Browsing by Subject "Jurassic"
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Item Facies variability in deep water channel-to-lobe transition zone : Jurassic Los Molles Formation, Neuquen Basin, Argentina(2014-05) Tudor, Eugen Petrut; Steel, R. J.; Olariu, CornelThis study focuses on the facies changes from the lower slope to toe-of-slope to basin floor over a 10 km outcrop belt, in down-dip and oblique-strike directions to the basin margin. The Jurassic Los Molles Formation in Neuquen Basin, Argentina represents the slope and basin floor of basin margin clinoforms, coeval with the shallow water and fluvial deposits named Las Lajas and Challaco formations respectively. The shallow and deep water deposits are diachronously linked in an Early-Mid Jurassic source-to-sink system developed in a back-arc basin during the incipient development of the Andes Mountains. Satellite images, high resolution panorama pictures and measured sections were used to correlate and interpret the spatial variability and overall geometry of the base of slope to basin floor units. The observations of this study refine the model for the channel-to-lobe transition zone with increase recognition and quantification of facies and architecture variability. The Los Molles basin margin was coarse grained and was ideal to observe changes in the geometry and depositional facies of channel-to-lobe deposits from updip to downdip continuous over an 8 km outcrop belt. The described channel-to-lobe transition zone clearly shows a downdip change in bed boundaries from dominantly erosive to non-erosional (bypass) to depositional and with a range of distinct facies changes. In the transition zone the sand to shale ratio is high (N:G: 65-70 %), with gutter casts and deep scours, with a high degree of amalgamation, gravel lags, mud rip-up clasts and laterally migrating beds. Within the same depositional unit (deep water lobe), at the base of the slope, the dominant sandstone beds change from amalgamated structureless and normal graded sandstone beds in the channelized lobe axis to parallel laminated and normally graded in the channelized lobe off-axis areas. Similar facies changes have been observed along proximal to distal direction. The lateral change of the dominant structures in the beds indicates changes in the flow regime and depositional style.Item A fluid inclusion and cathodoluminescence approach to reconstruct fracture growth in the Triassic-Jurassic La Boca Formation, Northeastern Mexico(2011-12) Kaylor, Autumn Leigh; Laubach, Stephen E. (Stephen Ernest), 1955-; Eichhubl, Peter; Catlos, Elizabeth; Ketcham, RichardOpening-mode fracture shapes are typically the result of brittle deformation and proportional growth in fracture height, length, and width. Based on the typical fracture shape, it is assumed that fracture tips are free to propagate in all directions. Some natural rock fractures have been shown to form as a result of slow non-elastic deformation processes. Such fractures may propagate to a finite length or height and accommodate further growth by aperture widening only. To determine the growth conditions of a fracture in the Triassic-Jurassic La Boca Formation of northeastern Mexico and to test fracture growth models, I combined fluid inclusion microthermometry and SEM-based cathodoluminescence cement texture analysis to determine the relative timing of fracture cement precipitation and related fracture opening for five samples collected along its trace. Fracture growth initiated at a minimum age of 70 Ma as two separate fractures with branching fracture tips that coalesced to a single continuous fracture under prograde burial conditions at a minimum age of 54 Ma. At this stage, fracture growth was accommodated by both propagation (i.e. increase in trace length) and by an increase in aperture during maximum burial and early exhumation. Samples collected at the fracture tips recorded temperatures reflecting fracture opening starting with maximum burial at a minimum age of 48 Ma at one tip and of 38 Ma at the other tip. Synkinematic fluid inclusions in crack-seal cement track continued fracture opening close to the fracture tips without a concurrent increase in trace length after 38 Ma until about 21 Ma. I attribute the observed change in fracture growth mechanism to a change in material response. The stage in aperture increase without propagation corresponds to an increase in elastic compliance or in non-elastic flow properties. Non-elastic flow can be attributed to solution-precipitation creep of the host rock. Dissolution of host quartz grains and subsequent quartz precipitation is consistent with the abundance of quartz fracture cement formed during exhumation. Cement textures from fractures in the La Boca Formation mimic those found in subsurface core, which allows application of the results to a variety of geologic environments.Item New records of early Jurassic gladius-bearing coleoids (Prototeuthidina and Loligosepiina) from Alberta, Canada(2016-05) Marroquin, Selva Mariana; Martindale, Rowan C.; Bell, Christopher J; Sprinkle, James T; Molineux, Ann MThe Ya Ha Tinda Lagerstätte from Alberta, Canada, preserves organic-rich black shales that capture the carbon isotope excursion (CIE) characteristic of the Toarcian Oceanic Anoxic Event (183 Ma). Excavations at Ya Ha Tinda uncovered fifteen Vampyropoda fossils from Pliensbachian and Toarcian strata. Vampyropoda are a group of cephalopods with eight arms and an internalized chitinous shell called a gladius. Ya Ha Tinda represents the largest record of Vampyropoda gladii found outside of Europe for the Early Jurassic, and provides the unique opportunity to place Vampyropoda in a chemostratigraphic context. The goals of this thesis were threefold: (1) to taxonomically identify the Vampyropoda found at Ya Ha Tinda, including two specimens that were collected by Russell Hall from the same formation in the 1980s, (2) to compare the distribution and paleoecology of Vampyropoda to coeval European Lagerstätten, and (3) to conduct a detailed analysis of how Vampyropoda taxa were influenced by the Toarcian Oceanic Anoxic Event. In total, only fourteen specimens have sufficient morphological details preserved for taxonomic descriptions. Twelve specimens are identified to the genus level; six Paraplesioteuthis cf. sagittata, three Loligosepia cf. aalensis, one Geopeltis cf. simplex, one Parabelopeltis cf. flexuosa, and one Jeletzkyteuthis sp. The remaining two specimens belong to the suborder Loligosepiina, one to the family Geopeltididae. With the exception of Paraplesioteuthis, this is the first time all of these taxa have been found outside of Europe. The discovery of L. cf. aalensis within Pliensbachian strata pushes back the earliest occurrences of these taxa from the Early Toarcian to the Late Pliensbachian. Vampyropoda discovered at Ya Ha Tinda are nearly identical to those found in German deposits of the coeval Posidonia Shale, but, unlike European Lagerstätten, the fauna from Alberta has a relatively even proportion of Prototeuthidina and Loligosepiina. There are no taxa found within the CIE interval. The absence of taxa from this interval may be a result of taphonomy, sampling bias, or water chemistry. My results highlight several challenges associated with Vampyropoda taxonomy and provide paleoecological data of Early Jurassic specimens that are vital for understanding the taphonomic controls influencing the fragmentary Vampyropoda fossil record.Item The role of bacteria in the deposition and early diagenesis of the Posidonienschiefer, a Jurassic oil shale in southern Germany(1988-05) Hiebert, Franz Kunkel; Folk, Robert L.The Jurassic (Toarcian) Posidonienschiefer of southern Germany is famous for its well preserved vertebrate fossils and its high organic content. The majority of the Posidonienschiefer (10 meters thick in the study area) consists of the Bituminous Shale, a fossiliferous laminated illite claystone. Two thin (30-40 mm) clayey pyritic biomicrosparites, the Upper and Lower Schlacken, interrupt the Bituminous Shale. Geologists who have studied the Posidonienschiefer disagree about the exact nature of its depositional environment. The argument centers on the interpretation of an impoverished benthic fauna and whether or not the water column directly above the sediment-water interface was anoxic or normally oxygenated. Kauffman (1981) proposed that an algal/fungal mat located at or near the sediment/water interface marked the boundary between aerobic and anaerobic conditions during deposition. The purpose of my research was to investigate the geologic conditions during deposition and early diagenesis of the Bituminous Shale and the Schlacken and to search for evidence of microbial activity. A detailed petrologic investigation of these two lithologies found no evidence of an algal/fungal mat, but did reveal the important contribution of microbial activity in the formation of pyrite and calcite cement. The Bituminous Shale was deposited in a low-energy tropical seaway. The upper water-column supported a diverse marine fauna. The aerobic/anaerobic boundary in the water column may have been located several millimeters above the sediment/water interface. Pore waters of the ocean-floor mud were dysaerobic to anaerobic. Occasional oxygenation events allowed opportunistic benthic organisms to colonize the sea-floor. Compaction of the Bituminous Shale occured prior to cementation of original porosity. Framboidal pyrite was formed during sulfidic diagenesis under anaerobic, but open, sediment/pore water conditions. Euhedral pyrite formed later as communication between pores became restricted during sediment compaction. The skeletal grains of the Schlacken formed as a winnowed lag deposit of Bituminous Shale sediment. During the early stages of sulfidic diagenesis the winnowed beds were rapidly cemented in a concretion-like sheet. Early cementation preserved delicate algal spores and clay fabric. Fossil bacteria were discovered in the calcite cement of the Schlacken by modified petrographic techniques, and confirmed with the scanning electron microscope. Experiments in which live bacteria were gradually entrapped in halite produced a crystal fabric identical to that of the fossiliferous calcite cement of the Schlacken. The microbial production of bicarbonate and ammonia during sulfidic diagenesis played a significant role in altering local geochemical conditions in the Schlacken sediment and initiated the precipitation of calcite cements. Fossil bacteria in the cements of the Schlacken are direct evidence of the presence and entrapment of bacteria during cementation, but do not conclusively prove their active role in the formation of calcite.Item Sequence stratigraphic analysis of marginal marine sabkha facies : Entrada Sandstone, Four Corners region(2010-08) Makechnie, Glenn Kenneth; Kocurek, Gary; Mohrig, Davis; Steel, RonaldThe Middle Jurassic Entrada Sandstone of the Four Corners region, USA, is composed predominantly of very fine-grained, red, silty sandstone with poorly defined sedimentary structures. The origin of this facies is enigmatic, even though it is common both on the Colorado Plateau and globally, and is spatially situated between deposits recording unambiguous marine and aeolian environments. Eleven sections were measured along an 85 km transect from the Blanding Basin in southeastern Utah to the San Juan Basin in northwestern New Mexico. Outcrop and laboratory analyses distinguish eight facies: (1) silty shale, (2) shallow subaqueous reworked, fine- to medium-grained sandstone, (3) brecciated, very fine-grained sandstone, (4) crinkly laminated, very fine-grained sandstone with preserved wind ripples and abundant silty laminae, (5) weakly laminated, fine-grained sandstone with occasional silty laminae, (6) planar-laminated, fine-grained, wind-rippled sandstone, (7) cross-stratified, fine- to medium-grained aeolian cross-stratified sandstone, and (8) micritic limestone. Lateral and vertical relationships of these facies show a proximal to distal transition from cross-bedded wind-lain facies to loess-dominated sabkha facies with increasing abundance of water-lain facies basinward. The well known Todilto Limestone (facies 8) is situated stratigraphically below loess-dominated sabkha facies (facies 4 and 5) within the Entrada Sandstone, reinforcing previous interpretations that the unit represents a catastrophic flooding event and not a local groundwater flux.