Symmetrical Ripple Marks Research Articles

Cratonal tidal deposits of the Paleoproterozoic Mesnard Quartzite, Lake Superior region: interpretation challenges

The Mesnard Quartzite of Paleoproterozoic age is believed to represent among the first preserved cratonal continental rocks in the Lake Superior region, was deposited above Archean basement in the Marquette structural trough and is estimated to contain more than 99% sandstone (now quartzite) beds. The Mesnard Quartzite is interpreted to have been deposited largely by tidal currents. Evidence of tidal current deposition includes lack of wave-influenced structures such as hummocky cross-beds, and presence of strongly unidirectional paleocurrents from cross-beds, mud-drapes on cross-bed foresets, evidence of current reversal including herringbone cross-bedding and sigmoidal cross-bedding. Sandstone bedding types include homogeneous or structureless beds, parallel-laminated beds, cross-beds, and symmetric ripple marks. Unidirectional paleocurrents from cross-beds are largely subparallel to the axis of the Marquette structural trough, leading to the interpretation that the trough was subsiding during deposition. Cross-bed set thickness and symmetric ripple marks both support the interpretation of shallow water marine deposition, likely only a few meters. Although the Mesnard Quartzite is on the order of 110 m thick, there is no evidence of internal sequence boundaries or flooding surfaces, and low accommodation during deposition is inferred. The climate during deposition is interpreted to be humid, and tropical to subtropical based on the quartz arenite composition. Deposition likely occurred in a failed rift basin or in an intracratonic extensional basin.

Paleoenvironmental significance and provenance of the Cretaceous calcareous deposits from the Djerem sub-basin (Adamawa, Cameroon) during the Gondwana evolution: sedimentary structures and geochemical constraints

The calcareous deposits were affected by syn-sedimentary (lamination, bedding, convolute, ripple marks, pseudo-nodule) and post-sedimentary structure (diaclase, fault, and desiccation crack). Lamination and bedding expressed by color variation and granulometry are linked to a calm period of sedimentation and seasonal fluctuations and are periodically affected by seismic shocks giving the convolute bedding and pseudo-nodules. The presence of symmetrical ripple marks and desiccation cracks indicate respectively back and forth water movements and periodic phases of emersion characteristics of lacustrine to swampy environments. According to geochemistry data, the major element composition classifies the studied samples as shales and wackes. High LREE/HREE ratios (9.07–13.08; average: 11.77); slight positive and negative Eu anomaly (0.98–1.11); Al2O3/TiO2 ratios (16.52–21.87; average: 19.73), Th/Co ratios (1.24–4.18; average: 2.26), La/Sc versus Th/Co and Zr versus TiO2 plots indicate sediments derived from felsic rocks. The K2O/Na2O ratios (0.24–1.90; average: 1.01); index of compositional variability (0.88–1.35); PIA (64.94–98.42) and the Zr/Sc versus Th/Sc diagram indicate that the source rocks have experienced globally a moderate recycling and sorting as well as moderate weathering. The chemical index of alteration values and the SiO2 versus Al 2 O 3 +K 2 O+Na 2 O diagram could indicate relatively warm and arid to semi-arid climate during deposition of sediments. The Sr/Ba (0.22–2.01; average: 0.58) ratios indicate a variant salinity. The Y/Ho (<40) and U/Th (<0.75) suggest respectively nonmarine environment and oxic conditions. The Djerem continental sub-basin would not have undergone marine influences yet known in the Nigeria basins of Benue.

The largest species of &amp;lt;italic&amp;gt;Asianopodus&amp;lt;/italic&amp;gt; footprints from Junggar Basin, Xinjiang, China

A discovery has been made where thirteen footprints, two invertebrate traces and symmetrical ripple marks were found in a greyish green calcareous fine sandstone bed. The discovery was found in the Lower Cretaceous Shengjinkou Formation of Urho area, northwestern Junggar Basin, Xinjiang and it formed three trackways and four isolated tracks. Classic methods were used to study the dinosaur footprints in the Urho area, which included measuring the stratigraphic sections and the footprint parameters in the field. Similar standards of measurements were used for the measurement of footprint length and width, digit length and width, the divarication between digits, the length and width of anterior triangle, pace and stride lengths. In addition, photos of each footprint were taken and the orientation and distance between the neighboured tracks were measured. Distributions and outlines of the footprints were drawn in the computer by using CorelDRAW. On the other hand, silicone rubber demould was used for the well preserved specimens for a detailed research and from the detailed study of the footprints and trackways, two different types of theropod footprints were identified. The large three-toed footprints were a new species, named Asianopodus niui ichnosp. nov., which are large sized tridactyl (47.0–56.0 cm long and 31.0–42.0 cm wide, width/length between 0.66–0.75), the divarication between digits II and IV is 49.00°–55.34°, subsymmetrical, digit III is slightly longer than digit II, but more longer than digit IV, the length/width of anterior triangle between 0.34–0.37, V-shape tracks with a distinct bulbous heel impression, without manus and tail impressions, the phalangeal formula is 3-3-2, the pace length is between 164–180 cm, the stride length is 328–336 cm. It is significant to note that this is the largest Asianopodus footprints that have been discovered. The medium-sized tracks belong to Asianopodus pulvinicalx , which are middle sized tridactyl (21.0–27.0 cm long and 18.0–20.0 cm wide, width/length between 0.72–0.86), the divarication between digits II and IV is 46.63°–51.40°, subsymmetrical, digit III is more longer than digit II and digit IV, the length/width of anterior triangle between 0.44–0.54, V-shape tracks with a distinct bulbous heel impression and without manus and tail impressions, the phalangeal formula is 3-3-3, the pace length is between 76.5–83.2 cm, the stride length is 160 cm. According to the footprint length, pace and stride length, relative stride length of the tracks, we calculated that the body length of A. niui ichnosp. nov. is 5.89 m and the speed is 8.14 km/h, while the body length of A. pulvinicalx is 2.52 m and speed is 6.48 km/h. Based on the symmetric ripple marks and invertebrate traces on the surface of the footprints layer, we infered that both the trackmakers of the Urho dinosaur footprints walked in the shore and shallow lacustrine environment.

Tafonomia de biválvios em calcários oolíticos da formação Teresina (Bacia do Paraná, Permiano Médio, Prudentópolis, PR)

Herein, it is presented the first detailed taphonomic study on bivalve mollusk shells preserved in the oolitic limestones of the Teresina Formation (probably Kungurian-Roadian, Lower-Middle Permian) in the eastern margin of the Parana basin. The selected beds are located in two quarries (informally named PRU 1 and PRU 2) in Prudentopolis municipality (Center-South Parana State), and positioned approximately in the middle of the formation and probably in the Pinzonella illusa Zone. The PRU 1 limestone (~30 cm thick), which is partially silicified and intercalated with predominantly pelitic rocks, is classified as a bivalve oolitic grainstone. The basal contact is erosive and the top shows symmetrical ripple marks, which are draped by shale with mud cracks. There are two fining-upwards successions characterized by dense to dispersed packing of the shells, which are usually disarticulated, randomly oriented (many nested/stacked) and mixed with some pelitic intraclasts. Microhummocky cross-stratification occurs a little below the top of the bed. The PRU2 bed is classified as ooid-bivalve rudstone (~5 cm thick), where all shells are disarticulated and fragmented, showing dense packing. The bivalves probably inhabited a muddy substrate and were mixed (as parautochtonous and allochthonous bioclasts) with ooids during high-energy storm events, including posterior shell displacement as a result of bioturbation. Thus, the calcareous beds represent amalgamated proximal tempestites with a complex taphonomic history, strong temporal/spatial mixing of bioclasts and limited paleoecological resolution. They are a typical example of shell beds generated in a huge epeiric sea, which was not necessarily connected to the ocean and where very low depositional-slope gradient, very slow subsidence and minimum sediment accommodation space caused frequent sediment reworking by storm related processes.

Ostracodes as indicators of brackish water environments in the Catskill Magnafacies (Devonian) of New York State

Leperditiid and beyrichiid ostracodes in marine margin deposits of the Devonian Catskill magnafacies of New York State document brackish conditions in what were previously considered freshwater floodplain deposits. The very low diversity ostracode fauna (two species) includes an abundance of anomalously thin-shelled individuals that belong to the genera Leperditia? and Welleria. This ostracode fauna is one of the oldest reported from brackish water in the stratigraphic record, and it documents the early ecological radiation of ostracodes into waters of reduced salinity. Associated fossils, including the bivalve Archanodon and the conchostracan Euestheria, have been considered to be freshwater forms, partly because of their occurrence in these rocks that were assumed to be fluvial in origin. Nearly everywhere, however, the ostracodes are associated with cosmine-bearing fish fragments, lingulid brachiopods, and the feeding trace Spirophyton, which have a marine or brackish-water affiliation. The presence of open carapaces on bedding planes as well as adults and several instar stages of each ostracode species indicate lack of postmortem transport. At least one ostracode-bearing mudstone contains the mineral amesite; associated calcareous mudrocks interbedded with these units contain symmetrical ripple marks, mudcracks, and locally intense manganese-oxide staining. Collectively, these features indicate extreme environmental conditions and suggest highly variable salinities in shallow pools or embayments along the paleoshoreline. The presence of brackish-water faunas, perhaps estuarine in origin, indicates an aspect of the diversity of Devonian paleoenvironments along this portion of the Catskill deltaic complex not previously recognized. The deposits are widespread, although not ubiquitous, within thick sequences of fine-grained flood-basin deposits. Because most previous paleoecological studies of fossils from these rocks assumed the sediments to be of freshwater origin, the new interpretation of brackish-water communities presented here could have important implications for understanding the evolution of early terrestrial organisms. For example, some animals may have evolved in brackish estuarine environments rather than in freshwater rivers as assumed in previous studies. Recognition of this brackish facies elsewhere in the Catskill succession will not only aid future identification of paleoshoreline position but may also help clarify whether early terrestrial organisms made the transition to land via brackish or freshwater environments.

Geology of Great Abaco Submarine Canyon (Blake Plateau): Observations from the research submersible “Alvin”

Nine dives in the research submersible “Alvin” were made into Great Abaco Submarine Canyon to depths ranging from 1850 to 3666 m. Our observations indicate that the walls of this canyon are distinctly terraced, consisting of nearly vertical to overhanging rock cliffs and intervening, less steep sediment-covered slopes. The wall rock consists mostly of massive, shallow-water limestones and dolostones of Cretaceous age, coated on exposed surfaces with manganese oxides. These rocks are heavily jointed/fractured and thus very blocky to angular in appearance, with sponges and other sessile organisms commonly attached. Talus slopes and sedimentary breccia deposits containing angular boulders are present at the base of these steep escarpments. Short-term bottom current measurements in the axis of the eastern part of the canyon indicate that currents are relatively weak, reaching velocities of only 10 cm/sec. This relatively placid setting is further corroborated by the abundance of turtle grass ( Thalassia) found along the canyon axis. However, abundant subdued, symmetrical ripple marks and large scour depressions at the base of boulders, indicate that high-energy events sporadically impact the canyon axis. Contemporary erosional activity along the axis of the western (headward) part of the canyon appears to be more significant, as evidenced by asymmetrical ripple marks, sand waves and bioerosion. Great Abaco Canyon has evolved with time via a variety of processes, including: (1) faulting: (2) subsidence; (3) defacement; and (4) erosional down-cutting. The location, orientation and initiation of this canyon appear to be structurally controlled by the Great Abaco Fracture Zone during pre-Santonian time. Regional subsidence during the Mesozoic allowed the walls of Great Abaco Canyon to build vertically by accretion of shallow-water limestones, whereas joint-controlled defacement has widened the canyon while maintaining steep walls. Erosional down-cutting in the canyon axis by carbonate sediment gravity flows also appears to have been important episodically, particularly during the Miocene and Pleistocene.

Intertidal sediments in the Auchenhew Beds (Triassic) of Arran

Synopsis Study of a 45 m succession of the Auchenhew Beds (Triassic) of western Arran, with a record of halite pseudomorphs, symmetrical ripple marks, flaser, wavy and lenticular bedding, bimodal palaeocurrents and particular vertical arrangements of lithologies and structures, has provided evidence of intertidal conditions, probably within the context of a relatively arid, low energy, muddy shoreline. The new observations lend support to earlier suggestions of a marine connection and influence at this time and necessitate some reconsideration of the environmental significance of trace fossils previously described from this area.

The Orientation of Fossil Bivalves in a Pene-littoral Sequence (The Rhaetian of South Wales)

ABSTRACT Primary flow-vector criteria (symmetrical ripple marks) derived from a sequence of sublittoral to possible shoreface sediments of Late Triassic age in South Wales yield well-defined orientations which are nearly identical at a number of widely separated outcrops and attest the prevalence of northwest-southeast oscillatory flow in this region. The bivalves Modiolus hillanus (J. Sowerby) and Rhaetavicula contorta (Portlock) occur abundantly in this sequence and at a few horizons these fossils display preferred orientations which are broadly consistent with the ripple data. However the fossil orientation patterns are not, in general, directly compatible with published experimental data and it is suggested that this discrepancy results from subsequent modification of origina wave-formed patterns in response to weak unidirectional (current) flow, possibly induced by tidal or longer period currents, which otherwise has left no discernible evidence in the fine-grained sediments.

Sedimentary and structural features of the Bell shale correlate (Early Devonian), Strahan quadrangle, western Tasmania

The sequence of interbedded mudstone, siltstone and very fine sandstone in the Strahan Quadrangle, correlated with the Bell Shale is at least 500 m thick. Two broad associations can be defined. The ratio of very fine sand to mud in the lower association is between 3:2 and 2:3 whereas in the upper association only occasional thin sandstone beds occur. Sedimentary structures such as lenticular bedding, symmetrical ripple marks and the lack of deep water sedimentary features suggest that deposition took place in shallow water. This is supported by palaeontological evidence. Some scouring and upward fining beds suggest that deposition from waning currents occurred sporadically within this environment. The Bell Shale correlate has been folded during two phases. The first phase produced folds plunging between 300 and 900 NW and the second phase produced folds plunging shallowly WNW or ESE. Cleavage was developed only locally during each phase. Folds of both phases produced a primary axial surface cleavage and the second phase produced crenulation cleavage in some areas. The amplitude of folds produced during the first phase decreases towards the centre of the basin. The locus of strain during the second phase of deformation was along the Firewood Siding Fault. The existence and timing of these deformation events demonstrates the uniformity in orientation of Tabberabberan structures throughout north and western Tasmania.

Sediment Dispersal Patterns of Oolitic Calcarenite in the Sundance Formation (Jurassic), Wyoming

ABSTRACT The Hulett Sandstone Member of the Sundance Formation (Callovian and Oxfordian, Jurassic) exposed along the eastern flank of the Bighorn Basin, Wyoming, occurs as lenticular and discontinuous units of oolitic calcarenite, whose dominant internal structure is trough cross-stratification. This crossbedding type indicates that the units may have formed as large-scale, asymmetrical sand waves. The near north-south bipolar distribution of crossbed inclination directions at individual localities suggests that tidal current action, probably in the subtidal zone, was responsible for the formation of the units. The primary mode of the directions measured perpendicular to small-scale symmetrical ripple marks is aligned near north-south and corresponds with crossbed data. An exhumed calcarenite body of the Hulett Sandstone that appears to be a composite fossil sand wave was found. The length of this body is roughly transverse to the local bipolar crossbed modes. The shape and structure of the body supports the view that it and other less well exposed bodies of the Hulett Sandstone are large-scale, composite sand waves.

Evidences of environment of deposition of red High Falls Shale, New York

ABSTRACT Field investigation of the Silurian High Falls Shale at Pine Hill, New York, has yielded well-preserved Lingula specimens. The distribution of Lingula shells on shale bedding planes suggests the action of gentle waves on tidal mud flats. High hematite content of the shales, shallow-penetrating mudcracks, and symmetrical ripplemarks are included among other evidence that the High Falls was deposited under littoral marine conditions.

Sedimentary structures in Huronian rocks of Ontario

A study of cross-bedding in the southern part of the Huronian outcrop belt indicates that the sediments were transported in a southerly direction (vector mean 160°). Symmetrical ripple marks show a similar preferred orientation (178°), and, together with dessication cracks, indicate shallow water conditions. Dimensional fabric of embedded clasts in massive polymictic conglomerates of the Gowganda Formation probably reflects glacial transport in a southerly direction. Clastic dykes in the Espanola and Serpent Formations are interpreted as injection structures from the underlying Bruce conglomerate. The dykes and ball and pillow structures may indicate tectonic instability during sedimentation. In the Cobalt-Gowganda area both directional sedimentary structures and preferred orientation of clasts in the Gowganda conglomerates indicate a southerly direction of transportation. A study of cross-bedding in the region around the Sudbury Basin suggests the presence of a positive area there in early Huronian times. In the Bruce Mines-Blind River area dimensional fabric of the Gowganda conglomerates suggests that the glaciers moved in a northeast-southwest direction in response to a regional palaeoslope. However, directional structures in associated waterlaid sediments indicate transport to the southeast, possibly reflecting a local southeasterly slope in the Huronian sedimentary basin. The presence of flute casts, groove casts, and graded bedding in the waterlaid sediments of the Gowganda Formation suggests the possibility of the activity of turbidity currents, but many other lines of evidence support a glacial origin for the conglomerates and laminated argillites.

Shallow Marine Currents on the Early (?) Triassic Wyoming Shelf

ABSTRACT Analysis of 690 ripple mark and 40 cross-stratification paleocurrent directions from 37 localities in the Red Peak Formation of Wyoming indicates three major current directions: northeast, southwest, and northwest. These are interpreted to respectively represent wave drift, rip, and longshore currents moving on, off, and along a northwest-southeast oriented coast that bordered the shallow, open marine, Wyoming shelf embayment during the Early (?) Triassic. Commonly used interpretive techniques based on calculated vector means of paleocurrent directions are inadequate for the polymodal distributions in the Red Peak. Instead, the major directions at each locality are determined and the resulting pattern interpreted visually. The use of vector means and moving averages would not define and could possibly mask the complex current patterns to be expected in the shallow, open marine environment. Ripple marks have been used infrequently as paleocurrent direction indicators. The long standing misconception equating external shape with mode of formation has inhibited their use. Evidence from the Red Peak supports other studies which have suggested that and asymmetric ripple marks are not wave-and current-formed respectively. Rather, most ripple marks possess some degree of asymmetry, either in shape or internal structure, and are formed by currents (including most waves in shallow-water). Truly symmetric ripple marks formed by standing oscillatory waves are probably rare. Being current-formed, ripple marks should prove increasingly important in reconstructing paleocurrent systems and paleogeography.

Shallow-Marine Currents on Early(?) Triassic Wyoming Shelf: ABSTRACT

Analysis of 690 ripple-mark and 40 cross-stratification paleocurrent directions from 37 localities in the Red Peak Formation of Wyoming indicates three major current directions: northeast, southwest, and northwest. These are interpreted to represent respectively, wave drift, rip, and longshore currents moving on, off, and along a northwest-southeast-oriented coast that bordered the shallow, open-marine, Wyoming shelf embayment during the Early(?) Triassic. Commonly used interpretive techniques based on calculated vector means of paleocurrent directions are inadequate for the polymodal distributions in the Red Peak. Instead, the major directions at each locality are determined and the resulting pattern interpreted visually. The use of vector means and moving averages would not define and could possibly mask the complex current patterns to be expected in the shallow, open-marine environment. Ripple marks have been used infrequently as paleocurrent direction indicators. The long-standing misconception equating external shape with mode of formation has inhibited their use. Evidence from the Red Peak supports other studies which have suggested End_Page 544------------------------------ that and asymmetric ripple marks are not wave- and current-formed, respectively. Rather, most ripple marks possess some degree of asymmetry, either in shape or internal structure, and are formed by currents (including most waves in shallow water). Truly symmetric ripple marks formed by standing oscillatory waves probably are rare. Being current-formed, ripple marks should prove increasingly important in reconstructing paleocurrent systems and paleogeography. End_of_Article - Last_Page 545------------

Paleocurrents and Shoreline Orientations in Green River Formation (Eocene), Raven Ridge and Red Wash Areas, Northeastern Uinta Basin: REPLY

Paleocurrent data from ripple marks and cross-stratification are related to orientations of shorelines and sandstone-body trends in the lacustrine and fluvial setting of the Green River Formation (Eocene) in the Red Wash field and the adjacent outcrops along Raven Ridge in Utah and Colorado. At 11 localities along Raven Ridge, the northeastern margin of the Uinta basin, 125 paleocurrent directions were measured from cross-stratification and asymmetrical ripple marks in the Douglas Creek and Garden Gulch Members and the lower part of the Parachute Creek Member. Vertical stratigraphic variation of paleocurrent directions at each locality is small, indicating that the over-all current system was stable. A plot of measurements of 84 asymmetric and 68 symmetric ripple marks shows that their distribution is similar, which is interpreted to be the result of their formation by the same current system. Based on arcs of azimuths, there is essentially no difference between paleocurrent directions from cross-stratification and from ripple marks. The dominant paleocurrent directions are toward the north, south, and southeast. Of all observations, 25 per cent range from 331° to 30°, and 51 per cent range from 121° to 210°. The shorelines in the northeastern Uinta basin area are interpreted to have been generally at right angles to the dominant paleocurrent directions. Therefore, essentially all of the shorelines had bearings of 31° to 120°. An arc of 61°-90° would contain about 40 per cent of the bearings of the shorelines, based on the paleocurrent data. Trends of single sandstone bodies, the total footage of sandstone, sandstone plus siltstone, and net sandstone in the Red Wash field, and the trends of major facies in the northeastern Uinta basin support the generalizations about the orientations of shorelines and sandstone-body trends.

Paleocurrents and Shoreline Orientations in Green River Formation (Eocene), Raven Ridge and Red Wash Areas, Northeastern Uinta Basin, Utah: ABSTRACT

Paleocurrent data from ripple marks and cross-stratification are related to orientations of shorelines and sandstone-body trends in the lacustrine and fluvial setting of the Green River Formation (Eocene) in the Red Wash field and the adjacent outcrops along Raven Ridge in Utah and Colorado. At 11 localities along Raven Ridge, the northeastern margin of the Uinta basin, 125 paleocurrent directions were measured from cross-stratification and asymmetrical ripple marks in the Douglas Creek and Garden Gulch Members and the lower part of the Parachute Creek Member. Vertical stratigraphic variation of paleocurrent directions at each locality is small, indicating that the over-all current system was stable. A plot of measurements of 84 asymmetric and 68 symmetric ripple marks shows that their distribution is similar, which is interpreted to be the result of their formation by the same current system. Based on arcs of azimuths, there is essentially no difference between paleocurrent directions from cross-stratification and from ripple marks. The dominant paleocurrent directions are toward the north, south, and southeast. Of all observations, 25 per cent range from 331° to 30°, and 51 per cent range from 121° to 210°. The shorelines in the northeastern Uinta basin area are interpreted to have been generally at right angles to the dominant paleocurrent directions. Therefore, essentially all of the shorelines had bearings of 31° to 120°. An arc of 61°-90° would contain about 40 per cent of the bearings of the shorelines, based on the paleocurrent data. Trends of single sandstone bodies, the total footage of sandstone, sandstone plus siltstone, and net sandstone in the Red Wash field, and the trends of major facies in the northeastern Uinta basin support the generalizations about the orientations of shorelines and sandstone-body trends.

Asymmetrical ripple marks and the origin of water‐laid Cosets of Cross‐Strata

AbstractAsymmetrical ripple marks formed by flowing water are described and classified on geometrical form, physical size, and hydrodynamic conditions of origin. By analysing models it is demonstrated that the character of cosets of cross‐strata resulting from the migration of asymmetrical ripple marks is dependent on ripple geometry and sediment supply. Predicted cross‐stratification patterns are shown to agree well with those described from water‐laid sediments. The use of cross‐strata in deducing sedimentation rates and depths of deposition is pointed out. Models are developed from natural situations for interpreting cross‐stratal dip direction patterns in shallow marine and deltaic sediments.

Ripple Marks Cut on Ordovician Limestone Pebble Conglomerates, Stansbury Range, Utah

Ripple marks formed in consolidated sediments without movement of the constituent particles occur in the Lower Ordovician Garden City Formation in the Stansbury Mountains, 40 mi W. of Salt Lake City. These marks were cut in lithified limestone pebble conglomerates. The symmetric ripple marks have wave lengths of 2-3 in. and amplitudes up to 1/2 in. The overlying beds are argillaceous limestones braided with siliceous silty laminae. The evidence indicates that the first silty sediment was set in wavy motion on the floor of a body of water, and the loose silt scoured the troughs and rounded the crests. The position of the troughs did not change or did not change fast enough to erase the marks. The braided lamination, as viewed in cross section, probably was caused by similar wavy motion of the water acting on unconsolidated clay and silt size sediments. Ripple marks in these sediments could not be retained completely and the remnants of such marks were further distorted during compaction. Another type of bedding surface mark in which the uppermost pebbles have been truncated by eroding silt is also present. These marks are rounded ridges up to 4 in. high, 18 in. wide, and from 23-57 in. from crest to crest. These ridges have sinuous trends and are discontinuous. They are formed of well-rounded limestone pebbles cemented by calcareous muds. The overlying beds are limestone braided with siliceous silt. The truncation of the uppermost pebbles in this case may have been a final modifying process on pre-existing ridges.

Facies and Sedimentary Environments of Albert Shale, New Brunswick

Three lithologic units can be recognized in the Mississippian Albert Formation in New Brunswick, Canada: the Dawson Settlement, Frederick Brook, and Hiram Brook Members. Typically non-marine redbeds lie above and below the grayish beds of the oil- and gas-bearing Albert, although marine evaporites also overlie the Albert in the east. Mud balls and mud cracks, plant remains, and abundant wave-formed ripple-marks evidence the continental origin of all three divisions of the formation. In fact, resemblance to the Green River (Eocene) lake-deposited oil shale of the West is remarkable. Trends of the symmetrical ripple-marks (which more-or-less parallel the shoreline) support lithofacies data, and serve to define the limits of Mississippian Lake Albert. Semi-arid to arid evapo itic conditions marked the last stages of Albert deposition. The principal source of hydrocarbons appears to have been the abundant palaeoniscid fish of the Frederick Brook Member, which are here confirmed as fresh-water lake inhabitants. The possibility of finding similar oil- or gas-bearing lake beds in hitherto-neglected areas of continental deposition is apparent.

Longitudinal Ripple Marks in Mud and Sand

ABSTRACT In the shallow Waddensea, off the coasts of Holland, Germany, and Denmark, the bottom is uncovered during ordinary low tide. Observations and measurements of ripple marks in both mud and sand, correlated with observations of environmental factors such as wind, waves, and currents, indicate that ripple marks in mud generally are symmetrical and are formed with their crests parallel to the direction of movement of currents. Normal, asymmetrical current ripples in sand patches are formed with their crests perpendicular to the crests of the symmetrical ripple marks in the surrounding mud. In addition to these transverse ripple marks, longitudinal ripple marks also are formed in sand, in some places in combination with the transverse marks. It is suggested that fossilized longitudinal ripple marks may be used as an indication of extremely shallow water in which currents of long duration acted in a constant direction. Moreover, they may provide indications of the orientation of nearby banks or shores.