G528Sedimentary Basin Analysis

Paleocurrent analysisFebruary 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

• cross-bedding: considerable azimuth dispersion; multiple measurements vital

• ripple-drift cross-lamination: considerable azimuth dispersion; ripples commonly form in response to back-eddies, etc.

UnidirectionalPlanarCurrent flows down-dip of structure

Uniparldown indicators

ripple foresets

dune foresets; Sinian, near Aksu, China

more ripple foresets

Uniplanup

• 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

UnidirectionalPlanarCurrent flows up-dip of structure

Uniparlup indicators

Pebble imbrication examples

Modern gravel beach, Baja

LaHood facies, Jefferson R. CanyonBasal 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)

Flutes and flute castsUnilinparl indicator examples

Unilinparl indicator examples

flute casts, Gaspe Peninsula, Quebec

Furrow cast, Cretaceous, central UTMore flute casts, Gaspe Peninsula, Quebec

unilinparl examples – trough cross bedscurrent

unilinperp indicatorsUnidirectionalLinearTrend of indicator perpendic. to flowe.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)

100

Pitch angle = 35CW from strike to pc indicator)Bedding soleStrike – pitch =280-35 = 245

280

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