Jurassic Stratigraphy

The Callovian to Ryazanian (late Middle Jurassic to earliest Cretaceous; here referred to as 'Late Jurassic' sediments of the Dutch Central North Sea Graben, comprising the Central Grabeli and Scruff Groups, form several potential reservoirs. Source rocks are present within both these groups and the Carboniferous and Late Triassic-Early Juras-sic. The structural development of the area is illustrated by a number of cross-sections. A structural subdivision has been made into platform, intermediate platform, outer graben, and inner graben. The complex history of rifting, wrench-faulting, halokinesis, and structural inversion strongly influenced the palaeogeography and ensuing sandstone developments. A few selected burial-history plots are presented as examples of the relationship between the structural subdivision and the hydrocarbon potential of the area.

Strata of the Bardas Blancas Formation (lower Toarcian–lower Bajocian) are exposed in northern Neuquén Basin. Five sections have been studied in this work. Shoreface/delta front to offshore deposits predominate in four of the sections studied exhibiting a high abundance of hummocky cross-stratified, horizontally bedded and massive sandstones, as well as massive and laminated mudstones. Shell beds and trace fossils of the mixed Skolithos-Cruziana ichnofacies appear in sandstone beds, being related with storm event deposition. Gravel deposits are frequent in only one of these sections, with planar cross-stratified, normal graded and massive orthoconglomerates characterizing fan deltas interstratified with shoreface facies. A fifth outcrop exhibiting planar cross-stratified orthoconglomerates, pebbly sandstones with low-angle stratification and laminated mudstones have been interpreted as fluvial channel deposits and overbank facies. The analysis of the vertical distribution of facies and the recognition of stratigraphic surfaces in two sections in Río Potimalal area let recognized four transgressive–regressive sequences. Forced regressive events are recognized in the regressive intervals. Comparison of vertical distribution of facies also shows differences in thickness in the lower interval among the sections studied. This would be related to variations in accommodation space by previous half-graben structures. The succession shows a retrogradational arrangement of facies related with a widespread transgressive period. Lateral variation of facies let recognize the deepening of the basin through the southwest.

The Late Jurassic (157–150 Ma) Morrison Formation of the Western Interior of the United States contains abundant altered volcanic ash. On the Colorado Plateau, this formation accumulated behind and downwind of a subduction-related volcanic arc along the western margin of North America. The ash in these distal fallout tuffs probably drifted eastward from coignimbrite ash clouds related to collapse calderas. Altered volcanic ash is particularly abundant in the Brushy Basin Member of the upper part of the Morrison Formation. In one 110-m-thick section in eastern Utah, 35 separate beds were deposited in a 2.2 m.y. period. Alteration occurred when glassy volcanic ash fell into fluvial and lacustrine environments, where it was diagenetically altered to various mineral assemblages but most commonly to smectitic clay. Periodically, ash fell into saline, alkaline lakes, and diagenetic alteration of the glassy ash produced a crudely zoned deposit on the Colorado Plateau. Altered volcanic ash beds in the outermost part of the lacustrine deposits are argillic (with smectitic clay), whereas zeolitic (clinoptilolite, analcime) and feldspathic (K-feldspar and albite) alteration dominates the interior zones. Feldspathic ash layers contain secondary silica, and consequently immobile element (e.g., Al, Ti, and high field strength ­elements) abundances were strongly diluted in these rocks. In contrast, the argillic ash beds experienced strong SiO2 depletion, and, as a result, they are enriched in the relatively immobile elements. The compositions of the zeolitic ash beds are intermediate between these two extremes and experienced the least alteration. As a result of these changes, immobile element concentrations are less reliable than ratios for determining the original magmatic composition of the ash. Most of the altered ash (regardless of type) was also depleted in water-soluble elements like the alkalies, U, and V. The latter two elements were oxidized during diagenesis of the ash, became soluble, and were partially leached away by groundwater. Locally, U and V in groundwater were reduced upon contact with organic materials and formed important ore deposits.

Several aspects of the mineralogy and geochemistry of the altered volcanic ash beds yield information about their original magmatic compositions. The volcanic ash beds typically have small phenoclasts of quartz, sanidine, plagioclase, biotite, zircon, apatite, and Fe-Ti oxides. Titanite is present in ∼40% of the ash beds; pyroxene and amphibole were found in less than 5%. Phenocryst assemblages, mineral compositions, inferred high ƒO2, rare earth element patterns, and immobile element ratios all suggest the parent magmas for the altered tuffs were subduction-related dacites and rhyolites. Small numbers of tuffs have Fe-rich biotite, amphibole, and/or clinopyroxene; both pyroxene and amphibole are alkali rich. These tuffs lack titanite, but some contain anorthoclase and F-rich apatite. Combined with enrichments in Nb and Y, these features show some tuffs had an A-type character and were related to some type of within-arc extension.

Paleowind directions, and distribution, radiometric ages, and compositions of the volcanic ash beds and of plutons in the western United States suggest that the most likely eruption sites were in the subduction-related Jurassic magmatic arc, which extended across western Utah and central Nevada and southward into the Mojave of California and southern Arizona (present-day coordinates). Pb isotopic compositions show that at least some of the ash was erupted from magma systems (now exposed as plutons) in the Mojave Desert.

We conclude that a brief ignimbrite flare-up from 157 to 150 Ma, but focused on the time period from 152 to 150 Ma, in this region may have been driven by slab steepening and conversion to a strike-slip boundary after a preceding phase of folding and thrusting. The presence of ash beds with A-type characteristics mixed with those that have more typical subduction signatures confirms that the Late Jurassic was geologically a transitional time in North America when subduction was changing to transtensional movement along the western plate boundary.

The outcrop belt of the Upper Jurassic Morrison Formation in the northeastern Uinta Basin and southeastern flank of the Uinta Mountains is particularly rich in dinosaurian and non-dinosaurian faunas, as well as in fossil plants. The discovery of several well-preserved, relatively intact,
fossil logs at several locations in Rainbow Draw and one location in Miners Draw, both near Dinosaur National Monument (Utah), has provided an opportunity to study the local paleobotany, stratigraphy, and sedimentology of the Morrison Formation in northeastern Utah. The Morrison  Formation in northeastern Utah consists of four members. In ascending chronostratigraphic order, they are the Windy Hill, Tidwell, Salt Wash, and Brushy Basin Members. The lithology (including the presence of glauconite grains) and fossil assemblage of the lower two members (Windy Hill and Tidwell) indicate a marine to marginal marine (coastal plain) depositional environment,  whereas the lithology, fossil flora and fauna assemblage of the upper two members (Salt Wash and Brushy
Basin) indicate a fluvial–lacustrine depositional environment.
At least 10 fossil log sites in Rainbow Draw have been documented so far, and geologic mapping indicates that the logs and wood all occur in the same stratigraphic interval within the Salt Wash Member, approximately 17 to 27 m above the base of the member. The unit containing the logs and wood is about 11 m thick and consists of very fine to fine-grained sandstone and siltstone with indistinct bedding and no discernible sedimentary features. The logs are siliceous, some have a
coaly exterior, and they range in exposed length from 0.5 to 11 m and reach diameters up to 1.1 m. In the Miners Draw area, a single siliceous log is documented in the upper part of the Salt Wash Member within a silty sandstone unit that is 4 m thick; its exposed length is about 6 m. Although the correlation of the Miners Draw log-bearing interval to the interval in Rainbow Draw is uncertain, both units are lithologically similar and both occur in the upper part of the Salt Wash Member. The logs have been identified as araucariaceous conifers that pertain to the same taxon originally described as "Araucarioxylon hoodii" Tidwell et Medlyn 1993 from Mt. Ellen in the Henry Mountains of southern Utah. Concurrent systematic work will prompt a nomenclatural transfer of this
species to the genus "Agathoxylon."
Based on the abundance of large fossil logs and wood in the same stratigraphic interval in Rainbow Draw, we hypothesize that the area was covered by stands of moderately large trees of araucariaceous conifers. The sedimentological evidence suggests that the trees were not transported far from their original site of growth before they were deposited in a low-energy floodplain environment.

The Henry Mountains basin is outlined by Middle Jurassic formations to form a southward pointing, triangular basin. An interval of mostly eolian sandstone and some intercalated non-eolian beds lies between the Navajo Sandstone (below) and Carmel Formation (above). These beds were known as the Harris Wash Tongue of the Page Sandstone but are redefined here as Temple Cap Formation because their stratigraphic position and isotopic age data indicate that they are time equivalent to the
Temple Cap Formation of southwestern Utah. The Temple Cap Formation is preserved throughout most of the basin but is missing in some areas, especially along the Waterpocket Fold. Its irregular
thickness is likely because of deposition on a paleotopographic surface developed by the J-1 unconformity on the Navajo Sandstone. West of the Henry Mountains basin was a significant paleohigh over which the Temple Cap Formation is missing.
The Carmel Formation in the Henry Mountains basin conformably overlies the Temple Cap Formation and includes (in ascending order) the Judd Hollow, Crystal Creek, Paria River, and Winsor
Members. The lower two members comprise the first marine transgressive-regressive cycle of the Sundance sea that flooded into Utah from the north. The upper two members comprise the second marine transgressive-regressive cycle. Each member is recognizable throughout the basin; however, they become sandier in the northwestern part of the basin because of an influx of sand that dominated deposition from north-central Arizona across the Kaiparowits basin and into the northwest corner of the Henry Mountains basin. This is particularly evident in the first marine transgressive-regressive cycle in which the typically fine-grained red beds (sandstone and siltstone) of the Crystal Creek Member
are sharply replaced with mostly eolian sandstone. In the areas where eolian sandstone dominates, the name Thousand Pockets Member is used. The Judd Hollow Member is an important and recognizable marker unit (even in the areas of sand-dominated deposition) that provides the means to separate the eolian beds of the redefined Temple Cap Formation from the Thousand Pockets Member. In those areas, we reassign the Thousand Pockets Tongue of the Page Sandstone to the Thousand Pockets Member of the Carmel Formation.
We recommend that Page Sandstone be restricted to Arizona where the Judd Hollow Member thins to a feather edge or is missing, making it difficult to separate the Temple Cap Formation from the Thousand Pockets Member. Finally, we recommend abandoning the use of Gypsum Spring Formation south of the Uinta arch, and instead placing these rocks in the Temple Cap Formation.

This progress report summarizes our work on most Middle Jurassic strata (Gypsum Spring, Temple Cap, Twin Creek, Arapien, and Carmel Formations) in the Utah sector of the Sevier fold and thrust belt. Throughout Utah and surrounding states, we have measured more than 150 sections, interpreted about 165 well logs, obtained about 50 new radiometric ages, and identified characteristic Bajocian, Bathonian, and Callovian palynomorph assemblages.

Here we assess the evidence for the placing of magnetic and fossil biozonal boundaries in Upper Tithonian to Lower Berriasian (Jurassic–Cretaceous boundary) sedimentary rocks on the Black Sea coast south of Theodosia (Ukraine): that is, in magnetozones M19n to M17r.

We consider our earlier-published results from these sections in relation to the correlative pattern that has become well established further west in Tethys. Additionally, this is compared and contrasted with other, alternative, results from Crimea that have been published in recent times.

Paleosols in the Morrison Formation (Upper Jurassic) from the Western Interior and Colorado Plateau regions occur in fluvial/overbank and marginal-lacustrine depositional facies associated with aggradational settings, and at sequence-bounding unconformities that mark divisions between major aggradational and degradational successions. Pedogenic features within these horizons preserve important contextual information about the local and regional paleoclimate and paleoenvironment in which the soils formed. Floodplain and lake-margin paleosols show evidence that most of the Morrison basin was characterized by a semi-arid to tropical wet -dry paleoclimate with fluctuating groundwater conditions, a low precipitation to evaporation ratio, and weak to moderately seasonal precipitation. Paleosol ichnofauna show evidence of a diverse and opportunistic flora and fauna that exploited changing conditions and existing nutrient and moisture regimes. Changes in paleosol type and degree of development over the basin indicate the overall regional paleoclimate was drier in the western and southern portions of the basin. Vertical trends indicate paleoclimatic conditions over the basin became steadily more humid through time. Laterally continuous, well-developed, deeply weathered paleosols formed during times of little or no deposition and mark regional unconformities. The paleosols at these sequence-bounding unconformities serve as useful regional stratigraphic markers to trace genetic packages across the Morrison depositional basin and to determine regional accommodation trends. D

Sediments of Kimmeridgian and Tithonian age are well exposed on the Boulonnais coast of northern France between Equihen and Cap Gris Nez and on the south coast of England at and adjacent to Kimmeridge Bay. Both successions were deposited on a marine shelf and lie within the Subboreal faunal province which enables detailed correlations to be made between them based on ammonite assemblages. They are, however, lithologically markedly different due to their environmental settings: close to a land area in the case of the Boulonnais and
within a depositional basin in the case of Kimmeridge. The succession adjacent to the Kimmeridgian-Tithonian boundary exposed in the Boulonnais is highly condensed and laterally variable with more attenuated successions occurring close to the former Anglo-
Brabant Massif land area. The boundary occurs at the end of a succession of up to six regressive-transgressive events that onlap the land area. This is in contrast to that at outcrop at Kimmeridge, where the Kimmeridgian-Tithonian boundary is marked by a correlative
conformity in an unbroken basinal succession. The cliff and foreshore exposures in the Kimmeridge area provide the only unbroken succession in the Subboreal faunal province of the beds adjacent to the Kimmeridgian-Tithonian boundary.

The Morrison Formation in Utah and Colorado contams many volcanic ash layers and ashy beds, now altered rnostly to bentonite, that have yielded isotopic ages. The Brushy Basin Member. at the top of the formation, gives single-crystal. laser-fusion and step-heating, plateau 4 0~r / 3 '~r ages on sanidine that range systematically between 148.1 5 0 . 5 (1 std. error of mean) at the top of the nlember to 150.3 rt 0.3 Ma near the bottom. The Tidwell Member, at the base of the Morrison Formation, contaills one ash bed about 3 m above the J-5 unconforlnity that occurs in at least two widely separated sections. This ash has been dated by "~r / '~A r dating of saliidi~le and gives ages of 154.75 rt 0.54 Ma (Deino NTM sample, laserfusion), 154.82 31 0.58 Ma (Demo RAIN sample, laser-fusion), 154.87 ! C 0.52 (Kunk NTM sample, plateau). and 154.8 ! C