Upper Part Of Sequence Research Articles

Sequence stratigraphy and paleoenvironmental significance of the Neoproterozoic Bambui Group, Central Brazil

The Bambui Group is a ca. 2,000 m-thick mixed biochemical-siliciclastic sedimentary succession that accumulated in the Sao Francisco Basin, central Brazil. Its age is contentious, but deposition during the Cryogenian-Ediacaran (ca. 635–550 Ma) is consistent with a new sequence stratigraphic model presented herein and previously published geochemical and structural data. Six stratigraphic sequences are recognized that record depositional evolution from an epeiric sea to foreland basin during the Marinoan snowball glaciation (ca. 635 Ma) to shortly after the Gaskiers (ca. 580 Ma) glaciation. Although this stratigraphic architecture differs from the classic lithostratigraphic and more recent chemostratigraphic and sequence stratigraphic interpretations of the Bambui Group, it is supported by correlation to other Cryogenian-Ediacaran successions globally. Collectively, these six sequences preserve a robust record of seawater redox structure leading up to the oxygenation threshold that triggered the evolution of multicellular animals in the Ediacaran.Sequence 1 is composed of Marinoan glacial deposits and an overlying cap carbonate with aragonite fan pseudomorphs and barite. Sequence 2 is composed of lime-mudstone that changes laterally into loess-rich peritidal phosphorites, suggesting oxygen stratification in shallow paleoenvironments. Sequence 3 is a carbonate succession with abundant stromatolites implying widespread oxygen production. The top of Sequence 3 is marked by a regional subaerial unconformity developed during uplift associated with the onset of the Brasiliano-Pan African Orogeny. Sequence 4 signals a return to loess derived sedimentation. Distal deposits contain abundant organic matter with framboidal pyrite reflecting bacterial sulfate reduction in low sulfate Ediacaran seawater. Sequence 5 is composed of well-developed aggradational parasequences composed of alternating deep subtidal organic-rich siltstones and shallow-water carbonate grainstones. Their well-defined cyclicity is interpreted to record extreme sea level fluctuations during the onset of the Gaskiers glaciation. Stromatolites suggest an oxygen-stratified water column. Sequence 6 marks the change from an epeiric sea to a foreland basin. Interbedded basinal glauconitic siltstone and shale loess-rich are correlative with shoreface sandstones. Authigenic glauconite requires suboxic bottom waters to precipitate and thus reflects oxygenation of distal settings. Most of the deposition ended by ca. 560–550 Ma when the entire Bambui Group was deformed during the assembly of West Gondwana.In sequences 1, 2, 3, 4, 5, and the lower portion of 6, the lateral continuity of lithofacies, predictable layer-cake stratigraphy, and lack of fluvial coarse siliciclastic deposits are characteristics of epeiric sea successions. The appearance of immature sandstone, poorly sorted conglomerate, braided fluvial deposits, and synsedimentary deformation in the upper part of Sequence 6 are interpreted as the onset of foreland basin development. Stacking relationships of redox sensitive, authigenic lithofacies in both the epeiric sea and foreland phases support recent research suggesting seawater redox conditions were in a state of flux during the apex of the purported Neoproterozoic Oxygenation Event. Organic matter and authigenic pyrite in Sequences 4 and 5 reflect increased primary productivity in surface waters fertilized by nutrients such as P and Fe adsorbed on loess particles. This mechanism could be expanded to a global scale and have contributed to the increase in atmospheric and oceanic oxygen levels. These findings redefine and highlight the significance of the Bambui Group for Neoproterozoic Earth history.

Effects of sea level and upwelling on development of a Miocene shallow-water tropical carbonate ramp system, Ponce, Puerto Rico

ABSTRACT A Miocene (Langhian–Tortonian, ca. 15–10 Ma) tropical ramp system exposed in southern Puerto Rico is characterized by shallow-water facies consisting of heterozoans, red algae, large benthic foraminifera (LBF), and corals, which occur as isolated corals, segment- and cluster-type reefs, and reworked accumulations. Photozoan association components are limited to corals (Montastraea, Porites, Goniopora, and Agaricia) and LBF (amphisteginids, soritids, gypsinids, miliolids) that have been documented to tolerate elevated nutrients, turbidity, and cooler water conditions. Similar shallow-water carbonate systems are found throughout the Caribbean, and this regional development is thought to have resulted from the well-documented upwelling in the Caribbean during the Miocene. Sea-level fluctuations also exerted a major control on facies distributions and shifts in the Puerto Rico ramp, including a vertical facies pattern that occurs in each of three sequences. Basal parts of sequences, deposited during sea-level rises, are dominantly composed of mollusks, echinoderms, red algae, LBF, bryozoans, and solitary corals that formed in low-energy seagrass-bed environments with local associated higher-energy shoal environments. Coral facies occur only in upper parts of sequences and formed in shallow-water, low- to high-energy environments closely associated with seagrass beds during late highstands and sea-level falls. A similar vertical facies pattern occurs in time-equivalent sequences elsewhere around the Caribbean. Strontium-isotope age data indicate two sequence boundaries reflecting sea-level falls formed at about 12.3 Ma and 11.1 Ma. Correlation with time-equivalent unconformities in other well-dated areas in the Caribbean and to sea-level lows on eustatic curves suggests a global signature for sequence development. The connection between the Caribbean and the Pacific along the Central American Seaway (CAS), impacted by local tectonic episodes and sea-level fluctuations during the Miocene, affected nutrient influx and upwelling in the Caribbean, which may be reflected in the vertical facies pattern in shallow-water carbonate sequences. Times of restricted connection during sea-level falls and lows resulted in reduced nutrients and upwelling, which may have been more conducive to coral development. Time-equivalent tropical carbonate systems in the Mediterranean and Indo-Pacific show similarities to those in the Caribbean, indicating influence of global processes (cooling, temperature gradients, oceanographic circulation). Differences between areas indicates the importance of local and regional controls, which in the Caribbean was dominantly the opening and closure of the CAS.

Astronomically forced sequence development of terrestrial massive sand-sheet, inland sabkha and palustrine units, Lower Miocene Hadrukh Formation, eastern Saudi Arabia

The Lower Miocene (lower Aquitanian) Hadrukh Formation (30–65 m thick), Al-Lidam area, Eastern Province, Saudi Arabia, was cored to examine sequence development within a non-marine siliciclastic-prone coastal plain in a semi-arid distal foreland setting. Facies are dominated by structureless quartz sands (terrestrial sand-sheets/shrub-coppice dunes or nebkhas), along with gypsiferous/calcareous quartz sands and gypsites (inland sabkhas), brecciated and sheet-cracked sandy carbonates/calcareous sands (palustrine), and some coarse lithic-clast quartz sands (fluvial) and associated paleosol-capped red and green-gray mudrock (interfluves). However, they lack the intercalated eolian laminated and cross-bedded sands of this hyper-arid region today, reflecting semi-arid Early Miocene conditions.Spectral analysis of core gamma-ray logs shows astronomical forcing within the eccentricity, obliquity and precessional bands. Tuning indicates the Hadrukh Formation is a composite sequence (~1.3 myr duration) containing three (400 kyr) major sequences and eight minor sequences. Fluvial facies are locally developed at the base and near the top of the composite sequence. Massive sand sheet facies dominate the sequences, and sandy evaporitic facies and palustrine sandy carbonates are best developed in upper parts of sequences but interfinger within short distances with other facies. Subsidence and differential warping caused thickness and facies variation but the succession shows a strong astronomical signal. Astronomical forcing controlled semi-arid vs. more humid phases of deposition, as well as the position of base level, elevation of the ground-water table and presumably density of shrub cover. The three major sequence boundaries developed during 400 kyr eccentricity minima and cooling, while minor sequence boundaries formed during short eccentricity (100 kyr) cooling events.

Cyclic carbonate–siliciclastic sedimentation in a shallow marine to coastal environment (latest Kimmeridgian–early Tithonian, Galve sub-basin, Spain)

The sedimentological analysis of the uppermost Kimmeridgian to lower Tithonian Villar del Arzobispo Formation (Galve sub-basin, NE Spain) has allowed characterizing different carbonate, mixed and siliciclastic facies deposited in coastal to shallow marine areas. Its age calibration has been specified on the basis of new biostratigraphic and chemostratigraphic data. Four successive sequences (30–80 m thick) characterized by the relative dominance of carbonate and siliciclastic facies reflect a sedimentary change from reciprocal to mixed carbonate–siliciclastic sedimentation. Sequences 1 and 2 are reciprocal sedimentation cycles from carbonate-dominated (muddy coastal plain-carbonate lagoon) to siliciclastic coasts (muddy coastal plain-wave dominated delta). Sequence S3 is also reciprocal, from a mixed carbonate–siliciclastic coastal plain-lagoon at the lower part to a wave-dominated delta on top. A mixed carbonate–siliciclastic coastal plain-lagoon characterizes the entire sequence 4. Climate and tectonics controlled carbonate deposition and siliciclastic supply. Reciprocal sedimentation in sequences 1–3 was controlled by orbitally-driven climate changes (from arid to humid conditions) on the age range of long-term eccentricity cycles. A greater imprint of extensional synsedimentary tectonics at the upper part of sequence 3 and during sequence 4 is thought to be the main factor producing the gradual opening of the sub-basin to the wave influence and a more continuous siliciclastic supply, then controlling the change from reciprocal to mixed sedimentation and obliterating the possible imprint of orbitally-induced climatic changes.

Genesis of the Northern Adriatic Sea (Northern Italy) since early Pliocene

The Northern Adriatic Sea is a shallow and very flat shelf area located between the northern Apennines, the southern Alps and the Dinarides; its present day physiography is the result of the filling of a relatively deep Quaternary foredeep basin, developed due to the northeastward migration of the Apennine chain. Multichannel seismic profiles and well data have allowed documenting the stratigraphic architecture, the depositional systems and the physiographic evolution of the Northern Adriatic sea since early Pliocene time. In particular, three main depositional sequences bounded by regional unconformities were recognized. The Zanclean Sequence 1 documents first the drowning of late Messinian incised valleys and then the southward progradation of a shelf-slope system, which is inferred to be related to a tectonic phase of the Apenninic front. The Piacenzian-Gelasian Sequence 2 records a relatively rapid transgressive episode followed by minor southward progradation; the top of the sequence is associated with a major late Gelasian drowning event, linked to the NE-ward migration of the Apennine foredeep. The Calabrian to upper Pleistocene Sequence 3 testifies the infilling of accommodation previously created by the late Gelasian drowning event, and it initially accumulated in deep-water settings and then in shallow-water to continental settings. The upper part of Sequence 3, consisting of the paleo-Po deltaic system, is composed of seven high-frequency sequences inferred to record late Quaternary glacio-eustatic changes. These high-frequency sequences document the stepwise filling of the remaining accommodation, resulting in the development of the modern shelf.

Sequence Stratigraphy of the Upper Cretaceous Sego Sandstone Member Reveals Spatio-Temporal Changes In Depositional Processes, Northwest Colorado, U.S.A

Abstract The Upper Cretaceous Sego Sandstone Member of the Mesaverde Group has been extensively studied in the Book Cliffs area of Utah and Colorado, and has been the focus of stratigraphic reconstruction aimed at developing an understanding of the evolution of the Western Cretaceous Interior Seaway. The Sego Sandstone Member was deposited in a marginal marine, tide-influenced environment of the Cretaceous Seaway. This study documents the sequence stratigraphy of the Sego Sandstone Member in northwestern Colorado, just north of Rangely, and compares and contrasts it with equivalent strata in the Book Cliffs area in Utah. The Sego Sandstone Member in the study area contains three sequences characterized by progradational and aggradational stacking patterns. The stratigraphically lowest sequence consists of a prograding, tide-influenced delta overlain by marine mudstones, which represents a retrogradation and flooding surface. The second sequence is composed of multiple parasequences and consists of an incised valley filled with stacked tidal bars which then pass into a largely aggradational stacking pattern, composed of barrier-island deposits with back-barrier, flood-tidal-delta deposits, and wave-dominated-shoreface deposits. The third sequence is a broad, tide-dominated distributary-mouth system with a sharp, incisional basal contact. The three sequence boundaries documented in northwestern Colorado are consistent with the three main sequence boundaries identified in the Book Cliffs. However, whereas barrier islands and flood-tidal deltas are characteristic of the Sego Sandstone Member in northwestern Colorado, similar deposits are not as prevalent in the Book Cliffs of Utah, suggesting different depositional processes and paleogeography. Tidal and fluvio-deltaic processes are the dominant controls on deposition of the Sego Sandstone Member north of Rangely, Colorado. The transition from a tide-dominated fluvio-deltaic system to a mixed wave–tide-influenced coastline indicates a fundamental change in processes and depositional environment in the upper part of Sequence 2. Such change from a prograding fluvio-deltaic system to a more passive tide-modified coastline is not observed in the Book Cliffs, and may be the result either of large scale transgression or of relocation of the river system through large-scale avulsion, which is not observed in the Book Cliffs. Our study shows significant stratigraphic variability between rocks exposed in the Book Cliffs versus time-equivalent rocks exposed in northwestern Colorado in the Upper Cretaceous, which has implications for the regional basin architecture and stratigraphic correlations.

Human remains from a new Upper Pleistocene sequence in Bondi Cave (Western Georgia)

A new sequence containing human remains from a previously unstudied cave, Bondi Cave, has been discovered in Georgia, with deposits dating to the Caucasian Upper Pleistocene. This site lies in the basin of Rioni-Kvirila Rivers, in the Imereti region of north-western Georgia. The site has yielded a long sequence with human occupations dated from~39 ka14C (uncalibrated) and thus covers the time span of the Middle Palaeolithic (MP)-Upper Palaeolithic (UP) transition in the region. Changes in the techno- logical features between the lower and upper part of the sequence indicate that Bondi Cave could potentially highlight the tempo and mode of the population replacement. Indeed, recent studies in the southern Caucasus (notably at Ortvale Klde, Western Georgia) suggest a very rapid occupation by modern humans replacing existing Neanderthal populations (Adler, 2002; Meshveliani et al., 2004; Bar-Yosef et al., 2006; Adler et al., 2008). The rich UP levels at the upper and middle parts of the new sequence offer data on modern human subsistence and technological behaviors, and on the humans who occupied this cave, as a human tooth has also been discovered in this part of the sequence.

Gilbert-Gauss geomagnetic reversal recorded in Pliocene volcanic sequences from Georgia (Lesser Caucasus): revisited

Abstract We carried out a detailed paleomagnetic, rock-magnetic, and Thellier paleointensity study of an ~3.6 My Pliocene lava flow succession in southern Georgia. An earlier study (Camps et al., 1996) revealed that several consecutive lava flows record an intermediate polarity direction at the base of the section followed by a thick reverse polarity zone. The transitional field was interpreted as an excursion within chron 2Ar or an upper Cochiti-Gilbert reversal. New paleomagnetic data reported here have been obtained from nearby lava successions. In total, about 170 standard paleomagnetic cores belonging to 22 lava flows were collected during the 2005 sample collection campaign. Rock-magnetic experiments showed that the remanence is carried by Ti-poor titanomagnetite in most of the samples. The fraction of grains with multidomain magnetic structure does not seem to be important. Characteristic remanent magnetization was successfully determined on all samples. The direct correlation with the original (Thoki) sequence, previous preliminary measurements of natural remanent magnetization (Sologashvili, 1986), and field observations allowed us to establish a new magnetic stratigraphy. The lower part of section is characterized by intermediate magnetic polarity followed by thick reversely magnetized lavas. The upper sequence, represented by 18 consecutive flows yielded normal magnetic polarity. The mean paleointensity of the intermediate field is drastically reduced with respect to the post-transitional field strength. Based on all available radiometric ages and new paleomagnetic data, it may be speculated that Gilbert-Gauss (R-N) reversal was recorded at the upper part of sequence. Lower intermediate polarity flows possibly represent a form of precursor of this reversal that is similar to the Matuyama-Brunhes geomagnetic transition.

An infrared spectral reflectance study of hydrothermal alteration minerals from the Te Mihi sector of the Wairakei geothermal system, New Zealand

Hydrothermal alteration minerals and their zoning with depth in the Te Mihi sector of the Wairakei geothermal field have been characterised in this study by using field portable short-wave infrared spectroscopy. Observed variations in the spectral reflectance of hydrothermally altered volcanic and sedimentary rocks indicate a downward gradation in the dominant alteration minerals from mordenite, smectite, illite, to illite + chlorite (epidote). The variations result from temperature and permeability controlled fluid-rock interactions. Mordenite is abundant in the shallowest samples. Dioctahedral, aluminous smectites (discrete phase or as mixed-layer species) are common in the upper part of the sequence, but decrease in abundance below 200 m. In the smectite-rich zone, beidellite is common and locally may predominate over montmorillonite. Illite, including the illite component in mixed-layer illite/smectite, becomes a significant component below 150–200 m. Illite has an octahedral cation composition close to that of muscovite, with insignificant amounts of Fe and Mg. Chlorite is nearly undetectable in the upper part (0–200 m) of the altered sequence, and may become a minor component below 200 m. Chlorite is abundant only in andesitic rocks. Chlorite from the lower alteration sequence is Fe-rich, probably with the Mg# values around 0.2–0.4. Epidote is abundant locally at depth.

Late Cambrian stratigraphy of the Heritage Range, Ellsworth Mountains: implications for basin evolution

Deposition of the Upper Cambrian succession of the Ellsworth Mountains was influenced by major, episodic tectonically-driven changes to the depositional basin geometry. We subdivide the succession into four stratigraphical sequences based on the recognition of three sequence-bounding unconformities. The upper part of Sequence 1 is composed of the laterally equivalent Liberty Hills, Springer Peak and Frazier Ridge formations, a siliciclastic fluvial to marine deltaic association displaying NW-directed palaeocurrents. A switch in the position of the Late Cambrian depocentre from the north-west to the south coincided with cessation of terrigenous clastic deposition and accumulation of Sequence 2, the limestones of the Minaret Formation. Previously unreported talus breccias from the Independence Hills provide important clues to basin configuration at this time. A brief period of emergence of the Minaret Formation is inferred, prior to rapid subsidence and disconformable deposition of Sequence 3 (the ‘transition beds’) in outer-inner shelf environments. Localized intra-basinal uplift occurred prior to the deposition of Sequence 4 (the lower Crashsite Group), the base of which is locally an erosive unconformity, with a correlative conformity exposed elsewhere. We interpret the Upper Cambrian succession as representing the ‘rift-drift’ transition from initial rifting (preceded by Middle Cambrian volcanism) to thermal subsidence along the South African sector of the palaeo-Pacific margin of Gondwana.

Sedimentology, microfacies and stratigraphic distribution of foraminifers of the lower “Pseudoschwagerina” Limestone (Rattendorf Group, late carboniferous), carnic alps (Austria/Italy)

The Lower “Pseudoschwagerina” Limestone (Rattendorf Group) is a cyclic sequence composed of fossiliferous, shallow marine limestones and thin clastic beds. In the Schulterkofel section three depositional cycles are well developed. Clastic intervals at the base of all depositional cycles formed during relative sea-level lowstands, during transgression well bedded fossiliferous limestones and massive algal mounds were deposited. Bedded cherty limestones with marl intercalations are interpreted as deposits formed during relative sea-level highstands with water depths of some tens of meters. In the Lower “Pseudoschwagerina” Limestone (LPL) fusulinid rich limestone beds occur in different stratigraphic levels, particularly at the base and on top of clastic intervals. Taphonomic features and composition of fusulinid assemblages indicate that reworking and even redistribution of fusulinids play a subordinate role. They are considered as parauthochthonous assemblages, accumulated during periods of low sediment input. Based on fusulinid biostratigraphy, the lower part of the LPL (depositional sequence 1) is correlated with thesokensis Zone, because of the occurrence ofRuzhenzevites ferganensis without species of the genusDaixina (Bosbytauella). The main part of the LPL (uppermost part of depositional sequence 1 to sequence 3) belongs to thebosbytauensis-robusta Zone. For the Carboniferous/Permian (C/P) boundary only an interval can be estimated between the upper part of sequence 3 (Schwagerina versabilis-Rugosofusulina arianica) and tectonically isolated limestone beds of the overlying, predominantly clastic Grenzland Formation (Sphaeroschwagerina-Pseudoschwagerina = middle Asselian). In the Carnic Alps the C/P boundary lies therefore immediately below the base or within the lowermost clastic Grenzland Formation.

Genetically related alluvial deposits across active fault zones: an example of alluvial fan-terrace correlation from the upper Quaternary of the southern Po Basin, Italy

Integrated studies of alluvial fan systems and related intramontane valley deposits provide potentially useful tools for understanding the character of alluvial sedimentation and its control in fault-controlled piedmont zones. An example is reported from the uppermost Quaternary alluvial deposits of the northern Apennines and the southern margin of the Po Basin. A distinctive cyclicity characterizes both the outcropping and the buried units in the study area. In intramontane valleys individual cycles consist of regular alternations of sections displaying high and low terrace preservation. In the subsurface, a rhythmic alternation of coarse-grained bodies and laterally extensive pelitic horizons occurs at various scales and represents the basic cyclic motif of the depositional system. The correlation between the terrace sequences and the depositional cycles in the subsurface allows the differentiation of distinct sequences with a characteristic internal architecture. In the lower part of sequences, poorly preserved terrace deposits are laterally correlatable with alluvial fan gravel bodies, whereas highly preserved terrace deposits are correlated laterally with predominantly floodplain sediments in the upper part of sequences. Reliable correlation lines have been constructed using 14C dating. Thrust tectonics and climate exerted a major control on sequence development. Deep valley excavation, with subsequent low terrace preservation, occurred in response to relief rejuvenation during successive thrusting events. On the other hand, decreasing erosion related to periods of minor tectonic activity led to terrace preservation in the intramontane valleys. The coeval episodes of alluvial fan growth and abandonment, recorded at the basin margin, are interpreted to reflect changes in the efficiency of the drainage system, due to variations of tectonic uplift in the adjacent areas. Alternating phases of deglaciation and renewed glaciation, however, are inferred to have played a major role in controlling the amount and type of sediment delivered to the alluvial fan complexes.

Configuration of Shelf Sandstone Oil Reservoirs, Upper Cretaceous (Turonian) Turner Sandy Member of Carlile Shale, Powder River Basin, Wyoming: ABSTRACT

Oil production in the Upper Cretaceous Turner Sandy Member of the Carlile shale on the east flank of the Powder River basin is established in two types of stratigraphic traps characterized by distinct geometries and reservoir properties. One type is medium-grained sandstone bodies as much as 4 m thick that have filled elongate (as much as 10 km), narrow (< 1.5 km), east-west-trending erosional depressions of low relief. Trough and tabular cross-stratification indicates an eastward direction of transport. This type of sandstone is interpreted as previously deposited sediments reworked by transgression following global drop in sea level in middle Turonian time. The sands were redistributed and concentrated by offshore-directed downwelling flows on inner shelf in irregularly spaced depressions controlled by recurrent movement of basement fault blocks. Although this type is limited in areal extent, reservoirs in it are good (porosity and permeability are as much as 20% and 100 md, respectively). In contrast, the other type of trap is generally very fine-grained sandstone that occurs in shoaling-upward sequences as much as 12 m thick. Shale intercalations decrease upward, and planar lamination and hummocky cross-stratification are prevalent. Upper parts of sequence are bioturbated and contain trace fossils of Skolithos ichnofacies. Thesemore » sequences form widespread (70 km/sup 2/) bodies that were deposited below fair-weather base on the storm-wave-dominated outer shelf. Although the overall sandstone bodies are extensive and resulting production is large, reservoir in these bodies are characterized by thin beds and vertical and lateral discontinuity on a centimeter scale. In addition, sandstones are tight (average porosity and permeability are 15% and 0.5 md, respectively) because of original fine grain size and presence of authigenic clays.« less

Cardium Formation at Seebe, Alberta--Storm-Transported Sandstones and Conglomerates: ABSTRACT

End_Page 644------------------------------The Upper Cretaceous (Turonian) Cardium Formation is one of the most productive oil-bearing formations in Alberta. As a result of detailed mapping of two outcrops near Seebe, Alberta, we have defined five coarsening-upward sequences within this formation. The sequences begin with bioturbated mudstones and coarsen into sandstones, commonly with conglomeratic veneers on top. In sequence 2, a 20-cm thick conglomerate has been molded into symmetrical gravel waves. The sandstones are dominated by hummocky cross-stratification, occurring in the upper parts of sequences 1, 2, 3, and 5. The implication is that these sequences terminate in water deeper than fair-weather wave base (10 to 15 m). The trace-fossil assemblage, dominated by abundant Zoophycos and long horizontal Rhizocorallium, suggests similar depths. Foraminifera recovered from sequences 3 and 4 comprise a highly diverse assemblage of agglutinated species, also supporting deposition below fair-weather wave base. In view of the physical and biologic evidence for deposition in a few tens of meters of water, and the total absence of medium-scale cross-bedding, we suggest that the entire Cardium Formation at Seebe was deposited below fair-weather wave base. The sandstones were emplaced by storm-generated density currents, the same storm both generating the flow and imprinting hummocky cross-stratification on the deposit. Finally, we suggest that the conglomerates were also moved rapidly seaward by density currents, there being no evidence for the traditional beach or transgressive lag interpretation of these rocks. End_of_Article - Last_Page 645------------

Upper proterozoic supracrustal rocks of North America: A brief review

Abstract Well-preserved successions of supracrustal rocks of Upper Proterozoic age are present around the margins of North America but are rare in the continental interior. The thickest and most complete assemblage is present in the Cordilleran region, particularly in the central and northern parts. In these regions, a three-fold subdivision of the supracrustals is possible. The oldest assemblage (Sequence A) is older than 1.2 Ga, the middle assemblage (Sequence B) is between ca. 1.2 Ga and 0.8 Ga and Sequence C is between ca. 0.8 Ga and 0.57 Ga, the base of the Cambrian. Here, Upper Proterozoic is used as roughly equivalent to Sequences B and C. Sequence A is only briefly considered. Detailed lithostratigraphic correlation is possible between the rocks of Sequences A and B and generally thinner counterparts in the Canadian Arctic, on the northern margins of the Canadian shield. Sequence C is not represented by sedimentary rocks on the northern edge of the continent. The break between Sequences A and B, the Racklan orogeny, is not recognised in the southern part of the Cordillera but there is evidence of widespread mafic igneous activity at about the same time (ca. 1.2 Ga ago). Upper Proterozoic rocks are poorly represented in the continental interior; the Keweenawan succession provides an exception. It rocks fill an intracratonic rift, possibly related to early movements during the Grenville orogeny. The Upper Proterozoic history of the eastern part of North America is much simpler in that it comprises a single supracrustal assemblage corresponding to Sequence C and the upper part of Sequence B, all formed after the Grenville orogeny. Thus the different tectonic histories of the eastern and western margins of North America resulted in development of different sedimentary accumulations. With the exception of the Racklan orogeny, which affected the northern part of the Cordillera, the Upper Proterozoic assemblages of both the eastern and western margins of the North American continent reflect sedimentary accumulation and subsidence that culminated in the opening of Phanerozoic oceans.

Cardium Formation (U. Cretaceous) at Seebe, Alberta—storm-transported sandstones and conglomerates in shallow marine depositional environments below fair-weather wave base

As a result of detailed mapping at Seebe, Alberta, we have defined five coarsening-upward sequences in the Turanian Cardium Formation. The sequences begin with bioturbated mudstones and coarsen into sandstones, commonly with conglomeratic veneers on top. In sequence 2, the conglomerate is up to 20 cm thick, and has been molded into sand waves a few centimetres high and with wavelengths of about 1 m.The sandstones are dominated by hummocky cross-stratification (HCS), a broad, low-angle undulating stratification now believed to be formed below fair-weather wave base by long-period storm waves. Sandstones with HCS occur in the upper parts of sequences 1, 2, 3, and 5, suggesting that all of these sequences terminate in water deeper than fair-weather wave base (10–15 m). The trace fossil assemblage of abundant Zoophycos and long, horizontal Rhizocorallium suggests similar depths. Foraminifera in sequences 3 and 4 show a high diveristy of agglutinated species, also suggesting deposition below fair-weather wave base.In view of the physical and biological evidence for deposition in a few tens of metres of water, and the total absence of medium-scale cross-bedding, we suggest that the entire Cardium Formation at Seebe was deposited below fair-weather wave base. The sandstones were emplaced by storm-generated density currents, the same storm both generating the flow and imprinting HCS on the deposit. Finally, we suggest that the conglomerates were also moved rapidly out to sea by density currents, there being no evidence for the traditional beach or transgressive lag interpretation of these rocks.

Encroûtements calcaires dans les altérations des marnes éocènes de la falaise de Thiès (Sénégal). Organisation morphologique et minéralogie

The calcareous duricrusts of the weathering zone of Eocene marls from the cliff of Thiès (Senegal) were subject to a very detailed study by macromorphological, micromorphological (microscope and stereoscan) and mineralogical means. The authors show on field that vertical and lateral sequences of calcareous accumulations are similar to those described in mediterranean countries. However, the detailed study of the mechanism of calcareous accumulation (calcitization) emphasizes that the latter takes place principally due to the development of calcite microcrystals (crystalliplasma). This calcitization is maximum to the top of profiles and to the lowest part of sequences ; this accumulation is accompanied by an epigenesis of quartz, dolomite and clay by calcite. To this geochemical environment of calcitization is opposed a decalcitization environment at the bottom of profiles and at the upper part of sequences. This decalcitization at the bottom of profiles is mild and accompanied by neoformation of quartz, dolomite and attapulgite. In these two environments, there are inverse and reversible epigeneses between calcite on the one hand, and the other constituents on the other hand. Decalcitization and calcitization are related to structures ranging from plane to concentric, then massive.

Deposition of turbidites and agitated-water siltstones : A study of the Upper Carboniferous Westward Ho! Formation, North Devon

The Westward Ho! Formation consists dominantly of mudstones and siltstones. Two distinct groups of facies can be contrasted. The first contains very fine grained black mudstones and parallel bedded graded siltstones and fine sandstones which show all the sedimentary structures now associated with turbidites. By contrast, the second group of facies contains irregularly rippled and cross-bedded siltstones and sandstones, the sedimentary structures indicating agitated water. The sequence of facies shows that the turbidites occur only in the lower parts of coarsening-upward (shallowing) sequences, or are associated with black mudstones in very quiet water. The upper parts of the regressive sequences contain only agitated-water facies. Two groups of siltstone-filled scours occur. In the lower group, the scours are of wide extent and the sequence above each scour fines-upward. In the upper group, similar fining-upward sequences above scoured surfaces occur, but the upper parts of the sequences contain many small channels which have a fill finer than, or identical with, the surrounding sediment. The fine nature of the fill suggests a slump-scar origin rather than channelling. The Westward Ho! Formation is unique in containing three extremely rapid vertical transitions from turbidite to agitated-water facies. Progressive increase in agitation in the area of turbidite accumulation is suggested, rather than regional progradation of a slope and hence more gradual transition into agitated-water facies.