cross-stratification


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cross-stratification

[′krȯs ‚strad·ə·fə′kā·shən]
(geology)
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These fine-grained to gravelly sandstones, 3.7 m thick, are dominated by trough cross-stratification structure, which indicates the transport and sedimentation of sand material in the lingoid subaqueous dunes, under the influence of traction currents.
Nevertheless, in sandy gravel sediments of subaqueous dunes, the cross-stratification mainly consists of grain flow deposits with little or no grain fall deposition on most of their lee faces (Kleinhans, 2004), suggesting that the sediment was delivered in pulses: rolling grain deposits for the coarsest sediments, however, are also common (Kleinhans, 2004).
After describing the stratigraphic sections of the Guaduas formation in the studied area, 12 lithofacies were found: 1) Sandstone with bidirectional cross-stratification, 2) Sandstone with irregular wavy bedding, 3) Rhythmic successions of sandstones and claystones, 4) Sandstones with heterolithic bedding, 5) Claystones with Sandy lenses, 6) Claystones with thin lamination, and plant debris, and coal, 7) Claystones and siliceous limestones with paleosols, 8) Sandstones with through-cross stratification, 9) Sandstones with planar cross-stratification, 10) Sandstone with planar stratification, 11) Sandstone with climbing cross-stratification, and 12) Massive sandstones (Table 2).
From Figure 4 in the Umbita's section, it is clear that lithofacies 6 (claysones with thin lamination, plant debris, and coal) is the most abundant and usuallyrelates with facies 1 (Sandstone with bidirectional cross-stratification) andfacies 8 (Sandstones with through-cross stratification).
Thickness of the sandstone beds range from 4 to 200 cm; they are lenticular, showing pinch and swell morphology, hummocky cross-stratification, parallel lamination (Plate 1a), trough cross-bedding, flute casts, load casts (Plate.1b) and longitudinal ridges.
The limestone succession displays pinch and swell morphology, parallel-lamination, hummocky cross-stratification, low-angle cross- bedding, and longitudinal ridges at their bases.
Origin of hummocky and swaley cross-stratification - the controlling influence of unidirectional current strength and aggradation rate.
The Formation is made up of purple polygenic and polymodal conglomerates with a poorly-sorted sandy matrix supported fabric displaying a variable crude cross-stratification. The clasts, which are usually subrounded, consist of volcanic rocks (80%), reddish sandstone, greywackes and quartz.
This unit shows different types of cross-stratification magnified by clast alignments locally.
Sandstone beds have trough and planar cross-stratification in Nagri Formation, near Bahadar Khel old bridge.Interpretation: The laterally persistent sheets of sandstone, dominated by large trough cross- stratification with subordinate small scale, planar and trough cross-stratification are interpreted as the deposit of sand-dominant braided fluvial system.
Furthermore, current ripple cross-lamination, trough cross-stratification and planar stratification were formed by deposition associated with migrating current ripples, dunes, and upper stage plane beds, respectively (Bridge, 1993).
(1988): Wave-formed, coarse-grained ripples and their relationship to Hummocky cross-stratification. Journal of Sedimentary Petrology, 58(4): 607-622.