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Paleocurrent Analysis February 7, 2011 Paleocurrent Analysis (Paleocurrent Indicator Analysis)

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Paleocurrent Analysis February 7, 2011 Paleocurrent Analysis (Paleocurrent Indicator Analysis) G528 Sedimentary Basin Analysis Paleocurrent analysis February 7, 2011 Paleocurrent analysis (paleocurrent indicator analysis) • Measurement of the orientation of key sedimentary structures formed during transport of sediment by moving fluid Type of Indicators • catalogued in six groupings by the following conventions • a) root term plan or lin for planar of linear • b) prefix uni or bi for unidirectional or bi-directional • c) suffix down or up for direction with respect to dip of planars • d) suffix parl or perp for direction with respect to strike of linears • e.g., tabular cross-beds = Uniplandown Uniplandown indicators Unidirectional Planar Current flows down-dip of structure • cross-bedding: considerable azimuth dispersion; multiple measurements vital • ripple-drift cross-lamination: considerable azimuth dispersion; ripples commonly form in response to back-eddies, etc. Uniparldown indicators dune foresets; Sinian, near Aksu, China ripple foresets more ripple foresets Uniplanup Unidirectional Planar Current flows up-dip of structure • clast imbrication: generally measured in gravel, could measure sand imbrication in lab • flame-structure vergence: could form in response to local slope variations • axial surface of contorted bedding: could form in response to disharmonic folding Uniparlup indicators Pebble imbrication examples Modern gravel beach, Baja LaHood facies, Jefferson R. Canyon Basal Six Mile Crk Fm, Coberly Gulch Uniparlup indicators, cont. Overturned soft-sediment folds, K Baja Flame structures Unilinparl • Unidirectional • Linear • Trend of linear feature is parallel to flow • e.g.: (a) flute casts, and some furrow casts (b) trough sets of cross-strata (c) some prod, bounce or skip marks (d) some oriented clasts with ‘upstream’ and ‘downstream’ end (e.g., tree with root wad) Unilinparl indicator examples Flutes and flute casts Unilinparl indicator examples flute casts, Gaspe Peninsula, Quebec More flute casts, Gaspe Peninsula, Quebec Furrow cast, Cretaceous, central UT unilinparl examples – trough cross beds current unilinperp indicators Unidirectional Linear Trend of indicator perpendic. to flow e.g., (a) current-ripple marks (b) axes of flow rolls bilinparl indicators • bidirectional (ambiguous) indicators • linear features • feature is parallel to current direction • e.g., a) parting lineation b) clast lineation (woody debris, elongate fossils, etc.) c) groove casts, and some furrow casts d) channel fills bilinparl examples – parting lineation Cretaceous Colorado Gp. Frying Pan map area near Dillon, MT bilinparl example: floating elongate clasts bilinparl ‘floaters’ bilinperp ‘rollers’ bilinperp: rolling elongate clasts bilinparl (‘floaters’) vs. bilinperp (‘rollers’) Measurement of paleocurrent indicators in undeformed strata • Planar features: measure strike and dip • Linear features: measure trend of feature Measurement of paleocurrent direction in deformed strata • Planar features: – record strike and dip of feature and strike and dip of tilted beds – restore beds + pcurrent indicators to horizontal about strike • Linear features: – Measure pitch angle from strike to indicator – note CW or CCW – note stratigraphic facing direction (looking up or down) – add or subtract pitch from strike as appropriate Measurement of unilinparl feature (flute cast) Pitch angle = 35 CW from strike to pc indicator) Bedding sole Strike – pitch = 280-35 = 245 280 100 Dealing with trough cross-strata Trough x bedding – K Colorado Gp. Frying Pan map area From: DeCelles et al., 1983 Cut orientation vs. trough orientation cut vs. trough orientation Inferring flow direction from trough geometry .
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