QUOTE (Ames @ Feb 7 2006, 03:53 PM)
From my limited O' level geology, It's a matter of scale.
Festoons are from little ripples, cross bedding is from dunes.
Festoons look like eye shapes, cross bedding is where whole sets of bedding planes are cut off by similar sets of bedding planes at a different angle like curtain(drapes) swags.
I am looking to the rock men to put me right and maybe explain the significance...
Nick
I've been holding off on giving this little sedimentology lecture, but now seems like the appropriate time:
I tend to only use the term "festoon" for very large scale trough cross bedding, like you might find in eolian sanstones, where the cross bedding has recorded the migration of the foresets (the down-current face of a bedform) of very large dunes (m's to 10's m high). Large relief dune foresets can also form in subaqueous settings, like subtidal sandwaves or bars in deep river channels. I think the NASA/JPL guys use the term more generically for trough cross bedding and lamination.
The smaller-scale features that the rovers have been identifying would fit into what I would call "cross lamination". They have identified: 1) trough cross lamination, where the lamina foresets are curved and intersect the lower lamina set boundary at a low angle and may be truncated at a high angle at the upper lamina set boundary; 2) planar tabular cross lamination, where the foreset lamina are inclined, but parallel, and intersect the upper and lower lamina set boundaries at approximately the same angle; and 3) low angle inclined cross lamination, which is just what it sounds like. There is also plenty of horizontal planar lamination.
The difference between these lamination types has to do with the morphology of the bedform which migrated to form the strata type. When straight-crested subaqueous current ripples migrate, they produce planar tabular cross lamination. When sinuous-crested subaqueous current ripples migrate, they form trough cross lamination. Low-angle inclined cross lamination can form by migration or encroachment of a low-relief sand sheet, or a traction carpet, in either subaqueous or eolian settings. Horizontal planar lamination can form in a number of very different settings: 1) upper flow regime (fast current velocity and/or low water depth) plane beds, which can form in fluvial, tidal, and sediment gravity flow settings; 2) antidune migration, where the entire bed is in transport (the planar lamination forms down current from the up-current migrating antidune); and 3) settling out from suspension. Horizontal and low-angle inclined lamination can also form by the migration of wind ripples (also called ballistic ripples) in an eolian setting. This is sometimes called "wind ripple translatent strata". These features have distinctive alternations of coarse and fine fractions between lamina, which often results in a "pinstripe" appearance in outcrop, and they are then called pinstripe lamina.
Any oscillatory flow (influence of waves) either alone or combined with unidirectional flow, tends to make bedforms that leave trough cross lamination as they migrate (sometimes just vertically aggrading). This "wave-ripple cross lamination" exhibits curved foreset lamination, but they tend to have no preferential dip direction between lamina sets.
The other important criteria for describing cross bedding and lamination is the angle of climb... that will be Lecture #2!
Dave Rubin's site has many excellent examples of cross bedding and cross lamination and other resources:
http://walrus.wr.usgs.gov/seds/
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