Thick-bedded Sandstone Research Articles

Стратиграфическое расчленение и корреляция палеогеновых отложений Северо-Восточной Болгарии

The formal lithostratigraphic units, grouped in the Frangen and Provadia Plateaus connected with the Lower Eocene subseries: Beloslav Formation with the Krivnen Member, can be traced in the Momino Plateau, in a part of the Rojak Plateau, and south of the Kamcija River around "Georgi Trajkov" dam. In the Kamčija Region only the Krivnen Member remains unchanged with respect to its lithologic characteristic. In the Beloslav Formation facial changes of its rock composition are observed, which are related to the increased role of the sandstones at the expense of the sands and in certain cases also at the expense of the decrease of the clayey component in the sandstones. Also the Upper Paleocene sediments of the Komarevo Formation can be traced in Northeast Bulgaria. ln this area the Avren Formation and the Ruslar Formation are widely spread. Slavejkovo Member (a new member) is distinguished in the upper part of the Avren Formation. This member consists of thick-bedded sandstones alternating with marls, which age may be determined, according to microforaminifer data reported in literature, as Middle(?) Late Eocene. The characteristic detritus limestones, lying over it, are set up in the Momino Formation, which belongs to Late Eocene. The Ruslar Formation includes in its lower parts all manganese bearing sediments. On the basis of microforaminifer data reported in literature the lower levels of the Ruslar Formation in the Frangen Plateau most probably belong to the Upper Eocene subseries. The other upper part of the formation belongs to Oligocene. Cuisian Nummulites and Assilines are grouped in a conglomerate-sandstone formation in the sediments outcropping in the defile of the Kozja Reka River. The biostratigraphic analysis of nummulite fauna, as well as microforaminifer data reported but interpreted from a modern stratigraphic point of view, make possible the correlation of the lithostratigraphic units and elucidate their space-time interrelations.

Palaeoenvironmental analysis of the Late Proterozoic Nagthat Formation, NW Kumaun Lesser Himalaya, India

The Late Proterozoic Nagthat Formation (∼ 750 m) of the Jaunsar Group exhibits extensive development of multistoried medium- to thick-bedded sandstone, impersistent conglomerate (pebbly beds) and siltstone-shale. It represents a shoreface to a proximal inner-shelf deposit, developed under the dominant influence of tidal and occasional storm conditions. The major lithofacies recognised are the following. (a) The coarse-grained siliciclastic facies (CGS), represented by discontinuous beds of coarse-grained sandstone with lenses of conglomerates. The important sedimentary features observed in this facies are medium- to large-scale trough, tabular cross-stratifications and crudely graded conglomerates etc. The coarse-grained sandstone is frequently punctuated by numerous broadly undulatory or locally more or less flat erosional surfaces. A proximal shoreface environmental set-up has been assigned to this facies. (b) The interbedded medium- to fine-grained siliciclastic facies (IMFS), consisting of decimetre- to metre-thick alternations of medium- to fine-grained subarkosic to sublithic arenite with subordinate quartz arenite and quartz wacke. The striking sedimentary features are the herringbone cross-stratification and rare hummocky cross-stratification (HCS). The rareness of the HCS suggests a frequent reworking of storm-laden sediments by fair-weather processes in the intermediate shoreface zone. (c) The fine-grained siliciclastic facies (FGS) represented by parallel to cross-laminated silty sandstone and shale, is interpreted as the distal shoreface to proximal inner-shelf deposit. The study of these facies reveals the three prominent associations, viz., fining upward CGS-IMFS-FGS and CGS-FGS, and coarsening upward FGS-IMFS. The first two facies associations have resulted mainly from tidal activity, whereas the last one provides evidence of storm activity. The overall coarsening upward stratigraphic sequence suggests gradual shallowing of the basin and thus indicates a prograding behaviour of the Nagthat Formation.

Waterflood Response of Reservoirs in an Estuarine Valley Fill: Upper Mannville G, U, and W Pools, Little Bow Field, Alberta, Canada (1)

Lower Cretaceous upper Mannville G, U, and W pools in Little Bow field are hosted by separate, parallel, elongate estuarine sandstone bodies within an incised valley fill. Each sandstone body is 3-4 km long, 300-500 m wide, and up to 22 m thick. G pool was discovered in 1972 and placed on primary production; oil production declined gradually and was accompanied by modestly increasing gas-to-oil ratios (GOR) and water-to-oil ratios (WOR). U and W pools were discovered in 1982 and 1983, respectively, and were produced by primary methods until initiation of waterflooding in 1985. Response to waterflooding these two pools has been a rise, then decline, in the GOR, followed by rapidly rising WOR, which is currently up to 10:1 in wells adjacent to water injectors. Production re ponse indicates control by mesoscale and microscale reservoir heterogeneities. Mesoscale heterogeneities include permeable sandstone beds several meters thick, that are continuous between adjacent wells, and stochastic shale beds up to 80 cm thick, which are not yet correlatable between wells. Rapid breakthrough of water occurred in producing wells adjacent to injectors due to channeling in thick permeable sandstone beds between shale beds. Microscale heterogeneities are principally mineralogically segregated laminae of cross-stratified sandstones. Permeability values from cores indicate variations of one order of magnitude among laminae. It appears that water passes preferentially through low-permeability sandstone laminae due to the higher mobility of water compared to oil.

Correlation Trends in Thin-Bedded Turbidites, Jackfork Sandstone, DeGray Lake, Arkansas

Michael B. De Vries and Arnold H. Bouma The methods of data collection for modern and ancient turbidite systems are necessarily different. Therefor, the integration of data sets from modern and ancient turbidite systems is desirable if lithofacies and surface morphological models are to be linked. To achieve this, two activities have to be dealt with: 1) Modern and subsurface turbidite systems must be extensively cored and described, and 2) outcrops in ancient turbidite systems must be correlated laterally in sufficient detail to establish a framework to facilitate the interpretation of subenvironments which build the turbidite system. The latter approach can be accomplished only in a few turbidite systems that are relatively well exposed, with outcrops exposing thick sections of turbidites which have an equally thick correlative. One ancient turbidite system meeting the above criteria is the Pennsylvanian Jackfork Group exposed around DeGray Lake, Arkansas. Two correlatable outcrops form the east and west wall of the Spillway, 214 feet (65 m) apart, while the third outcrop is 5500 feet (1675 m) to the west near the intake for the power-generating dam. Each outcrop was divided into lithofacies-based units and each unit was described in general terms and individual beds in each unit were measured. The lithofacies-based units established a rough correlation between large segments of each outcrop using changes in character. A more detailed assessment and quantification of these changes is then produced when dividing the units into smaller subunits. Stratigraphically thick sections of individual beds can often be correlated between Spillway walls. It is difficult to accomplish this using lithologic logs derived from measured sections. It is easier to plot the sandstone bed thickness values on a graph such as shown in Figure 1. Each successive bed is weighted the same on the vertical scale with the horizontal scale representing bed thickness. The thinner beds, represented by the left-deflecting spikes, show no characteristic pattern and do not necessarily follow the same trend as the thicker beds, represented by the right-deflecting spikes. The thicker beds can be divided into thickening-upward, thinning-upward and symmetrical successions of beds which may be called cycles. These cycles form distinctive patterns for each outcrop and unit. Pattern recognition can be used to correlate cycles from each outcrop similar to the correlation of subsurface well-logs. Individual beds which make up each cycle can then be correlated across the Spillway. This method allows for important lithofacies changes and thinning trends to be recognized. The lower one-third (approx. 140 feet [42 m] thick) of the Spillway section overlies massive channel deposits. The thinningand fining-upward nature of these beds and their position over channel deposits make these interpretable as a channel-levee-overbank succession. Paleocurrent direction for the channels in this part of the Jackfork Group is westerly. Detailed correlation of individual beds, using a plot similar to Figure 1, reveals an increasing degree of correlatability upward, from 40% in the lower part to over 80% in the upper part of the section. This increase may reflect more laterally extensive, sheet-type flow deposition. The second section is relatively thick-bedded and is sandwiched between very thin-bedded distal overbank deposits. This section is interpreted as a depositional lobe deposit. Mostly thickening-upward cycles can be recognized and used for correlation between Spillway walls. Individual bed and cycle thicknesses for each cycle show thinning trends across the Spillway which switch back and forth in direction, compatible to compensation cycles. A correlative section of turbidites, which were measured at the Intake, also shows thickening-upward trends. The overall thickness of this section is about 51 feet (16 m) thick while the Spillway sections are much thicker at about 80 feet (42 m) thick. Paleocurrent directions measured from sole markings show that this section is slightly oblique to directly down-current of the Spillway. Figure 1 shows good overall correlation between Spillway sections and the Intake section, but relatively poor bed-to-bed correlatability. The third section is 14 feet (4 m) thick and comprises thicker-bedded turbidites which are thinning-upward and are sandwiched between very thin-bedded basin plain or distal overbank deposits. This section is interpreted as a small channel deposit. The thicker beds typically split into several low angle amalgamations. A correlative section at the Intake is slightly thicker, but is thinner-bedded and shalier. As in all sections, the original correlation was based upon lithofacies. The thickness plot for this section shows that the beds included in the overbank deposits at the Spillway can be correlated with Louisiana State University, Department of Geology and Geophysics, Baton Rouge, Louisiana 70803

Textural and mineralogical properties of turbidite sandstone of Upper Miocene to Pliocene Tamugigawa Formation in the Niigata Basin.

The Upper Miocene to Pliocene Tamugigawa Formation, distributed in the western part of the Niigata Sedimentary Basin, Northern Fossa Magna, is confined trough-filled turbidite (TAKANO, 1990). Textural properties and mineral composition of this turbidite sandstone were investigated in this study. The textural analysis shows that the turbidite sandstone is composed of sand-sized and silt- to clay-sized populations. Both populations are clearly distinguished, and are normally distributed respectively. Accordingly the sandstone is very poorly sorted and very positively skewed. Although the vertical variation of grain size distribution throughout a thick-bedded sandstone bed is generally obscure, the upward increase of quantity of fine population sometimes results in the positive grading.The modal analysis shows that the sandstones are mainly lithic greywacke, and have similar composition in Q-F-R diagram. However, the detail investigation of minerals and rock fragments revealed that they are classified into the following two types: one including clastics from granite together with older sedimentary rocks and volcanic rocks, and the other which doesn't include clastics derived from granite. These sandstones were supplied from the Mikuni Mountains to the northwest. Early to middle Miocene volcanic rocks covering basement rocks in the Mikuni Mountains were firstly eroded and supplied, and then granitic rocks constituting the basement were supplied as the uplift of mountains advanced.

New Interpretations of Pennsylvanian and Permian Stratigraphy, San Juan Basin and Southeast Paradox Basin: ABSTRACT

The Honaker Trail, Paradox, and Pinkerton Trail Formations of the Hermosa Group are recognized throughout most of the San Juan basin. The Paradox Formation is extended southeastward beyond the limits of its evaporite facies into the basin, where it consists of thick shelf-carbonate rocks and thin black shale, sandstone, and siltstone interbeds. Where the Hermosa Group thins onto the marginal uplifts, the Paradox loses the thick carbonate rocks and becomes indistinguishable from the rest of the Hermosa. The Hermosa is correlated in the subsurface with the Madera and Sandia Formations to the southeast. The transitional Rico Formation, between the marine Hermosa Group and the continental Cutler Formation, is identified throughout the subsurface of the San Juan basin and is correlated with similar deposits out-cropping along the northern and eastern margins. The Cutler Formation includes the Organ Rock, Cedar Mesa, and Halgaito members throughout most of the basin. In the vicinity of the Hogback monocline, the Cedar Mesa Sandstone Member undergoes a gradational eastward facies change from cyclic evaporite and sandstone to thick-bedded sandstone. The subsurface Cedar Mesa is correlated in part with similar rocks in the outcropping Abo and Supai Formations.

Submarine Canyon, Slope, and Shelf Sedimentation in an Upper Eocene-Oligocene Progradational System (Limnos Island, North Aegean Sea, Greece): ABSTRACT

The only well-exposed outcrops of a post-Alpine late Eocene-Oligocene basin in the north Aegean Sea are at Limnos Island. These mostly consist of typical slope deposits overlain by remnants of shallow marine shelf and continental (braided-river) deposits. Three main slope lithofacies are distinguished. Canyon deposits consist of thick-bedded, massive, and pebbly sandstones (facies B), conglomerates (facies A), pebbly mudstone where the matrix shows flow (facies F), rock falls (nummulitic limestones), and zones of slump folds in sandstones (facies F). Channelized facies of massive sandstones, classical turbidites, and thin interbeds of sandstone and mudstone (facies E - probably overbank or levee deposits) are associated with canyon deposits. Pelagic and hemipelagic slope deposits consist of mudstone (facies G) and thin-bedded sandstone (facies D) with occasional small to medium-scale slump folds. These mud-rich slope deposits are incised by several channels filled with conglomerates, thick massive sandstones with well-developed dish structures, and turbidites interbedded with thin layers of mudstone (facies C). This unit composes a typical thinning and fining-upward sequence.

Seismic Reflection Character Analysis of Stratigraphic Traps in Cretaceous Cardium Formation, Alberta, Canada

The Cretaceous Cardium Formation, Alberta, Canada, produces oil and gas from thin sandstone stratigraphic traps. The formation can be subdivided into the Cardium sand and overlying Cardium zone. Over a 4,000 mi2 (10,000 km2) study area, the Cardium zone decreases in thickness from 80-120 ft (24-36 m) in the northwest to about 40 ft (12 m) in the east. Sandstone beds, some of which are stratigraphic traps, are interbedded with shale in the thicker northwest section. Sandstone beds are absent toward the east. The underlying Cardium sand increases in thickness from about 40 ft (12 m) in the west to 160 ft (48 m) in the east; this increase in thickness corresponds to an increase in shale and decrease in thick-bedded sandstones. An analysis of 97 seismic lines in the study area indicates that two major seismic reflection patterns comprise the Cardium Formation seismic interval. One pattern consists of two seismic reflections spaced 20-40 msec apart, and the other pattern consists of a single reflection. These two patterns can be mapped across a large area. The areal distribution of the double seismic reflection pattern corresponds approximately to the occurrence of sandstone beds in the Cardium zone. Reflection patterns of 26 synthetic seismograms generated from sonic logs are comparable to the reflection patterns observed on the field seismic data. This relationship allows interpretation of the field seismic records in terms of stratigraphic characteristics. Thickness of the Cardium zone and distribution of sandstone beds in the Cardium zone control the seismic reflection patterns. The double reflection pattern occurs where the Cardium zone is greater than 80 ft (24 m) thick and contains sandstone beds; one reflection is generated at the interface of the Cardium zone and an overlying shale, whereas the other reflection is generated at the interface of the Cardium zone and Cardium sand. The single reflection pattern occurs where the Cardium zone is less than 80 ft (24 m) thick and lacks sandstones. Because the seismic reflection patterns are regionally mappable units, seismic reflection character analysis can be used to highgrade areas for prospect generation and to depict the trend of potential stratigraphic traps. This study further demonstrates that sandstone stratigraphic traps and associated facies can be more accurately mapped using a combination of well logs and field seismic records than by using well control along.

Multiscale Anatomy of a Reservoir: Geological Characterization of Pembina-Cardium Pool, West-Central Alberta, Canada

Geological reservoir elements that control a reservoir's conduit system are mineralogy, grain size, packing, pore network, lithohydraulic units, lithologic continuity, fractures and faults, and rock-fluid interactions. All occur at different scales. We have focused on four scales typical of a developed reservoir to characterize geological reservoir heterogeneity in the Pembina-Cardium pool: (1) megascale, the field-wide view; (2) macroscale, the interwell continuity; (3) mesoscale, the neighborhood of the well; and (4) microscale, the domain of grains and pores. 1. Megascale observations. The reservoir top is characterized by a ridge-and-swale topography oriented northwest-southeast. A similar but more subdued topography is also characteristic of the unconformity between conglomerates and sandstones. Conglomerates are distributed irregularly and are generally shaly in beds thinner than 1 m. Thick conglomerates are common on the east and south flanks of underlying ridges and impart northwest-southeast fluid production trends and yield zones of reduced pressures. In thick conglomerates, high flow capacities are detrimental to hydrocarbon recovery as they result in bypassing of injection fluids. The conglomerate is a lithohydraulic unit as it is a body of strata distinct from neighboring strata by depositional, diagenetic, and tectonic imprintin s, and by a characteristic set of petrophysical properties. 2. Macroscale characteristics. Reservoir lithofacies and lithohydraulic units are laterally discontinuous and form irregular patterns that can possess an irregular northwest-southeast orientation. Fluid production trends oriented northeast-southwest in areas of thin, shaly conglomerates appear to be the product of fractures. In these areas, reorientation of injector wells to line drives results in higher oil recoveries. 3. Mesoscale properties. In cores, five lithofacies characterize the field. They are (1) dark-gray mudstone and siltstone, (2) bioturbated, thin and very thin-bedded shale, siltstone, and very fine and fine-grained sandstone, (3) thin and very thin-bedded shale, siltstone, and very fine and fine-grained sandstone, (4) medium to thick-bedded, very fine and fine-grained sandstone, and (5) conglomerate. As each lithofacies possesses contrasting, but partially overlapping petrophysical properties, they are lithohydraulic units. Lithofacies 1 acts as the reservoir seal and restricts flow within the reservoir. Lithofacies 2 has high porosities, low permeabilities, and dominates the bottom half of the reservoir. Lithofacies 2 has a high storage capacity (^phgrh), but its flow capacity (kh) i low. Lithofacies 3 reduces cross-flow because its vertical permeabilities are very low. Lithofacies 4 has modest permeabilities and high porosities, and is common to the top half of the reservoir. This lithofacies' flow capacity in T47, R7 W5M is 17 times greater than that of lithofacies 2. Lithofacies 5 has modest porosities, but very high permeabilities resulting in intervals with very high flow capacities. 4. Microscale investigations. In the sandstones, the mineral framework consists predominantly of quartz and chert whereas chert prevails in the conglomerates. These mineralogic differences lead to reduced cementation by silica on cherts and result in the preservation of pore spaces resembling original pores in chert-rich zones. Although quartz grains are modified by megaquartz overgrowths, chert particles are altered by pressure solution, fracturing, and crushing. In both sandstones and conglomerates, framework grains are cemented by siderite, calcite, kaolinite, and illite. Experiments with reservoir cores flooded with CO2-enriched brines and data from waterflooding indicate that siderite and calcite are unstable under reservoir conditions. This behavior can result in perm ability losses from release of insoluble fine materials and weakened responses during polymer floods. Mercury capillary-pressure data from 28 measurements of lithofacies 4 sandstones indicate that mercury End_Page 1233------------------------------ recovery efficiency increases with decreasing permeability and porosity. This behavior, if confirmed elsewhere in the field, may explain the performance of areas that have reduced permeabilities and porosities but are responding well to waterflooding.

Proximal to distal sedimentological variation in a linear turbidite trough: implications for the fan model

ABSTRACTDetermination of geographically dependent sedimentological variation (‘proximality’) in ancient flysch deposits formed an important part of early turbidite studies. Attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models. However, there are many turbidite formations, such as the Lower Cretaceous Cumberland Bay Formation (CBF) of South Georgia, which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes. The CBF is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back‐arc basin, principally by currents flowing WNW, parallel to the basin margin. Four lithofacies associations are recognized on the basis of sandstone/shale ratio. The two finergrained associations are constant in character across the CBF outcrop. In the coarse‐grained associations there is a change in character WNW, down the palaeocurrent direction. This is brought out by decreasing sandstone bed thickness and percentage amalgamation, but these changes are not always marked or consistent. In contrast, the internal character of the sandstone beds changes strongly, with a marked proportional increase in Tband Tcdivisions within the bed downcurrent. The evidence suggests that the system was aggradational rather than progradational: tectonic control of the basin margins prevented major migration of the depositional system, and most areas remained in the same position relative to source through time. Comparison of the CBF with other turbidite formations suggests two end‐member states which will produce radically different vertical sequences. Progradational systems will produce strong vertical facies changes, where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source, however there will be no lateral change in the character of any particular facies type. In contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical.

Upper Carboniferous Fine-Grained Turbiditic Sandstones from Southwest England: A Model for Growth in an Ancient, Delta-Fed Subsea Fan

ABSTRACT The Upper Carboniferous Crackington and Bude Formations of southwest England comprise laminated black mudstones, gray siltstones, thin-, medium- and thick-bedded sandstones, and contorted, silty sandstones. The thick-bedded sandstones only occur in the Bude Formation, which overlies the Crackington. The sandstones were deposited predominantly by turbidity currents in a relatively shallow basin. They are unusual for two reasons: 1) sediment coarser than fine sand is entirely absent; 2) unlike many previously described ancient-subsea-fan deposits, organized bundles of sandstones displaying asymmetric upward-thickening or upward-thinning sequences are no more common than symmetric sequences, and both types of organized bundles are much less common than disorganized (nonsequential) bundle , particularly in the Bude Formation. Sediment was supplied directly from a delta to the north, by resedimentation of delta-front sands, and slumping of upper pro-delta muds and silts to form a lower pro-delta turbidite fan. Fan growth is not readily explained by current models for single-sourced, tripartite subsea fans displaying radial growth. It developed, instead, a bipartite growth pattern, which was strongly influenced in its upper part by external variables, including grain size, distributary channel switching and other deltaic processes, active tectonism within the basin, and possibly sea-level fluctuations. The fan grew by down-slope construction of independent levee/channel complexes, each of which headed in the upper pro-delta and even possibly the delta front. Slumping and mobilization of argillaceous facies resulted in the random occurrence of contorted beds in all parts of the fan. Regionally extensive mudstones, known as key shales, within the fan deposits probably reflect large-scal , upslope, delta-lobe abandonment. The Crati and Ebro Fans of the Mediterranean Sea would appear to be good modern analogs for the Crackington-Bude fan system.

Conical fold terminations in the bannisdale slates of the english lake district

The structure of an area of Silurian rocks in the non-metamorphic Caledonides near Kendal, Cumbria (U.K.) is briefly described. The main folds are periclinal and die away in the southeast of the area. The uniform thickness of sandstone beds around folds shows that they deformed dominantly by buckling and only locally is flattening important. A model is developed to explain the conical attitude of bedding within the periclinal terminations. The model is erected on the basis of buckling alone and is developed to examine the effects of flattening. Inhomogeneity in buckling and flattening over the area is demonstrated using the model and discussed. The pattern of folding is compared to that predicted by earlier, published, experimental work.

Geology of northeastern Great Barrier Island (needles point to Rangiwhakaea bay), New Zealand

Abstract Indurated, complexly deformed greywacke basement on northern Great Barrier Island is intruded by numerous Miocene porphyry dikes. The basement consists mainly of thin-bedded sands tonel mud stone and thick-bedded sandstone lithofacies with minor conglomerate, chert and sedimentary breccia. Conglomerate contains a variety of acidic igneous pebbles, and sandstones are dominated by volcanic detritus. Sparse belemnites indicate a Late Jurassic (Puaroan) age. Quartz porphyry and dacite dikes are more common and older than hornblende-pyroxene and two-pyroxene andesite porphyries. The dikes are hydrothermally altered, contain very minor copper mineralisation, and were mostly intruded along steep N and shallow NW trending joints. They are comparable with the Kai-iti Porphyrites of northern Coromandel Peninsula and probably have a similar age, i.e. about Middle Miocene. The relationship of dikes to the andesitic volcanics (Coromandel Group) of central and southern Great Barrier is uncertain.

Recognition of a Thin Stratigraphic Trap by Seismic Reflection Character Analysis: ABSTRACT

The Cretaceous Cardium Formation, Alberta, Canada, which produces oil and gas from thin stratigraphic traps comprising coastal and End_Page 307------------------------------ offshore shelf sand-ridge deposits, appears as railroad tracks on seismic sections. Ninety-seven seismic lines were examined over a 10,000 km2 area. Here, the Cardium is divided into the Cardium Sand and the overlying Cardium Zone, both of which are 15-50 m thick. The Cardium Sand systematically grades eastward from (a) shoreface-strandplain massive sandstones to (b) inner-shelf sandstones encased in shale. The Cardium Zone grades eastward from (a) marginal marine/inner-shelf sandstones (< 10 m thick) encased in shale to (b) shelf shales. Two major reflection patterns characterize the Cardium Formation. One consists of two high-amplitude reflections spaced 20-30 m apart, and the other consists of a single reflection; further subdivision is possible on the basis of reflection amplitude. Areally, these patterns correlate with the regional distribution of sedimentary facies described above. Reflection patterns of 26 1-D seismic models generated from sonic logs correlate with those of the field seismic data thus allowing interpretation of the field data in terms of sedimentary facies. Thickness of the Cardium Zone and number and thickness of sandstone beds in the Zone were found to control seismic reflection patterns. The double reflection pattern occurs where the Cardium Zone is more than 24 m thick and contains shelf sandstone beds encased in shale. A single reflection, generated from the Cardium Sand, occurs where the zone is less than 24 m thick and lacks sandstones. These relationships can be used to detect and map potential sandstone reservoirs on seismic records. End_of_Article - Last_Page 308------------

Seastacks Buried Beneath Newly Reported Lower Miocene Sandstone, Northern Santa Barbara County, California: ABSTRACT

Three large, isolated exposures of a light-gray, coarse-grained, thick-bedded sandstone unit occur in the northern San Rafael Mountains of Santa Barbara County, California. These rocks are moderately fossiliferous and contain Vertipecten bowersi, Amussiopecten vanvlecki, Aequipecten andersoni, Otrea howelli, shark teeth, whale bones, and regular echinoid spines. The fossils indicate that the sandstone unit, although previously reported as upper(?) Miocene, correlates best with the lower Miocene Vaqueros Formation. This unit was deposited in angular unconformity on a Cretaceous, greenish-gray turbidite sequence of interbedded sandstone and shale, and onlaps the unconformity erosion surface from west to east, the unit being thicker in the west and older at its base. The underlying Cretaceous sandstone beds are well indurated, and during the eastward transgression of the early Miocene sea, they resisted wave erosion and stood as seastacks offshore of the advancing coastline, thus creating a very irregular topographic surface upon which the Vaqueros Formation was deposited. Some seastacks were as much as 4 m tall, as indicated by inliers of Cretaceous rock surrounded by 4-m thick sections of the Vaqueros Formation. End_of_Article - Last_Page 665------------

Stratigraphy and sedimentology of Tokomaru Formation (Late Miocene — Early Pliocene), eastern Raukumara Peninsula, New Zealand

Abstract Tokomaru Formation is a thick, extensive, Upper Miocene — Lower Pliocene (Kapitean-Opoitian), sheet-form, sand-dominated sequence within the East Coast Deformed Belt, Raukumara Peninsula, New Zealand. The rocks appear to be autochthonous, lying with small unconformity on mid — upper Tertiary allochthonous rocks in the east, and overstepping with large unconformity onto complex Cretaceous strata in the west. The formation is fine grained, but has at least 1 localised, basal conglomerate lens, some medium-grained calcarenites, and a tuff bed. From composite sections, the formation can be subdivided into a lower 100 m of homogeneous, thickbedded, fine-grained sandstone, followed by at least 200 m of alternating, frequency graded, very fine sandstones and coarse siltstones. The sands are largely volcanic derived, but only 1 definite tephra (rhyolitic in composition) was found. Foraminifera indicate upper to mid bathyal water depths, but transported inner shelf forms are also present. Locally abundant, mostly fragmentary macrofossils of low diversity appear to have been transported. Abundant trace fossils include Ophiomorpha, Zoophycos, Paleodictyon, Chondrites, and a simple V-shaped Tigillites-type burrow, which are consistent with an upper to mid bathyal environment. More than 1 sediment gravity-flow mechanism was responsible for sediment emplacement. The conglomerates were possibly emplaced by high-density turbulent flows with subsequent tractional reworking. The thick-bedded sandstones and alternating sequence were deposited by many turbidity currents. Deposition took place in the arc-trench gap of the Neogene New Zealand magmatic arc.

Upper Jurassic and Lower Cretaceous Sedimentary Sequence, Baltimore Canyon Trough, United States Atlantic Margin: ABSTRACT

The extent and character of Upper Jurassic and Lower Cretaceous rock units in the Baltimore Trough are revealed by geologic data from 29 exploratory wells. These data, released to the public in 1982, have been used previously to define regional rock-stratigraphic units. In this study, four detailed stratigraphic cross sections were constructed to show rock-unit correlations based on lithology and electric logs. Thin-section photo-micrographs document the mineralogic composition of these units. The stratigraphic terminology used herein is informally adopted from the nomenclature used on the Scotian Shelf. Most of the Upper Jurassic section consists primarily of Mic Mac gray shale and siltstone with minor amounts of very fine to medium-grained quartzarenite, red-brown shale, and lignite. This interval also End_Page 1923------------------------------ contains some anomalously thick sandstones and siltstones, which are generally limited to the north and have been tentatively assigned to the Mohawk(?) unit. These sandstones are mostly medium to coarse-grained, calcite-cemented quartzarenites. Upper Jurassic Abenaki limestone as much as 2,210 ft (675 m) thick was penetrated by most of the eastern wells. The limestone is mostly wackestone to grainstone, with varying amounts of oolites and fossils. Thick-bedded sandstones characterize the Lower Cretaceous Mississauga unit. These sandstones are mostly fine to medium-grained, calcite-cemented quartzarenites. The overlying Naskapi unit consists of calcareous shale. Thick sandstone beds dominate the uppermost Logan Canyon unit, which consists mostly of fine to coarse-grained, cal ite-cemented quartz arenite. End_of_Article - Last_Page 1924------------

Sedimentologic and Tectonic Factors Controlling Growth and Dimensions of Submarine Fans: ABSTRACT

The morphology, development, composition, and structure of submarine fans reflect five controlling factors. (1) Grain-size distribution--sandy debris results in radial fans, whereas mud-dominated debris promotes elongate fans. (2) Eustacy--during high sea-level stands, fan growth is curtailed as sediment, especially coarse sediment, is trapped in paralic settings, whereas during low stands, fan growth is accelerated owing to direct access of sediment to canyon heads and the flushing of debris from shelf areas. (3) Basin architecture--confined or unconfined. (4) Penecontemporaneous tectonics from seismic triggering to migrating source terranes. (5) Postdepositional tectonics that bias or alter lithofacies preservation--bias in sediment subduction results in the preferentia loss of distal facies. Some examples that demonstrate these five controlling factors include the following. (1a) The Navy and Arguello fans of the California continental borderland are sandy, radial fans, whereas (1b) the Bengal and Indus fans are muddy elongate fans. (2) The Eocene Farello fan, southern California borderland, is composed of thick intervals of sand and conglomerate lithofacies, corresponding to low sea level stands, and thin fine-grained intervals, corresponding to high sea level stands. (3) Relatively unconfined fans are most typical of passive or trailing continental margins, e.g., the Bengal, Amazon, and Mississippi fans, although even here diapir fields, seamounts, and fracture zones offer local impediments. In contrast, fans in confined basins are characteristic of convergent or transf rm continental margins, e.g., borderland, forearc, and backarc basins, although passive margins may also have confined basins. The Laurentian fan, for example, is blocked by the Kelvin Seamounts. (4) Large infrequent earthquakes (one to several per millenium) may trigger large depositional events, e.g., the 1929 Grand Banks shock. Shingling of depositional units occurs in pull-apart basins of strike-slip fault systems; such sequences may have stratigraphic thicknesses (not vertical accumulations) of 10 km/m.y. (33,000 ft/m.y.). Examples include the Neogene Ridge basin in California and the Cretaceous Izumi basin in Japan. Where a canyon head lies seaward of a transform fault, a large fan develops with multiple sources, e.g., the Monterey and Delgada fan systems. Where a canyon lies landw rd of a transform boundary, multiple small fans of one source terrane are strung out along a continental margin, e.g., the Baranoff fans of southeastern Alaska. (5) Inferred subduction complexes, e.g., the Torlesse of New Zealand, are composed principally of thousands of meters of tectonically thickened (thrust faulted) thick-bedded sandstone lithofacies. This composition may be due to decoupling of the more distal, fine-grained facies from the proximal facies and its transportation into the mantle along with the underlying oceanic crust. End_of_Article - Last_Page 488------------

Turbidite facies in an ancient subduction complex: Torlesse terrane, New Zealand

The Torlesse terrane of New Zealand is an ancient subduction complex consisting of deformed turbidite-facies rocks. These are mainly thick-bedded sandstone (facies B and C) with subordinate mudstone (facies D and E), comparable to inner- and middle-fan deposits of a submarine fan. Strata were deposited in trench-floor and trench-slope settings that received sandy sediment from slope-cutting submarine canyons. The dominance of sandstone suggests that some mudstone may have been selectively subducted. Construction of a detailed sediment dispersal model is not possible because tectonic deformation has largely destroyed original facies relationships and paleocurrent patterns.

Bioturbation Patterns in a Channel-Levee-Overbank Sequence of Paleocene Submarine-Canyon Fill, Point Lobos, California: ABSTRACT

Bioturbation patterns in the Paleocene submarine canyon fill (Carmelo Formation) at Point Lobos, California, differ for channel, levee, and overbank deposits. Variation in (1) such ichnoassemblage characteristics as taxonomic composition, diversity, abundance, and behavioral/preservational types, and (2) the overall degree of biogenic reworking of the sediment are particularly significant. The ichnoassemblage of the channel-levee-overbank sequence includes Arenicolites, ?Aulichnites, Chondrites, ?Helminthoida, ?Neonereites, Ophiomorpha, Scolicia, Thalassinoides, escape structures, and two unidentified traces. All the trace fossils were produced by infaunal organisms burrowing at various depths below the sediment-water interface. The channel deposits are characterized by relatively low diversity and density (in comparison with the overbank deposits), and mainly consist of traces of deep-burrowing animals (e.g., Ophiomorpha). The overbank deposits have a relatively diverse and dense ichnoassemblage produced by both deep- and shallow-burrowing animals. The levee deposits are similar to the overbank deposits in trace diversity, but are intermediate between the channel and overbank deposits with respect to their overall degree of bioturbation. Bioturbation patterns in these three subenvironments differ as a consequence of the chance of preservation of biogenic sedimentary structures. Each bed type (e.g., mudstone) contains similar ichnoassemblages regardless of the depositional subenvironment in which the bed type occurs. This pattern indicates that the distribution of the infaunal organisms producing the traces was influenced more by factors associated with a particular lithology (e.g., texture or organic content) than by environmental factors peculiar to a specific subenvironment . Therefore, the relative abundance of the various bed types ultimately preserved within each subenvironment corresponds to the bioturbation patterns characteristic of the channel-levee-overbank sequence. For example, physical sedimentary structures (e.g., Bouma a-b intervals in thick sandstone beds) and biogenic sedimentary structures (e.g., escape structures and truncation of traces) in the channel environment indicate frequent episodic events of extensive erosion, followed by rapid deposition of sand bed several tens of centimeters thick. Thus, traces produced by deep-burrowing organisms in thick sandstone beds are most frequently preserved. In the overbank deposits, an abundance of traces left by shallow-burrowing organisms (e.g., Arenicolites) in mudstone beds, the type of behavior (e.g., feeding burrows) represented by such traces, and the higher degree of bioturbation of all bed types indicate relatively slow, continuous deposition. Therefore, the change in conditions of sedimen ation (e.g., frequency of significant erosional/depositional events, amount and/or type of sediment eroded/deposited, ratio of erosion to deposition) results in the preservation of a different bioturbation pattern within the more tranquil overbank subenvironment. The bioturbation pattern characteristic of the levee deposits represents End_Page 483------------------------------ a transition between the bioturbation patterns of the channel and overbank deposits--that is, it results from a transition in the factors that dictate the preservation of biogenic sedimentary structures. End_of_Article - Last_Page 484------------