Primary Structures I

Advanced Tectonics
James S. Aber


Primary Structures in Clastic Sedimentary Strata

Alluvial sediment. Planar and cross-bedded sandstone of former river channel and overbank environment. Brule Formation (Tertiary), West Ash Hollow Road, Crawford, Nebraska.
Alluvial sediment. Wavy-bedded sand of climbing ripples above plane-bedded sand. Camera lens cap for scale. Brule Formation (Tertiary), West Ash Hollow Road, Crawford, Nebraska.
Small channel filled with clay fragments; channel is cut into plane-bedded sand. Camera lens cap for scale. Brule Formation (Tertiary), West Ash Hollow Road, Crawford, Nebraska.
Alluvial sediment. Cross-bedded sandstone layers alternate with fine silty beds. Sand was deposited in river channels, and silt accumulated on floodplains. Oligocene strata, Toadstool Park, northwestern Nebraska.
Alluvial sediment. Cross-bedded sandstone represents a channel fill on top of fine-grained silty sediment of a floodplain. Oligocene strata, Toadstool Park, northwestern Nebraska.
Alluvial sediment. Base of channel fill contains coarse sand and fine gravel. Camera lens cap for scale. Oligocene strata, Toadstool Park, northwestern Nebraska.
Alluvial sediment. Rippled channel form with sand fill, Pennsylvanian, north Texas.
Alluvial sediment. Cross bedding in Brazos River conglomerate channel deposit. The conglomerate consists mainly of chert pebbles. Pennsylvanian, northern Texas.
Alluvial sediment. Interbedded clastic silt (red) and gypsum (white) of a floodplain environment. Triassic red beds, Palo Duro Canyon, near Amarillo, Texas.
Alluvial sediment. Rippled gypsum sand in channel fill, Triassic red beds, Palo Duro Canyon, near Amarillo, Texas.
Alluvial sediment. Rippled surface in sandstone of Sioux Quartzite (middle Proterozoic), Sioux Falls, South Dakota.
Alluvial sediment. Raindrop impressions, bottom side of sandstone bed, Sangre de Cristo Formation (upper Pennsylvanian), Cuchara, Colorado.
Beach at Galveston, Texas. Note series of bars and swales that parallel the beach front, as seen during low tide.
Beach at Galveston, Texas. Two sets of ripples cross to form an interference pattern. Ripple sets represent swash and backwash of waves on the beach front. Hat for scale.
Cross-bedded sand filling tidal channels. Galløkken, section B, Lower Jurassic, island of Bornholm, Denmark (Rolle 1977).
Interbedded sand and clay deposited in a tidal environment. Wavy and "flaser" bedding is typical of the ebb and flow of tides. Galløkken, section C, Lower Jurassic, island of Bornholm, Denmark (Rolle 1977).
Small unconformity in the sand and clay beds--note angular discordance behind the scale pole. Galløkken, section C, Lower Jurassic, island of Bornholm, Denmark (Rolle 1977).
Aeolian sediment. Wind-blown sand dunes with rippled surface. White Sands, New Mexico.
Aeolian sediment. Ripples on gypsum sand with animal footprints. White Sands, New Mexico.
Aeolian sediment. Wind erosion reveals curved cross beds within older dunes. White Sands, New Mexico.
Turbidity deposits. Interbedded sandstone (light) and shale (dark) layers of marine offshore environment. Sandstone beds were laid down by turbidity currents, whereas shale beds represent quiet marine sedimentation. Upper Devonian, Church Street Quarry, Elmira, New York.
Turbidity deposits. Closeup view of turbidites (light-colored sandstone) interlayered with dark shale of an offshore marine environment. Upper Devonian, Genesse Gorge, western New York.
Turbidity deposits. Rapid deposition of water-saturated sediment brought about soft-sediment deformation (collapse) in the upper portion of this section. Upper Devonian, south-central New York.
Tilted turbidites of the Tesnus Formation, Marathon Basin, west Texas. Thick sandstone turbidites are separated by thin shale interbeds.

Rock/Soil Creep

Soil creep is downslope (to right) in uppermost portion of section. This gives the illusion of folding but is really only a surficial phenomenon. Marathon Basin, west Texas.
Rock creep in garnet schist, Black Hills, South Dakota. Even "hard" rocks are subject to downslope creep in the weathered zone.
Soil-creep fold in tephra on side of volcanic cone, Eifel region, western Germany. Extreme, near isoclinal, folding is seen on the side of this extinct volcano.

Reference

Primary structures--part II.
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Notice: Advanced tectonics is presented for the use and benefit of students enrolled at Emporia State University. Others are welcome to view the course webpages. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor. All text and imagery © J.S. Aber (2014).