Raindrop Impressions Research Articles

Trackways in the New Red Sandstone of the Connecticut River Valley, USA, the cradle of ichnology

Abstract Fossil trackways in the Jura-Triassic rift basin of the Connecticut River Valley are important to geoheritage for their quality, preservation, and role in the history of geology, especially of ichnology. Discovered in the early 1800s, these trackways of small to large, bipedal, three-toed animals came to the attention of Edward Hitchcock, a prolific geologist and natural theologian. His first publication on the subject was in January 1836 and, in the same year, William Buckland included parts of the report, with illustrations, in his Bridgewater Treatise. Hitchcock and Buckland are credited with establishing the field of ichnology, the study of track and trails. Within the first decade, other traces documented include four-footed tracks, fish fossils, invertebrate burrows, plant impressions, ripple marks and raindrop impressions; few bones have ever been discovered in the valley. Early fossil-track collectors included Hitchcock, James Deane, Dexter Marsh and Roswell Field. Specimens quickly made their way to Europe, and publications brought the tracks to international attention, in both science and literature. In nineteenth-century discussions, the three-toed trackway makers – birds or dinosaurs? – were particular subjects of interest and debate. These fossil trackways remain well known and studied today, with several preserved field sites and museum collections.

True substrates: The exceptional resolution and unexceptional preservation of deep time snapshots on bedding surfaces

AbstractRock outcrops of the sedimentary–stratigraphic record often reveal bedding planes that can be considered to be true substrates: preserved surfaces that demonstrably existed at the sediment–water or sediment–air interface at the time of deposition. These surfaces have high value as repositories of palaeoenvironmental information, revealing fossilized snapshots of microscale topography from deep time. Some true substrates are notable for their sedimentary, palaeontological and ichnological signatures that provide windows into key intervals of Earth history, but countless others occur routinely throughout the sedimentary–stratigraphic record. They frequently reveal patterns that are strikingly familiar from modern sedimentary environments, such as ripple marks, animal trackways, raindrop impressions or mudcracks: all phenomena that are apparently ephemeral in modern settings, and which form on recognizably human timescales. This paper sets out to explain why these short‐term, transient, small‐scale features are counter‐intuitively abundant within a 3.8 billion year‐long sedimentary–stratigraphic record that is known to be inherently time‐incomplete. True substrates are fundamentally related to a state of stasis in ancient sedimentation systems, and distinguishable from other types of bedding surfaces that formed from a dominance of states of deposition or erosion. Stasis is shown to play a key role in both their formation and preservation, rendering them faithful and valuable archives of palaeoenvironmental and temporal information. Further, the intersection between the time–length scale of their formative processes and outcrop expressions can be used to explain why they are so frequently encountered in outcrop investigations. Explaining true substrates as inevitable and unexceptional by‐products of the accrual of the sedimentary–stratigraphic record should shift perspectives on what can be understood about Earth history from field studies of the sedimentary–stratigraphic record. They should be recognized as providing high‐definition information about the mundane day to day operation of ancient environments, and critically assuage the argument that the incomplete sedimentary–stratigraphic record is unrepresentative of the geological past.

First record: Dendroidichnites (D. irregulare) in Late Triassic marginal lacustrine deposits of the Passaic Formation, west-central New Jersey

ABSTRACT Fine-grained siltstones of the Late Triassic Passaic Formation, near Milford, New Jersey have yielded the first evidence of the trace fossil Dendroidichnites irregulare in the Newark Supergroup of New Jersey. Associated trace fossils include Helminthoidichnites, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia Ichnofacies. Associated sedimentary structures include desiccation cracks, raindrop impressions, cross-bedding, and tool marks. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure.

Experimental rain prints and gas escape structures as a framework for interpreting circular imprints in shales of the Pottsville Formation (Pennsylvanian, Alabama)

AbstractCoal‐bearing, fine‐grained clastic rocks of the Pennsylvanian Pottsville Formation contain well‐preserved vertebrate and invertebrate trace fossils and disarticulated plant fossils. These fossil‐bearing rocks also contain small, circular imprints that resemble raindrop impressions but have also been interpreted as structures formed by gas bubble escape from wet sediment. Because rain prints are associated with arid, subaerially exposed depositional conditions and gas escape structures with water‐saturated, organic rich sediments, the interpretation of the circular structures profoundly impacts the palaeoenvironmental reconstruction and interpretation of the life habits of the trace‐makers. In this project, rain prints and gas escape structures were produced to compare these two hypotheses regarding mode of formation and to gain insight into interpretations of similar circular pits in the geological record. The results indicate that gas escape structures form most readily in fine‐grained sediment under water‐saturated conditions, in contrast to rain prints, which form most readily in unsaturated sand. A comparison with experimental structures suggests that the Pottsville shales preserve gas escape structures, not raindrop impressions, suggesting that the environment was rarely desiccated. Because rain prints and gas escape structures are superficially similar, genetic interpretations should not be based on these similarities, but rather on the differences observed between them. Reexamination of circular pits in the sedimentary rock record might require adjustment of environmental interpretations.

First record of the trace fossil Protovirgularia in the Passaic Formation (Late Triassic), Newark Supergroup, near Milford, New Jersey

Fine-grained siltstones of the Late Triassic Passaic Formation, near Milford, New Jersey, has yielded the first evidence of the trace fossil Protovirgularia in the Newark Supergroup of New Jersey. Associated trace fossils include Helminthoidichnites, Lockeia, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia ichnofacies. Associated sedimentary structures include desiccation cracks, raindrop impressions, tool marks, and cross-bedding. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure.

Fossils or sedimentary structures? Carbonaceous spheroids from the shale of the Cryogenian Nantuo Formation in Shengnongjia area, South China

Black shale of the Cryogenian Nantuo Formation, Shennongjia area, South China contains spheroids which co-occur with fossils of macroalgae. These three-dimensional structures occur sparsely as isolated specimens or abundantly as clusters which show different types of boundaries between individuals. They are generally circular on bedding surfaces and elliptical on transecting sections, and range up to several millimeters in diameter. Both petrographic thin sections and scanning electron microscopy (SEM) reveal that these spheroids are mostly organic carbon in composition and exhibit sharp, well-rounded external margins composed of microcrystalline clay minerals. The carbonaceous nature of these spheroids and different elemental abundances between spheroids and matrix are also confirmed by Raman spectroscopy and energy dispersive X-ray spectroscopy (EDS). Organic carbon isotopic values of individual spheroids range from −28.55‰ to −28.83‰, with an average of −28.72‰, which show indistinguishable δ13Corg values with the surrounding sedimentary matrix but exhibit statistically significant difference to those of contemporaneous macroalgae. A range of physical origins for these spheroids—including concretions, fluid escape structures, raindrop impressions, or bubble impressions—are considered and cannot be entirely rejected. Alternatively, biogenic origin is also a possible interpretation from observations of their morphological, mineralogical, geochemical, and sedimentological characteristics. Although their nature remains unresolved, the documentation of these new carbonaceous spheroids from the Nantuo Formation of Shennongjia area adds to a growing list of fossil-like structure records in the Cryogenian Period.

Formation of a hidden cavity below droplets impacting on a granular substrate

Droplet impact on a granular layer results in various morphologies of the liquid–grain mixture. Some are concentrated and highly curved, some are extended and flatter. No matter how the morphology looks from the top, it is generally believed that its bottom is tightly connected to the concavely deformed granular target. In this paper we report the discovery of a hidden cavity below a droplet residual, formed upon impact on packings of hydrophilic grains and exposed by X-ray tomography. Its occurrence in the parameter space is explored. We elucidate the mechanism leading to this counterintuitive phenomenon using a dual-curvature model and an energy criterion. This research may shed new light onto the ongoing discussion about the origin of the so-called fossilized raindrop impressions.

SHORT-TERM EVOLUTION OF PRIMARY SEDIMENTARY SURFACE TEXTURES (MICROBIAL, ABIOTIC, ICHNOLOGICAL) ON A DRY STREAM BED: MODERN OBSERVATIONS AND ANCIENT IMPLICATIONS

A wide variety of sub-ripple-scale sedimentary surface textures are known from bedding planes in the sedimentary rock record. Many of these textures were traditionally ascribed an abiotic origin (e.g., due to rain drop impact, adhesion, etc.), but in recent decades the role of microbial mats and biofilms in sculpting and mediating some forms has become increasingly recognized. Microbial sedimentary textures are now well-described and understood from modern tidal environments and biological soil crusts, but descriptions from fluvial settings are less common, despite their known occurrence in ancient alluvium. This paper reports a suite of primary sedimentary surface textures which were observed forming in discrete bodies of standing water in the lower reaches of the ephemeral Murchison River, Western Australia. Microbial sedimentary signatures included bubble impressions (burst and intact) and roll-ups, in addition to reduced horizons. Many of these features exhibited rapid temporal evolution of their morphology in the dry days following an interval of heavy rain. Significantly, these microbial features were witnessed in close spatial proximity to other abiotic and biotic sedimentary surface textures including raindrop impressions, adhesion marks, desiccation cracks, and vertebrate and invertebrate traces. Such proximity of abiotic and microbial sedimentary surface textures is rarely reported from bedding planes in the rock record, but these modern observations emphasize the fact that, particularly in non-marine environments, such structures should not be expected to be mutually exclusive. An appreciation of the fact that primary sedimentary surface textures such as these develop during intervals of stasis in a sedimentation system is crucial to our understanding of their significance and diversity in the rock record.

New dinosaur tracks from the Lower Cretaceous (Valanginian-Hauterivian) Saniri Formation of Yeongdong area, central Korea: Implications for quadrupedal ornithopod locomotion

Newly discovered tracks of a large quadrupedal ornithopod and a theropod with trail trace, and a bird track from the Cretaceous Saniri Formation of Yeongdong area are described. One ornithopod trackway is very unusual in having large (length width ∼15 and >15 cm respectively) clover-leaf-like manus tracks situated anteromedial to the pes tracks with atypical negative, inward rotation of 45°. Ornithopod pes tracks are quadripartite with three separated, elongate-oval, nearly parallel sided, wide digit impressions and a separate suboval heel impression (mean length and width about 41 and 36 cm respectively: l/w ratio 1.13). Manus track morphotypes are a clover-leaf-shaped configuration of three digit impressions, representing digits II–IV, in triangular configuration and registered just in front of (anteromedial to) pes track digits II and III. The pes morphotype is typical of Caririchnium, but the manus morphotype is quite distinct from previously described ichnotaxa, thus justifying a new ichnotaxon: Caririchnium yeondongensis ichnosp. nov., probably representing a facultatively-quadrupedal Iguanodon-like trackmaker.Theropod tracks are composed of three tapered pes digit impressions with interdigital angles between digits II and IV 45°. Length and width about 22.8 cm and 15.5 cm, respectively. Pace, stride, and pace angles are about 51 cm, 101 cm, and 170°, respectively. Theropod tracks are characteristically associated with a nearly continuous tail trace, which is up to 360 cm in length, 4.5 to 6 cm in width, and broad “U” shaped in cross section. Expulsion rims and dragging striae occur intermittently. In addition, dinosaur skin impressions, poorly preserved large sauropod tracks, a bird track, invertebrate and plant fossils are found from the lake margin deposits also containing rain drop impressions and desiccation cracks. Dinosaur tracks of the Yeongdong area represent the oldest (Valanginian-Hauterivian) dinosaur tracks of Korea.

First record of an arthropod from the Passaic Formation (Late Triassic), near Milford, New Jersey

AbstractMudstones of the Triassic Passaic Formation, near Milford, New Jersey, have yielded the first evidence of an arthropod impression in that formation. Associated trace fossils include Helminthoidichnites, Lockeia, Scoyenia, Spongeliomorpha, and the reptile footprint Rhynchosauroides, representing the Scoyenia ichnofacies. Associated sedimentary structures include desiccation cracks and raindrop impressions. The Passaic sediments were deposited under shallow water lacustrine shoreline conditions subject to periodic subaerial exposure.

Late Early Permian continental ichnofauna from Lake Kemp, north-central Texas, USA

Continental trace fossils of Early Permian age are well known in the western United States from Wolfcampian (~ Asselian to Artinskian) strata, but few examples are known from Leonardian (~ Kungurian) deposits. A substantial ichnofauna from strata of the lower part of the Clear Fork Formation at Lake Kemp, Baylor County, Texas, augments the meager North American record of Leonardian continental trace fossil assemblages. Ichnofossils at Lake Kemp occur in the informally-named Craddock dolomite member of the Clear Fork Formation, which is 12–15 m above the local base of the Clear Fork. The trace-bearing stratum is an up-to-0.3 m thick, laminated to flaser-bedded, dolomitic siltstone that also contains mud cracks, raindrop impressions, microbially induced mat structures, and some land-plant impressions. We interpret the Craddock dolomite member as the feather-edge of a marine transgressive carbonate deposit of an irregular coastline marked by shallow bays or estuaries on the eastern shelf of the Midland basin, and the trace-fossil-bearing stratum at Lake Kemp is an unchannelized flow deposit on a muddy coastal plain. The fossil site at Lake Kemp yields a low to moderately diverse fauna of invertebrate and vertebrate traces. A sparse invertebrate ichnofauna consists of arthropod feeding and locomotion traces assigned to Walpia cf. W. hermitensis White, 1929 and Diplichnites gouldi Gevers in Gevers et al., 1971. Tetrapod footprints are most common and assigned to Batrachichnus salamandroides (Geinitz, 1861), cf. Amphisauropus kablikae (Geinitz and Deichmüller, 1882), and Dromopus lacertoides (Geinitz, 1861), which represent small temnospondyl, seymouriamorph, and basal sauropsid trackmakers. Both the traces and sedimentary features of the fossil horizon indicate a freshwater setting at the time of track formation, and the trace assemblage represents the Scoyenia ichnofacies and the Batrachichnus ichnofacies in an overbank environment with sheet flooding and shallow ephemeral pools on an extensive coastal plain. The Lake Kemp tetrapod track assemblage is characteristic of the global Early Permian tetrapod ichnofauna found in red beds, which is dominated by a handful of ichnogenera that include Batrachichnus, Limnopus, Amphisauropus, Dromopus, Varanopus, Hyloidichnus, Ichniotherium and Dimetropus, which are the tracks of temnospondyls, seymouriamorphs, diadectomorphs, “pelycosaurs”, “captorhinomorphs”, and araeoscelids. The Lake Kemp tracks also further document the continuity of the ichnogenera Batrachichnus, Amphisauropus and Dromopus from Wolfcampian into Leonardian time and thus support the concept that Wolfcampian and Leonardian red-bed tetrapod footprints represent a single biostratigraphic assemblage.

History of Ichnology: The Reverend William Buckland (1784–1856) and the Fugitive Poets

William Buckland not only was an eccentric scientist liked by virtually all his peers, but in the early 1800s he was also one of the foremost authorities on geology. Buckland's vivid sense of humor and the fact that he enjoyed working on rather strange subjects, such as coprolites, rain drop impressions, cave paleontology, and fossil footsteps, provoked a great deal of light-hearted jesting from both his friends and his students. A collection of humorous poems and caricatures were produced to pay homage to the “Geologic Wizard.” Among these fugitive poets and artists were Buckland's good friends Phillip Duncan, Philip Shuttleworth, Henry De la Beche, Richard Whately, William Conybeare, and Thomas Sopwith. Buckland was still the subject of humorous verse well into the 1950s by the contemporary South African poet William Plomer.

PALAEOBIOLOGY OF THE CLIMACTICHNITES TRACEMAKER

Abstract: Trace fossils such as Climactichnites offer rare insights into the palaeobiology of Cambrian soft‐bodied animals, especially those that inhabited emergent sand flats and are not known from body fossils. Analysis of field and museum Climactichnites, together with experiments on the preservation of similar modern trails, indicates that the tracemaker was an elongate, bilaterally symmetrical, dorsoventrally flattened, soft‐bodied animal with a muscular foot. These characteristics are consistent with the tracemaker being a primitive mollusc or mollusc‐like animal. Unlike most Neoproterozoic and Cambrian molluscs, the tracemaker could reach considerable size; at up to c. 69 cm long, it was one of the largest Cambrian animals of its time. When moving on the sediment surface, locomotion resulted from muscular waves generated along the sole of its flexible foot; the foot was extended and then clamped onto the substrate. Contraction of pedal muscles then pulled the body forward. Sedimentary structures associated with Climactichnites wilsoni, such as polygonal desiccation cracks, raindrop impressions, adhesion structures and gas escape structures demonstrate that the animal inhabited intermittently subaerially exposed environments. The tracemaker’s method of locomotion is similar to that employed by modern intertidal gastropods, which make Climactichnites‐like trails on exposed sand flats. However, these modern trails are not preserved because of erosion by wind, waves, tides and subsequent bioturbation. Abundant microbial sedimentary structures are associated with C. wilsoni, and together with low levels of vertical bioturbation, intimate that microbial binding may have mediated the preservation of these early mollusc trails.

Walking through volcanic mud: the 2,100 year-old Acahualinca footprints (Nicaragua) II: the Acahualinca people, environmental conditions and motivation

We analyzed bare human footprints in Holocene tuff preserved in two pits in the Acahualinca barrio in the northern outskirts of Managua (Nicaragua). Lithology, volcanology, and age of the deposits are discussed in a companion paper (Schmincke et al. Bull Volcanol doi: 10.1007/s00445-008-0235-9, 2008). The footprint layer occurs within a series of rapidly accumulated basaltic–andesitic tephra that is regionally correlated to the Masaya Triple Layer Tephra. The people were probably trying to escape from a powerful volcanic eruption at Masaya Caldera 20 km farther south that occurred at 2.1 ka BP. We subdivided the swath of footprints, up to 5.6 m wide, in the northern pit (Pit I) into (1) a central group of footprints made by about six individuals, the total number being difficult to determine because people walked in each other’s footsteps one behind the other and (2) two marginal groups on either side of the central group with more widely spaced tracks. The western band comprises tracks of three adjacent individuals and an isolated single footprint farther out. The eastern marginal area comprises an inner band of deep footprints made by three individuals and, farther out, three clearly separated individuals. We estimate the total number of people as 15–16. In the southern narrow and smaller pit (Pit II), we recognize tracks of ca. 12 individuals, no doubt made by the same group. The group represented in both pits probably comprised male and female adults, teenagers and children based on differences in length of footprints and of strides and depth of footprints made in the soft wet ash. The smallest footprints (probably made by children) occur in the central group, where protection was most effective. The footprint layer is composed of a lower 5–15-cm thick, coarse-grained vesicle tuff capped by a medium to fine-grained tuff up to 3 cm thick. The surface on which the people walked was muddy, and the soft ash was squeezed up on the sides of the foot imprints and between toes. Especially, deep footprints are mainly due to local thickening of the water-rich ash, multiple track use, and differences in weight of individuals. The excellent preservation of the footprints, ubiquitous mudcracks, sharp and well-preserved squeeze-ups along the margins of the tracks and toe imprints, and the absence of raindrop impressions all suggest that the eruption occurred during the dry season. The people walked at a brisk pace, as judged from the tight orientation of the swath and the length of the strides. The directions of a major erosional channel in the overlying deposits that probably debouched into Lake Managua and the band of footprints are strictly parallel, indicating that people walked together in stride along the eastern margin of a channel straight toward the lake shore, possibly a site with huts and/or boats for protection and/or escape.

Subaerial gutter marks. Raindrop impressions, scour marks and other sedimentary structures, Silurian tymochtee dolomite, Waterville, Ohio

Field work was carried out on a four meter thick sequence of unfossiliferous Silurian Tymochtee Dolomite along the southeastern margin of the Silurian Michigan basin. Bedding plane exposures are common. The studied section contains a swale, gutter marks, scour marks, anhydrite molds, raindrop impressions, flat-pebble conglomerate, and ripple marks. Rocks with raindrop impressions formed in a subaerial environment. Many of the gutter marks form linear arrays. They originated as independent rivulets in a subaerial environment during a rain storm. No evidence of sheet flow has been found. The raindrop impressions associated with the gutter marks formed contemporaneously with the formation of the latter or shortly after. The present report documents the only known presence of gutter marks and raindrop impressions found in any Silurian strata anywhere within Michigan, Ohio and Indiana. Gutter marks formed in a subaerial environment may be more common than realized previously. Lastly, this paper argues that the size, shape, patterns, and presence of raindrop impressions has environmental significance beyond that of indicating subaerial exposure of a sediment.

Nematode trails from the late triassic of Pennsylvania

A new occurrence of nematode trails has been identified in cyclic lacustrine reddish brown siltstone deposits of the Late Triassic Fassaic Formation at Birdsboro, Pennsylvania. During lake regression, shoreline sediments covered by a thin film of water allowed free movement of the nematodes as well as production of surficial sinusoidal trails. Continued desiccation of these deposits resulted in mudcracking. Tool marks and occasional raindrop impressions point to subsequent rainstorms that carried in sediment which covered these previous traces. The assignment of the fossil sinusoidal trails to the ich‐nogenus Cochlichnus suggests that nematodes may have an ichnologic record that dates back to the Pre‐cambrian, and they are an important producer of this ichnotaxon.

Tidal Sedimentation from a Fluvial to Estuarine Transition, Douglas Group, Missourian--Virgilian, Kansas

ABSTRACT The Tonganoxie Sandstone Member of the Stranger Formation (Douglas Group, Upper Pennsylvanian, Kansas) was deposited in a funnel-shaped, northeast-southwest-trending paleovalley that was incised during the uppermost Missourian sealevel lowstand and backfilled during the subsequent transgression. Quarry exposures of the Tonganoxie near Ottawa, Kansas, include 5 m of sheetlike, vertically accreted siltstones and sandy siltstones, bounded above and below by thin coals with upright plant fossils and paleosols. Strata range from submillimeter-thick, normally graded rhythmites to graded bedsets up to 12.5 cm thick with a vertical sedimentary structure sequence (VSS) consisting of the following intervals: (A) a basal massive to normally grad d interval; (B) a parallel-laminated interval; (C) a ripple-cross-laminated interval; and (D) an interval of draped lamination. The VSS-C intervals of thicker bedsets are characterized by climbing ripples that evolve from Type A (erosional-stoss) to Type B (depositional-stoss). Synsedimentary convolutions at the tops of many climbing-ripple sequences and a variety of water-escape structures indicate rapid deposition. The vertical sequence of sedimentary structures indicates each bedset was deposited by a waning current with significant suspended load. The Tonganoxie succession has many similarities to fluvial overbank/floodplain deposits: sheetlike geometry, upright plant fossils, lack of bioturbation and body fossils, dominance of silt, and a punctuated style of rapid sedimentation from suspension-laden waning currents. Missing, however, are thick clay drapes or evidence of prolonged exposure and desiccation, which generally characterize a floodplain sequence with seasonal overbanking. Physical and biogenic sedimentary structures--including tetrapod trackways, surface grazing traces, abundant raindrop impressions, wind ripples, runnel marks, runnel marks, and runoff washouts--indicate that subaerial exposure was periodic and brief, and may have followed each sedimentation event. Analysis of stratum-thickness variations through the succession suggests that tides significantly influenced sediment deposition. Strata ranging through three orders of magnitude systematically thicken and thin, recording the influence of an ebb-dominated, diurnal tidal system with a well developed semimonthly inequality. By conservative estimate of sedimentation rates based on neap-spring tidal cycles, the sequence aggraded at an average rate of approximately 3.8 m/yr. These unusually high rates appear to have prevailed for only a short time and were probably spatially restricted within the basin. A fluvial-to-estuarine transitional depositional setting is interpreted for the Tonganoxie by analogy with modern depositional settings that show similar physical and biogenic sedimentary s ructures, vertical sequences of sedimentary structures, and aggradation rates.

Modern and Fossil Raindrop Impressions as a Lesson in Interpretation of Ancient Sedimentary Features

Raindrop impressions which duplicate fossil raindrop impressions from Jurassic-Triassic sediments can easily be produced today. The modern and fossil imprints can then be used as the basis for both a qualitative and quantitative lesson in the interpretation of ancient sedimentary features.

Why Not Raindrop Impressions?

ABSTRACT Field observations show that most raindrop impressions are produced during the last vestiges of shower activity when the rain has almost ceased to fall. These fewer number of raindrops impact upon portions of the mud surface that have the right degree of saturation to retain impressions. This accounts for the isolated nature of most modern and fossil raindrop impressions. While not all so-called raindrop impressions are considered to have formed due to the impact of raindrops, one can not eliminate their important role.

Permian White Rim Sandstone Member of Cutler Formation: Coastal Dune Field, Utah: ABSTRACT

The White Rim Sandstone Member of the Permian Cutler Formation in Canyonlands National Park, Utah, was deposited in a coastal eolian environment. This conclusion is based on sedimentary structures, petrologic features, and stratigraphic relations. The White Rim consists of two major genetic units. These units may represent a coastal dune field and related interdune ponds. Distinctive sedimentary structures of the coastal dune unit include large to medium-scale, unidirectional, tabular planar cross-bedding; high-index ripples paralleling dip direction of the foresets; raindrop impressions; slump marks; and coarse-grained lag layers. Also, a predominant northwest to southeast orientation of the cross-bedding agrees with the paleowind direction proposed in this area for Perm an time. Distinctive sedimentary structures of the interdune pond unit include wavy, horizontally laminated bedding, adhesion ripples, and desiccation(?) polygons. These features may have been produced by water-table fluctuations. Petrologic characteristics of the White Rim help to define the depositional environment as coastal. A crinoid fragment was identified at one location; heavy minerals are present in both units; and small amounts of well rounded, reworked glauconite are present throughout the study area. The White Rim Sandstone Member was deposited during a period of changing sea level and migrating strandlines. Continental sedimentation was dominant in eastern Utah, along the ancestral Uncompahgre Mountains; and marine deposition was prevalent in western Utah. Rocks deposited in the two environments interfinger in the Canyonlands area. Previous authors have proposed that the White Rim represents either a shallow-marine or eolian facies equivalent to both the upper part of the Toroweap Formation and the Gamma Member of the Kaibab Limestone. Results from this study suggest that the White Rim more likely represents eolian deposition on a prograding shoreline characterized by a semiarid climate and predominantly onshore winds. End_of_Article - Last_Page 570------------