choice was included. At the quarry level, the predictability rate ranged from 76 to >90%. Using both probability results, only seven of the samples were misclassified. In a blind test of quarry samples, the DFA assignment was 80% correct. These tests confirm the objective reliability of class assignments based on DFA. Results based on this data set can be used to classify new samples in future geologic interpretations and economic exploitation of the deposits in the region.