Gale Crater Stratigraphic Measurements and Preliminary Interpretations Ryan Anderson April, 2009

Methods CTX DEM (~10 m per pixel) and accompanying image were provided by Larry Edwards Align the image with the DEM, open both with IDL and manually select points along bed edges or surfaces

Followed the technique described in Fueten et al. (2005) to get strikes, dips and error bars: –Fit a plane to the points using linear regression. –Calculate the strike and dip of the best-fit plane. –Propagate uncertainty in the plane coefficients through the calculation, multiplying by the student’s-t value for the 95% confidence interval. Many locations had large errors in strike and/or dip. Keep only the most reliable measurements: –Dip error < 1 degree and strike error < 20 degrees. –Note: this does not guarantee that what was measured is actually the strike/dip of a bedding plane! It only guarantees that the points selected were fit well by a geometric plane.

This image shows sets of points that resulted in low- error plane fits. Numbers indicate the order in which measurements were made. (e.g. #7 is the seventh set of points used to define a plane) –The number label is used to keep track of which strike/dip corresponds to which location on the map. 5 km N

Results 1. Most layers in the Gale mound are not flat-lying, but are not tilted more than ~5-6 degrees. Higher dips typically reflect the local slope of the mound rather than the dip of a bed. 2. Several very similar strike/dip measurements appear to indicate a set of parallel layers that are expressed along the NW mound. –Closer examination shows that these layers are quite planar, suggesting flat-lying, low energy deposition and subsequent tilt with little deformation. –Average dip: ~5 degrees, average strike: ~225 degrees 3. The fan-shaped unit near the proposed traverse is likely part of an extensive unit that drapes the lower mound. –The observed phyllosilicates represent an “erosional window” through this more resistant mantling unit. –The prominent light-toned ridge along the proposed traverse may also be related to this unit.

Dip (degrees) Points with Strike Error < 20 o & Dip Error < 1 o Strike (degrees) Strikes/dips associated with the unconformable light-toned yardangs. Locations that were meant to trace a bedding plane, but actually reflect local slope.

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The bracket indicates the span of the points used to define plane #75. The plot shows the DEM profile and the plane that fits those points. The plane is fitting the local slope and doesn’t correspond to a stratigraphic bed. The same appears to be true for plane #80, #57, and #56.

Results 1. Most layers in the Gale mound are not flat-lying, but are not tilted more than ~5-6 degrees. Higher dips typically reflect the local slope of the mound rather than the dip of a bed. 2. Several very similar strike/dip measurements appear to indicate a set of parallel layers that are expressed along the NW mound. –Closer examination shows that these layers are quite planar, suggesting flat-lying, low energy deposition and subsequent tilt with little deformation. –Average dip: ~5 degrees, average strike: ~225 degrees 3. The fan-shaped unit near the proposed traverse is likely part of an extensive unit that drapes the lower mound. –The observed phyllosilicates represent an “erosional window” through this more resistant mantling unit. –The prominent light-toned ridge along the proposed traverse may also be related to this unit.

There appears to be a set of layers with similar strikes and dips that is exposed around a significant portion of the NW mound, including the proposed MSL traverse and the canyon.

Dip (degrees) Points with Strike Error < 20 o & Dip Error < 1 o Strike (degrees)

Points used to define plane #7 As a visual method of evaluating how well the plane fits with the feature of interest, we examine the intersection between the plane and the DEM. The yellow line is the intersection of the best-fit plane for location #7 with the DEM. Next Slide

The intersection of plane #7 with the mound traces this contact very well, even though the points defining the plane are ~15 km away. –This layer is very planar! –Suggests that it was deposited flat-lying in a low-energy environment, and has been tilted with little deformation –Different appearance where points were selected versus this location: mantling/dust- cover? –This layer is just above a change in texture which seems to correlate with the shift to magenta on the CRSIM map Composition change  change in erosional characteristics? Rough, Magenta “Smooth”, Blue

The intersection of plane #7 with the DEM (dark blue line above) appears to be consistent with the group of similar strikes and dips that occur in a band around the NW mound, though it becomes difficult to tell if it is tracing a bedding plane in some locations. It is roughly parallel to the layers near the head of the canyon.

Results 1. Most layers in the Gale mound are not flat-lying, but are not tilted more than ~5-6 degrees. Higher dips typically reflect the slope of the mound rather than the dip of a bed. 2. Several very similar strike/dip measurements appear to indicate a set of parallel layers that are expressed along the NW mound. –Closer examination shows that these layers are quite planar, suggesting flat-lying, low energy deposition and subsequent tilt with little deformation. –Average dip: ~5 degrees, average strike: ~225 degrees 3. The fan-shaped unit near the proposed traverse is likely part of an extensive unit that drapes and skirts the lower mound. –The observed phyllosilicates represent an “erosional window” through this more resistant mantling unit. –The prominent light-toned ridge along the proposed traverse may also be related to this unit.

“Fan” has similar texture and spectral properties to a unit that drapes and skirts the lower mound.

This is a very extensive unit! Mound appears to overlap onto the unit here

Similar textures, with preserved bedforms, occur at the North and South edges of the mound in the area of study.

Similar textures appear at the base of the mound farther to the east as well…

Speculation: the fan-shaped unit is related to an alluvial fan formed during the period of erosion prior to the deposition of the light-toned yardangs. The alluvial fan formed on top of the extensive draping unit, and protected it from erosion. However, the alluvial fan eventually did erode away, leaving a fan- shaped mesa of the underlying surface. Infilled channel

Results 1. Most layers in the Gale mound are not flat-lying, but are not tilted more than ~5-6 degrees. Higher dips typically reflect the slope of the mound rather than the dip of a bed. 2. Several very similar strike/dip measurements appear to indicate a set of parallel layers that are expressed along the NW mound. –Closer examination shows that these layers are quite planar, suggesting flat-lying, low energy deposition and subsequent tilt with little deformation. –Average dip: ~5 degrees, average strike: ~225 degrees 3. The fan-shaped unit near the proposed traverse is likely part of an extensive unit that drapes the lower mound. –The observed phyllosilicates represent an “erosional window” through this more resistant mantling unit. –The prominent light-toned ridge along the proposed traverse may also be related to this unit.

This is not just the edge of a bed, it is a topographic ridge. The phyllosilicates appear in the accompanying trough. Strike/dip measurements on the ridge are similar to the “fan” unit’s strike/dip: the ridge may be related to the mound-skirting, draping unit.

“Fan-shaped” Unit Ridge Phyllosilicates Ridge layers seem to underly the “fan” unit.

Results 1. Most layers in the Gale mound are not flat-lying, but are not tilted more than ~5-6 degrees. Higher dips typically reflect the slope of the mound rather than the dip of a bed. 2. Several very similar strike/dip measurements appear to indicate a set of parallel layers that are expressed along the NW mound. –Closer examination shows that these layers are quite planar, suggesting flat-lying, low energy deposition and subsequent tilt toward the NW with little deformation. –Average dip: ~5 degrees, average strike: ~225 degrees 3. The fan-shaped unit near the proposed traverse is likely part of an extensive unit that drapes the lower mound. –The observed phyllosilicates represent an “erosional window” through this more resistant mantling unit. –The prominent light-toned ridge along the proposed traverse may also be related to this unit.

Future Work / Outstanding Questions Determine how the green (phyllosilicate) and magenta (sulfate) units are related to the rest of the stratigraphic sequence. (No good strike/dip measurements yet… may not be possible with CTX) –Are they conformable with the higher ~5 degree dipping unit? Improve understanding of the mantling/skirting unit –How did it form? –When did it form, relative to other units? –How extensive was it? Update and improve my unit map of the proposed landing site and traverse. –Need to look more at the crater floor units and how they are related (or not) to the mound. –What happens at the transition between crater floor and mound? Why is there a scarp? What is the basal unit between the ridge and the scarp? –Look more closely at HiRISE and/or MOC images. Come up with a plausible order of events to explain the stratigraphy and geomorphology of the mound and floor units