Large-scale Cross-stratification Research Articles

Facies Evaluation and Depositional Environments of Carbonates of the Bagh Group, Dhar District, Madhya Pradesh, India

A significant event of marine transgression took place in Central India during Late Turonian-Coniacian. Fossiliferous marine succession of Bagh Group is one of the few carbonate successions exposed in peninsular India which was in focus of the current study for understanding this event. The signatures of this event were identified in the carbonate succession. The carbonates of Bagh Group are composed of two formations: the lower part is represented by Nodular limestone Formation which is overlain by Bryozoan limestone Formation at the top. On the basis of grain size variation and sedimentary structures, the Nodular limestone is divisible into three facies: facies ‘A’, facies ‘B’ and facies ‘C’. A hardground exists between facies B and facies C. Lack of sedimentary structures and high mud content indicates low energy depositional setting for the Nodular limestone Formation. Similarly, Bryozoan limestone Formation is divisible into five facies: facies ‘D’, facies ‘E’, facies ‘F’, facies ‘G’ and facies ‘H’ based on grain size variation and sedimentary structures. All of these five facies are fossiliferous. Glauconites are present within facies ‘G’ and have two modes of occurrence - as infilling within Bryozoan limestone and as altered feldspar. Presence of both small- and large-scale cross-stratification in Bryozoan limestone with lesser mud content are indicative of high energy shallow marine conditions. Large-scale cross-stratifications are possibly representing tidal bars while the small scale cross stratifications are formed in inter bar setting. Presence of reactivation surfaces within facies ‘E’ also supports their tidal origin. Increase in depositional energy condition is also evident from dominated by packstone facies.

The first upper Silurian land-derived palynological assemblage from South America: Depositional environment and stratigraphic significance

A thick succession of quartz-rich sandstones cropping out in the middle part of the Villacorta Creek, on the western slope of the Sierra del Famatina (Valle Hermoso, La Rioja Province, northwestern Argentina) was recently recognized as the Villacorta Formation. The sandstone package rests disconformably on greenish sandstones of the Ordovician Suri Formation and is overlain along a marked angular unconformity by rocks of the upper Carboniferous Agua Colorada Formation (Paganzo Group). The Villacorta Formation is divided into four intervals, the lower one is formed by quartz-rich sandstones showing large-scale cross-stratification; the second part is similar but includes some levels of intraformational conglomerates and shows a decrease in both grain-size and the scale of the cross-bedding sets. The third section comprises medium- and coarse-grained sandstone with intercalation of fine-grained sandstones. The upper interval corresponds to two thickening and coarsening upwards successions. This last section was deposited in a deltaic environment where the top of mouth bars and lagoon deposits preserved palynological assemblages and phytodebris. A low-diversity palynological assemblage recovered from these mudstone levels includes abundant nematophyte cuticular-like fragments, cryptospores (monads, dyads, tetrads) and trilete spores (e.g., Scylaspora scripta, Emphanisporites protophanus). The stratigraphic relationships of the succession and the palynological assemblage suggest a late Silurian age for the Villacorta Formation. The exclusive presence of land-derived components in the organic residue allows to capture a snapshot of the terrestrial vegetation and constitutes the first record of this type of palynological association and nematophyte remains in this southern region. This assemblage improves previous paleogeographic reconstruction enlarging the extent of Silurian basins in Argentina. The shallowing successions of the Famatina region pass to marine deposits in the Precordillera area, indicating a westward transition to offshore environments.

Tidal bar cyclicity record in the Lower Permian: The Rio Bonito Formation, Paraná Basin, southern Brazil

Tidal bars register gravitational interactions between the Earth, Moon, and Sun and allow inferences about astronomical positioning through the cyclic variation of sandstone layer thickness throughout depositional events. Measurements of sandstone layers thickness in a tidal bar of the Rio Bonito Formation (Paraná Basin, southern Brazil) and further spectral analysis allowed the definition of the tide patterns imprinted on rock layers. The study was based on a remarkable tidal bar outcrop located in the Candiota paleovalley in the southern basin, Candiota town, Brazil, with sedimentary characteristics that suggest to a tidally dominated estuary depositional environment. The tidal bar shows a medium- to large-scale cross-stratification, reactivation surfaces, bidirectional structures, and tidal bundles. The spectral analysis resulted in a significant frequency peak (0.0329) which corresponds to a period of 30.45 layers per cycle. Six of these cycles are recognizable in the tidal bar by its cyclic thickening-thinning of layers, that range from 1.5 to 11 cm. This cyclic pattern is interpreted as being produced by the neap-spring tide cycle. Our study also allows us to infer that the south Paraná Basin Permian tide had a semi-diurnal pattern, with a neap-spring tide period of 14.9 days. Comparison with previous studies points to small changes in the duration of the neap-spring tide cycle in the last 900 Ma (15.2 days in the Cambrian, 14.9 days in the early Permian and 14.7 days today), explained by the distance and acceleration of the movement of the Moon's revolution and decrease in the speed of rotation of the Earth.

Wandering gravel-bed rivers and high-constructive stable channel sandy fluvial systems in the Ross River area, Yukon Territory, Canada

Mid-Cretaceous strata within the Tintina Trench, 3 km west of the community of Ross River, contain evidence of deposition in two distinct, alternating, fluvial settings. Coal-bearing, mud-dominated strata are commonly associated with high-constructive sandy channel systems, with extensive overbank, levee and splay deposits. Channels are between 3 and 30 m wide and 0.4–7 m thick. They show repetitive development of side and in-channel bar-forms, as well as up-channel widening of the rivers by selective erosion of associated overbank and levee deposits. Levees extended for several hundred metres away from the channels. In this setting low-angle inclined stratification and epsilon cross stratification may reflect lateral migration of crevasse channels or small streams. The paucity of exposure prevents recognition of the channels as products of multiple channel anastomosed systems or single channel high-constructive systems. Gravel-dominated strata, inter-bedded with, and overlying coal-bearing units, are interpreted as deposits of wandering gravel-bed rivers, with sinuosity approaching 1.4. In most exposures they appear to be dominated by massive and thin planar-bedded granule to small pebble conglomerates, which would traditionally be interpreted as sheet-flood or longitudinal bar deposits of a high-gradient braided stream or alluvial fan. Architectural analysis of exposures in an open-pit shows that the predominance of flat bedding is an artefact of the geometry of the roadside exposures. In the pit the conglomerates are dominated by large scale cross stratification on a scale of 1–5.5 m. These appear to have developed as downstream and lateral accretion elements on side-bars and on in-channel bars in water depths of 2–12 m. Stacking of strata on domed 3rd order surfaces suggests development of longitudinal in-channel bar complexes similar to those observed in parts of the modern Rhône River system. Mudstone preserved in some of the channels reflects intervals of channel abandonment or avulsion. Minimum channel width is from 70 to 450 m.

Paleohydraulic reconstruction of the largest Glacial Lake Missoula draining(s)

Glacial Lake Missoula (GLM) was the source of Earth’s largest known peak flood discharges onto the Channeled Scabland, northwestern United States. The original studies of GLM were based on geomorphological mapping of lake sediments and shorelines. More recently, studies have emphasized the computational fluid dynamics of downstream megaflood routing and characteristics in the Channeled Scabland and adjacent areas. However, actual details of the lake-draining processes GLM have received little scientific attention. In this paper, we employ two-dimensional hydraulic modelling to understand the paleoflow conditions for different scenarios of draining GLM at its highest stands. Values are calculated for three GLM drainage hydrographs (2.6; 13; 17 × 10 6 m 3 s −1) that previous studies had estimated for outlet flows. Consistent with regional mapping and with the original interpretation of fluvial geomorphology by other studies, the modelling shows that lake silt sequences in the gorges of GLM must postdate the most highly energetic outburst events from the lake. This is because the flows generated in the lake by megaflood outflows are sufficiently energetic to erode any accumulated silt deposits. In contrast, the gravels underlying the silts in these zones include boulder-sized clasts, large-scale cross stratification, and 70–100 m-high bars forms, all of which indicate very high-energy flood flow conditions. The modelling also shows that the paleohydraulic conditions which developed in the lake basin during the largest megaflood outflows would be capable of accounting for the observed bedrock scour at Rainbow Lake Pass, various “high eddy deposits” (eddy bars of flood gravel) in gorges, and the “giant ripple marks” (subaqueous gravel dunes) that were classically described in the 1940s. Previous studies interpreted the glacial lake silt/rhythmites and related deposits, such as the Ninemile Section, as evidence for numerous fillings and drainings of GLM. Our modelling suggests that these inferred events must have been relatively low in energy and magnitude relative to the earlier outburst(s) that emplaced the flood gravels, subaqueous gravel dunes, and scabland-like erosion surfaces.

The Holocene Pucón eruption of Volcán Villarrica, Chile: deposit architecture and eruption chronology

The Pucon eruption was the largest Holocene explosive outburst of Volcan Villarrica, Chile. It discharged >1.0 km3 of basaltic-andesite magma and >0.8 km3 of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 14C years BP.

The Early Cambrian Bradore Formation of Southeastern Labrador and adjacent parts of Quebec: Architecture and genesis of clastic strata on an early Paleozoic wave-swept shallow marine shelf

Strata of the Bradore Formation have been interpreted in terms of a marine transgression developed at the onset of spreading of Iapetus Ocean, with a basal member of fluvial origin, a middle member of lagoonal origin and an upper member of tidal-channel origin. In Labrador the formation rests directly on a low relief surface, developed on metamorphic rocks of the Grenville Orogen, with no well developed paleosoils. Re-examination of the architecture of pebbly sandstones in the basal (Blanc-Sablon) member indicate that these formed on an open, storm-influenced wave-swept shelf as part of a low relief sand sheet, with evidence of multiple accretionary events. Structure in sandstones in the middle (Crow Head) member is largely obscured by a high density of Skolithos linearis domichnia. Abundance of hummocky cross stratification indicates deepening, with a transition to an open, storm-influenced, shallow shelf. Large-scale cross stratification in the glauconite-bearing upper (L'Anse-au-Clair) member reflects shoaling, with a return to deposition of medium-scale marine sandwave complexes, with no clear evidence of any tidal signature. Strata of the Bradore Formation were at least partly lithified prior to deposition of reefal and platformal carbonates in the overlying Forteau Formation. Paleocurrent indicators in the upper and lower members suggests that during the initial drift phase of Iapetus this segment (St Lawrence Promontory) of the Laurentian margin was swept by both geostrophic and storm-influenced currents, largely oriented parallel to the paleocoastline.

Stratigraphic evolution of a fluvial–eolian succession: The example of the Upper Jurassic—Lower Cretaceous Guará and Botucatu formations, Paraná Basin, Southernmost Brazil

The Guará and Botucatu formations comprise an 80 to 120 m thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guará Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guará and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Paraná Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guará Formation to a wider sedimentary context in the Botucatu Formation.

Shallow-marine fan delta slope deposits with large-scale cross-stratification: the Plio-Pleistocene Zaimokuzawa formation in the Ishikari Hills, northern Japan

The Plio-Pleistocene Zaimokuzawa Formation comprises marine to fluvial deposits in one of the Neogene back-arc basin around the Japanese islands. Five sedimentary facies (facies A–E) and two facies associations (FA 1 and FA 2) were distinguished in the lower Zaimokuzawa Formation. Facies A is cross-stratified sandstone induced by tidal currents. Facies B is a couplet of wave-rippled fine sandstone and siltstone. Facies C is pebbly sandstone interpreted as the deposit from a concentrated density flow and a turbidity flow. Facies D is pebbly mudstone deposited from a mud flow or debris flow. Facies E is a slumped alternation of sandstone and mudstone. FA 1 and FA 2 are typical of the lower and upper parts of the lower Zaimokuzawa Formation, respectively. FA 1 is composed of alternations of facies A and C, including facies B, D and E. Facies C and D have lobe-like geometries in FA 1. FA 2 is composed of facies B and C, and lacks facies A. Facies B and C fill concave-upward slump scars in FA 2. The constituent facies, shapes of the deposits and paleocurrent directions suggest that the lower Zaimokuzawa Formation was formed by progradation of a shallow-marine fan delta slope influenced by tidal currents, and that its lower (FA 1) and upper part (FA 2) corresponds to middle, lower and upper slope deposits, respectively. Wave and tidal currents reworked and redeposited the sediments supplied by gravity-driven processes on the delta slope to form the multiple facies associations. Grain sizes of facies A and B, which are mainly composed of sand-sized particles in spite of the presence of intervening pebbly deposits of sediment density flows, imply competence of the reworking processes. The distribution of cross-stratified sandstone is restricted to the western area, indicating a local inequality in the influence of tidal currents.

Unusual coarse, clastic, wave-dominated shoreface deposits, Pliocene to middle Pleistocene, northern Chile; implications for coastal facies analysis

ABSTRACT Pliocene to mid-Pleistocene shallow-marine strata exposed at Gatico, northern Chile (22° 30 S, 70° 14 W) display unusual characteristics for coarse-grained wave-dominated shoreface deposits. A 15 m2000 m strike section of Pliocene to mid-Pleistocene strata exposed within small, late Pleistocene sea cliffs contain four facies: beach, foreshore channel, upper shoreface, and rip channel. Beach and upper shoreface facies predominate. The latter consists of locally bioturbated, coarse-grained, pebbly sandstone with (1) single sets of planar to tangential cross-strata, up to 3.5 m thick, with reactivation surfaces, and (2) sets of medium-to large-scale cross-strata that were deposited on inclined set surfaces (up to 15°). Some of the inclined set surfaces extend up-dip of cross-set terminations. Inclined parallel laminae and bioturbated beds lie subparallel to the inclined set surfaces. Interpretation of these unusual bedforms has been made through analogy with the present-day microtidal wave-dominated coastline at Gatico, by facies analysis, and by comparison with modern-day wave-dominated shorelines. The large-scale sets of cross-strata record the migration of simple, two-dimensional (2-D) dunes and represent the slipface deposits of nearshore bars. Parallel laminae represent the bar surface. Medium-scale dunes migrated up the inclined set-bounding surfaces, which lie subparallel to older preserved bar surfaces. Paleocurrent data indicate that bar migration was largely directed obliquely onshore towards the southeast. The bar slipface deposits are considered to record the recovery of a nearshore bar immediately following storm conditions. Bar recovery involves re-establishment of a slipface and rapid onshore migration. Erosion surfaces that appear as the extension of lower set boundaries beyond the termination of cross-strata sets, and reactivation surfaces, are considered to be storm-generated. The episodic nature of bar migration is indicated by the development of heavily bioturbated horizons. The small-scale ascending cross-sets may reflect fair-weather conditions with northwestward dune migration up the modified bar slipface. Despite the abundance of barred shorelines around the world at present, their deposits are not widely described from the rock record, suggesting that they are either not preserved or not recognized. The scale and internal characteristics of these nearshore bars closely resemble those of some tide-generated bedforms (e.g., compound cross-strata in marine sediment with bimodal-bipolar paleocurrents and evidence of episodic bedform migration). The similarity of these north Chilean deposits to documented tidal deposits suggests that an element of caution may be required in the identification of tidal deposits from the rock record.

Shallowing-upward cycles in pelagic troughs (Upper Jurassic, Subbetic, Southern Spain)

Abundant calcareous tempestites with chert and hummocky cross-stratification (HCS) occur in Upper Jurassic successions (Milanos Formation) of the Subbetic (Betic Cordillera, Southern Spain). These calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts, and have thicknesses between 25 and 75 cm. Beds show symmetrical wave ripple lamination near the top, and are usually capped with wave ripples. According to palaeogeographic reconstructions, these strata were deposited in a subsiding trough in the pelagic domain of the Southern Iberian Continental Margin, which was the northwestern-most border of the Tethys. Calcisiltite and calcarenite beds deposited in this pelagic trough form small-scale to large-scale shallowing-upward cycles. Shallowing-upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part, have also been recognized in some sectors of the Subbetic pelagic basin. The large-scale cycle occurs in the upper part of the Jarropa Radiolarite Formation, which directly underlies upper Oxfordian, Kimmeridgian and Tithonian strata of the Milanos Formation. Two types of large-scale shallowing-upward cycles have been recognized, according to the presence or absence of thick units of oolitic limestone with large-scale cross-stratification. The water depth during deposition ranged from below, but near, storm wave base at the base of the large-scale cycles to very shallow carbonate platform at the top.

Internal architecture of mixed tide- and storm-influenced deposits: an example from the Alcântara Formation, northern Brazil

The uppermost portion of the Itapecuru Group is exposed in the eastern margin of the Sao Luis Basin, northern Brazil, where it consists of two units: the Alcantara Formation (Cenomanian) and the Cujupe Formation (Late Cretaceous-early Tertiary? [Rossetti, D.F., Truckenbrodt, W., 1997. Revisão estratigrdfica para os depósitos do Albiano-Terciário Inferior (?) na Bacia de Sao Luis (MA), norte do Brasil. Bol. Mus. Paraense Emílio Goeldi (Sér. Ciênc. Terra), in press]). The Alcântara Formation, which contains the large-scale structures discussed in this paper, consists of deposits attributed to mid- to upper-shoreface, foreshore, tidal channel, and lagoon/washover environments attributed to a regressive, barred shoreline. Several types of large-scale cross bedding (i.e., simple foreset, compound, mixed, undulatory, and intricately bounded) were recognized in the shoreface facies association. These structures are interpreted to record the interaction of storm and tidal processes. The storm influence is suggested by a combination of factors, mostly including: (a) the genetic association with other storm-generated sedimentary structures (i.e., swaley cross stratification and undulating parallel lamination with internal truncations); (b) the deposition on prominent surfaces formed by storm erosion, which are defined by large-scale, either symmetrical or asymmetrical scours arranged in a regular, repeating pattern; (c) the sedimentary features formed under combined (unidirectional and oscillatory) flow processes (e.g., compound/mixed bedding with superimposed either swaley cross sets or complexly truncating cross sets with highly undulating boundaries; large-scale, undulatory and intricately bounded cross beddings); and (d) the lateral change in structural styles within short distances, which records frequent modification from asymmetrical to symmetrical/nearly symmetrical bedform profiles (more likely to occur under storm-generated combined flows). The tidal signature is locally recognized by regularly spaced, thick/thin sandstone bundles defined by reactivation surfaces and/or mud drapes, which are attributed to tidal (ebb/flood) cycles. The analysis of paleocurrent distribution suggests that vigorous, southwestward-oriented storm flows interacted with local tidal currents on the shoreface to promote the landward transport of significant volumes of sand, which resulted in the large-scale cross stratification described in this paper. In addition, a secondary, southeastward-directed (oblique- to shore-parallel) combined flow would have periodically interacted with the main flow. The origin of this oblique- to shore-parallel flow is attributed to either the refraction of the main storm waves as they approached the paleocoastline or the interference of a separate storm episode, which competed with the major, landward-moving one. This complex storm flow pattern can be related to past penecontemporaneous seismic activity (i.e., tsunamis), as suggested by the paleogeography of the study area during the Cenomanian, combined with the structural history of the Sao Luis Basin and the sedimentary features recognized in the Alcantara Formation.

Moraine formation in northwestern Ontario: product of subglacial fluvial and glaciolacustrine sedimentation

Long, arcuate, stratified end moraines in northwestern Ontario may represent major and rapid sedimentation events in glacial Lake Agassiz. Rapid lowering of the lake or lift of a marginal ice dam may have triggered widespread outbursts of subglacial meltwater which deposited these end moraines as coalesced or broad subaqueous lacustrine fans. Moraine cores are of undeformed gravel, sand, and silty sand that fine upward. Coarse beds are massive to weakly stratified. Large-scale cross-stratification may be present. Sandy rhythmic beds are laterally transitional to silt–clay rhythmites (varves). Similar facies occur in adjoining eskers.Moraine sediments have the expected characteristics of rapid deposition on subaqueous fans and, therefore, may not represent either prolonged sedimentation or stable ice margins. Rather, they may reflect rapid sedimentation associated with large discharges that induced transient instability in the drainage network and (or) surging prior to marginal sedimentation. This explanation for moraine formation questions the traditional view that large end moraines represent climatically controlled stillstands.

Facies Architecture in a Fluvial-Deltaic Sequence, Upper Crooked Fork Group (Pennsylvanian), Tennessee and Kentucky: ABSTRACT

The upper Crooked Fork Group on the Cumberland plateau in northeastern Tennessee and southeastern Kentucky includes the Wartburg Sandstone (Corbin of Kentucky) and surrounding fine-grained strata of the Breathitt Formation below the Poplar Creek Coal. Detailed study of outcrops employs lateral profiles and architectural element analysis. Examination of bounding surfaces within and between elements indicates a sixfold hierarchy based on areal extent and geometry. The Wartburg Sandstone consists largely of three-dimensional macroform bar complexes. Foreset macroform elements predominate, with minor lateral accretion elements present and no apparent internal cyclicity. The channelized base of the Wartburg, predominance of large-scale planar and trough cross-stratification, low paleocurrent variance, and presence of soft-sediment deformation elements and massive sandstone elements with large mudstone intraclasts suggest a low-sinuosity fluvial setting. Strata overlying the Wartburg display fine-grained lateral accretion elements and associated overbank-fines. This suggests an upward change in fluvial style in the upper Crooked Fork Group from a predominantly coarse-grained low-sinuosity setting to a fine-grained high-sinuosity setting. The abundance of downstream and lateral accretion elements, plant debris, rooting, and thin discontinuous coals; paucity of burrows and bioturbation; and presence of thin marine horizons combined with the facies architecture suggest a channelized depositional setting with some marinemore » influence, indicative of fluvial or fluvial-dominated deltaic deposits.« less

Pedogenic and geomorphic evolution of a buried dune palaeocatena at Magilligan Foreland, Northern Ireland

Field description and laboratory analyses of a complex stratigraphy recently exposed near Magilligan Point have elucidated the late Holocene evolution of Magilligan Foreland, N. Ireland. A marine beach ridge palaeocatena passes northwards into a penecontemporaneous aeolian dune palaeocatena; both include soils and peat-filled depressions and are overlain by an aeolian dune sand. A second dune sand is confined to the south of Magilligan Foreland. The thickness of the decalcified zone of the palaeocatena varies from 0.4 m near the crest of the palaeodune to 2 m below its slopes. The palaeodune core exhibits large-scale cross-stratification enclosing unusual small-scale reverse-flow cross-stratified sets. D 90 (90% depletion) values indicate similar weathering intensities in the palaeocatena and in the overlying sand but deeper weathering in the former. A mineral stability sequence has been constructed and unweathered sand identified, which indicates the original composition of the Magilligan deposits. Inter-profile differences in weathering rates are attributed to variations in local environments.

Estuarine Trace Fossil Assemblages in Tournaisian Clashmelcon Formation, Kerry Head, Southwest Ireland: ABSTRACT

The Tournaisian Clashmelcon Formation comprises lateral-accretion deposits which fine upward from large-scale cross-stratified sandstone to current-dominated heterolithic beds interbedded with current- and wave-dominated heterolithic beds and desiccated cryptalgal laminites. Large-scale cross-stratification occurs in bundles with structures indicative of unsteady and reversing currents. Wave-dominated heterolithic beds and hummocky cross-stratified sandstone become more abundant upsection. A transported brachiopod-crinoid-bryozoan assemblage which is locally abundant above erosion surfaces becomes more diverse upsection. The Clashmelcon Formation is interpreted as deposits of laterally migrating and aggrading estuarine channels associated with tidal flats which gave way to estuarine embayments dominated by waves. Trace fossils are best developed in heterolithic bedding and include vertical and horizontal burrows. Low in the Clashmelcon Formation, tops of lateral-accretion deposits are locally very bioturbated by a few types of trace fossils including Skolithos, Arenicolites, Teichichnus, Cylindrichnus and, rarely, Glossifungites. Upsection, a more diverse trace fossil assemblage occurs. Wave-dominated heterolithic sediments display a moderately diverse trace fossil assemblage including Teichichnus, Monocraterion, Diplocraterion, Fucusopsis, Planolites, Paleophycus, Chondrites, Neonereites and, rarely, Curvolithus, Rhizocorallium, Bifungites, and Treptichnus. Rhizocorallium is also locally abundant immediately below the basal erosion surfaces of lateral-accretion deposits. Arenicolites, Teichichnus, Fucusopsis, Planolites, Paleophycus, and Chondrites are found in variable abundance throughout much ofmore » the succession. Chondrites is locally very abundant in wave-dominated heterolithic beds and the lower parts of channel fills.« less

Evolution of Catskill (Upper Devonian) River Systems: Intra- and Extrabasinal Controls

ABSTRACT The Frasnian Catskill magnafacies of the Catskill region, New York State, represents alluvium deposited in a foreland basin adjacent to the Acadian fold-thrust belt. Facies characteristics vary in space and time in association with a series of transgressive-regressive sequences on the order of 102m thick. Away from the paleoshoreline and upsection, the channel-belt sandstone bodies within a given regressive sequence show 1) an increase in mean grain size from fine to medium and coarse sand; 2) an increase in amount of large-scale cross-stratification relative to small-scale cross-stratification and planar stratification; 3) a decrease in the inclination of lateral-accretion surfaces and more common truncation of their upper parts by chute channel fills; 4) an increase in th proportion of sandstone-filled channels relative to those filled with sandstone plus mudstone. The overbank sandstone-mudstone beds concomitantly increase in mean grain size. The proportion of channel-belt deposits relative to overbank deposits increases slightly with distance from the paleoshoreline, and the degree of overlap (connectedness) of channel-belt sandstone bodies increases markedly. Paleohydraulic analysis shows that channels had low sinuosity (less than 1.25) and were predominantly unbraided. Channels ranged from tens to hundreds of meters wide, and were meters in depth. Channel slopes ranged from approximately 0.0001 to 0.0003, increasing with mean grain size of channel deposits, but decreasing as bankfull discharge increased. Bankfull mean velocity varied little from 0.5 to 0.7 m/s. With distance from the paleoshoreline, channels decreased in sinuosity and increased in sediment size, slope, and braiding index. Increase in proportion and connectedness of channel-belt deposits with distance from the paleoshoreline probably reflects upvalley increase in channel-belt-width/floodplain-width ratio rather than variation in deposition rate/avulsion frequency ratio. Du ing regression and transgression these spatial facies changes are reflected in, respectively, coarsening-upward and fining-upward vertical sequences. Eustasy probably had a minor influence on Frasnian regressive-transgressive sequences compared to the effects of varying rates of tectonic uplift, sediment supply, and subsidence. There is no evidence of long-term climatic change, either in the distribution of river-channel geometries or in the distribution of paleosols and plant and animal activity on the extensive floodplains.

Aeolian sedimentation from the Galtymore Formation (Devonian), Ireland

Summary The Galtymore Erg (Devonian) of the Galtee Mountains, southern Ireland can be reconstructed by the coupling of field relationships with current hypotheses and observations from other ancient aeolian sequences and modern ergs. Stratigraphical relationships show that the Galtymore Erg formed at the northern, fault-controlled, margin of the Munster Basin. Northern sequences, close to the basin margin, are dominated by northerly derived, proximal braided stream deposits. Two types of aeolian-dominant sequences occur within the Galtymore Formation. A marginal erg facies, characterized by simple crescentic dunes showing S-N palaeocurrents is interbedded with fluvial deposits. To the S an erg facies is developed which is characterized by the vertical repetition of cosets of large-scale cross-strata and thin horizontal deposits. The cosets are interpreted as representing the downwind migration and climbing of large dunes and interdune areas, respectively. The geometrical relationship of the hierarchy of bounding surfaces suggests that the dunes of the erg facies were compound crescentic in form, with enclosed interdune areas. The distribution of stratification types and foreset dip dispersion similarly suggest crescentic dunes. Estimates of compound dune wavelength and some dune and interdune dimensions can be made. Crescentic dunes (simple and compound) were moved by south-southeasterly palaeowinds towards the foot of alluvial fans.

Lower Jurassic Navajo-Aztec-Equivalent Sandstones in Southern Arizona and Their Paleogeographic Significance

Thick sequences of Lower Jurassic rhyolitic and andesitic volcanic rocks in several mountain ranges of southern Arizona contain interbedded quartzarenites. Locally up to 250 m thick, these sandstone lenses, composed of well-sorted and well-rounded quartz grains, commonly contain large-scale cross-stratification and are considered to be eolian sand deposits. The eolian sands were blown up against the continental side of the Early Jurassic volcanic arc that trended northwest-southeast across the southwestern margin of the North American continent and/or plate at that time. Paleocurrent data suggest southerly eolian transport of the sands from the Colorado Plateau area. Correlation of these sandstones with the Lower Jurassic Navajo and Aztec Sandstones is indicated by the pa eocurrent data as well as radiometric dating of the interbedded volcanics. Eolian sand transport southward across central Arizona in the Early Jurassic indicates that the Mogollon highlands either did not then exist, or were merely low, discontinuous inselbergs on a broad back-arc ramp, more appropriately called the Mogollon slope.

Palaeocurrent directions in the lower Dala sandstone, west central Sweden

Abstract A new wave of speculation on the palinspastic and plate tectonic development of the Baltic shield has underlined the need for palaeocurrent data coupled with interpretation of depositional environments for the sedimentary units in the shield. This paper presents data from the lower part of the Middle Proterozoic (Jotnian) Dala sandstone. Spectacular largescale cross-stratification at Risberg and Mångsbodarne is interpreted as being of aeolian origin. Dip directions of foresets indicate transport towards the northerly quadrant, but, as this reflects a wind direction, it does not tell anything about the regional palaeoslope at the time of deposition. On the other hand, tabular and trough cross-stratified sandstones with abundant intraformational clasts occurring at Fjätfallen are interpreted as being of fluvial origin. Foreset dip directions and trough axes indicate that the palaeocurrent at Fjätfallen was towards the south. This, being a direction of river flow, probably reflects the regional palaeoslope. This palaeoslope direction is consistent with at least one recent model for the palinspastic development of the Baltic shield.