G528 Sedimentary Basin Analysis - University Of · PDF file• axial surface of contorted...
Transcript of G528 Sedimentary Basin Analysis - University Of · PDF file• axial surface of contorted...
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