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Load Casts at Pass Lueg
View looking up at the underside of a C unit in a Lofer cyclothem. The numerous bulbous forms which project downwards are called load casts. Load casts are often associated with turbidites. One bulb would be around 10-20 cm in diameter.
Cross-sets
Sedimentary structures like these cross-sets form through the migration of sand dunes on the sea bottom under the action of a current. They can be used to infer flow directions and energy, and even aid in the interpretation of the general depositional system. Pleistocene calcarenites from Le Castella, Calabria, Italy.
Asymmetric ripples
Current ripples are small asymmetric bedforms generated by movement of fine-grained sediment under unidirectional currents. The presence of climbing ripple lamination indicates rapid deposition from a sediment-laden flow. Upward transition from planar lamination to current ripple lamination is indicative of flow deceleration. Sequence of sedimentary structures (cf. Bouma sequence), grain size decrease, and cyclical repetition of bedding points to deposition from turbidity currents. Photo taken at the Varvito Park, Ito, Brazil, with exposures of Permian strata from the Itararé Group. Scale in cm.
Climbing ripples
When a fluid laden with sediment rapidly drops some of its load as it is flowing, a sedimentary structure named "climbing ripple lamination" can form. It is characterized by superimposed ripple crests (or ripple cross-sets) migrating in the direction of flow. Photo taken at the Varvito Park, Ito, Brazil, with exposures of Permian strata from the Itararé Group. Scale in cm.
Mud cracks and ripples
On the surface of this Cretaceous mudstone bed from the Sousa Basin of Brazil, one can see both polygonal cracks (indicative of subaerial exposure) and wave ripples (indicative of a thin cover of water).
Ripple cross-lamination
Cross-sectional view of a calcareous mudstone to fine-sandstone bed, showing planar lamination passing upward to ripple-cross lamination. The symmetric shape of the ripples, the presence of foreset laminae dipping in opposite directions, and some lamina draping suggest that the cross-lamination was likely generated by wave-ripples. The sequence from planar lamination to ripple cross-lamination can be indicative of decreasing velocity of bottom currents (or changes in sediment grainsize and supply rate). Diameter of lens cap is 6 cm. Photo taken at the I-70 roadcut, near Morrison, Colorado (Cretaceous Dakota Group).
Wave ripples
Wave ripples in surface view (scale is 40 cm long). The crests are straight and regularly spaced. However, the overlying thin layer (top of the picture) shows a more disorganized pattern, partly reflecting the underlying crest orientation but probably indicating changing wave conditions. The preservation of these bedforms (that typically form in shallow water) can help to reconstruct the environment of deposition of sedimentary layers. Outcrop of the Cretaceous Dakota Sandstone, exposed along Dinosaur Ridge, Golden, Colorado.
Raindrop impressions?
These circular impressions, exposed on the underside surface of a layer of fine sandstone, have been interpreted as raindrop impressions. Sedimentologists have cautioned that similar impressions can form in muddy substrates by the ascent of bubbles through the sediment, or by interaction with air bubbles trapped in the overlying fluid. Scale in cm. Outcrop of the Maastrichtian Laramie Fm., exposed along the Triceratops trail Golden, CO, US.
CountyRd301
Cross-bedding, Co. Rd. 301 near McCoy --- About one third of the way up the picture are beds at an angle to the main layers.
Clastic dykes
Clastic dykes are sedimentary features consisting of seams of sediment truncating the surrounding host rock or deposit. These dykes can form through different processes. For example, fluidized sand can be injected through fractures, cutting through underlying and overlying deposits. Alternatively, pre-existing open cracks can be passively infilled by accumulation of a different kind of sediment. This picture shows three sub-vertical sandstone dykes, with parallel orientation, cutting through sub-horizontal mudstone and sandstone layers of the Pennsylvanian Fountain Formation, at the Balanced Rock site of Garden of the Gods Park, CO. Although different interpretations have been presented to explain the dykes in this outcrop (e.g., infill after thermal contraction of unconsolidated deposits in a cold environment), fieldwork by GRI scientists suggests that they should be interpreted as injection features, possibly related to seismic activation of small normal faults. Pencil for scale (resting vertically on right dyke).
Hyperpycnite
You may have heard of turbidites. This type of deposit forms from pulse-like flows, where sediment travels rapidly down a slope because of a gravity contrast with the surrounding fluid. Turbidites typically show an upward decrease in gain size and a vertical succession of structures (massive/planar laminated/ripple cross-laminated) indicative of decreasing energy of the flow. There are, however, some deposits that differ from turbidites by showing a symmetric internal organization, with increasing and then decreasing grainsize, and with structures pointing to increasing and then decreasing energy of the flow. Hyperpycnites are such kind of deposit, forming when a river, laden with sediment during its flooding stage, flows into a basin. The flow typically waxes, reaches peak strength, and then wanes. Therefore, the resulting hyperpycnite reflects the waxing and waning cycle in its internal structure. This picture shows a nice hyperpycnite from the Pennsylvanian Minturn Formation, near McCoy (CO, USA). The bed displays reverse grading in its bottom part and a gradual upward transition from ripple laminated, to planar laminated, to massive (waxing phase). The upper part of the bed shows normal grading and hints of ripple-cross lamination (waning phase).