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Martian Analogue - Section 2
Note: All Mars photos in this section are credit NASA/JPL-Caltech/MSSS. The photos are from the mast cameras on the Curiosity rover (Mars Science Laboratory) taken during the interval Aug.6, 2012 - May 31, 2021. Some photos have been stitched from 2 or more photos retrieved from the NASA online archive of raw images.
Bedrock brecciation at Gale crater
Pairs of pictures from Earth and Mars are shown above. The photo pairs contrast unbrecciated bedrock terrain with intensely brecciated bedrock terrain. The last photo illustrates mixed-mode brecciation showing bedrock frost heave, delamination of foliated bedrock by ice extrusion along with glacially shifted bedrock fragments. This occurrence on the Avalon Peninsula was formed in a subglacial environment by the widespread penetration of pressurized subglacial groundwater into frozen sedimentary rock. A similar process might have operated at Gale Crater.
Many sites revealed by the Curiosity rover at Gale crater show sedimentary bedrock that has been severely brecciated to the extent that brecciation is a dominant feature of the geomorphology. Understanding the origin of the brecciation is an important aspect of understanding the geologic history of the Gale crater site.
There are several possible causes of severe bedrock brecciation on both Earth and Mars. Landslides can fracture bedrock and scatter the resulting fragments. Catastrophic flooding can fracture and shift bedrock. Brecciation of harder layers of bedrock can occur when harder layers are undermined by the removal of underlying softer layers through erosion or dissolution. Removal of large amounts of overburden by erosion can lead to bedrock brecciation when underlying layers rebound unevenly. Volcanic or tectonic action can brecciate bedrock. Impact events are a potential cause of large-scale bedrock brecciation. Bedrock brecciation can result from glacial loading or frost heave, acting alone or in combination.
The variability and complexity of the bedrock brecciation seen at Gale crater, combined with limitations in the available data, make any attempt at determining causes of brecciation intrinsically speculative. Instead of trying to determine the probable origin of brecciated bedrock features at Gale crater, the following discussion will focus on showing that subglacial frost-heave action is a possible origin. Specifically, it will be shown that many features seen on the Avalon Peninsula, and known to represent examples of brecciation by subglacial bedrock frost heave, resemble features seen at Gale crater.
Many sites revealed by the Curiosity rover at Gale crater show sedimentary bedrock that has been severely brecciated to the extent that brecciation is a dominant feature of the geomorphology. Understanding the origin of the brecciation is an important aspect of understanding the geologic history of the Gale crater site.
There are several possible causes of severe bedrock brecciation on both Earth and Mars. Landslides can fracture bedrock and scatter the resulting fragments. Catastrophic flooding can fracture and shift bedrock. Brecciation of harder layers of bedrock can occur when harder layers are undermined by the removal of underlying softer layers through erosion or dissolution. Removal of large amounts of overburden by erosion can lead to bedrock brecciation when underlying layers rebound unevenly. Volcanic or tectonic action can brecciate bedrock. Impact events are a potential cause of large-scale bedrock brecciation. Bedrock brecciation can result from glacial loading or frost heave, acting alone or in combination.
The variability and complexity of the bedrock brecciation seen at Gale crater, combined with limitations in the available data, make any attempt at determining causes of brecciation intrinsically speculative. Instead of trying to determine the probable origin of brecciated bedrock features at Gale crater, the following discussion will focus on showing that subglacial frost-heave action is a possible origin. Specifically, it will be shown that many features seen on the Avalon Peninsula, and known to represent examples of brecciation by subglacial bedrock frost heave, resemble features seen at Gale crater.
A panoramic view of a brecciated bedrock area at Gale crater is shown above. The brecciation includes both horizontal and vertical block displacements. Displacement and streamlining of features indicate possible glacial ice movement in a left-to-right direction at this site.
The sedimentary bedrock seen in the above photo has been extensively brecciated in a subglacial process. Many block displacements occurred in an upward direction, providing evidence of subglacial bedrock frost heave. Other blocks have been shifted horizontally by moving glacial ice. This site provides a wide-area example of subglacial bedrock brecciation driven by pressurized groundwater that entered frozen bedrock beneath a cold glacier. The brecciation occurred during deglaciation at the end of the Younger Dryas cold period.
Geometric correlation between nearby blocks
Both photos above show a moderate degree of bedrock brecciation. A key feature shared by the two occurrences is the correlation between nearby blocks. When bedrock is fractured by landslides or catastrophic flooding, or by impact, adjacent blocks would tend to be uncorrelated. Uncorrelated blocks bear no geometric relation to their neighbors because, after fracturing, the resulting rock fragments are randomly scattered over a wide area. When bedrock is fractured by subglacial frost heave, adjacent blocks commonly result from the breakage of larger blocks without subsequent scattering, leaving a visible geometric correlation between nearby separated fragments.
The above two images provide a panoramic view of a single area, with the second frame continuing rightward from the right-hand edge of the first frame. Typical of many of the brecciated bedrock sites imaged at Gale crater, there is a high degree of correlation between adjacent blocks, with little evidence of widespread scattering of blocks. Some blocks have been displaced vertically relative to their neighbors.
As with the preceding photos from Gale crater, most of the displaced bedrock blocks shown above are correlated with neighboring blocks in terms of shape, position and orientation. The bedrock at this site was affected by subglacial bedrock brecciation with upward displacement of blocks.
Chaotic brecciation
Each of the three images from Gale crater shown above reveals chaotic bedrock brecciation with diminished correlation between adjacent blocks. Nevertheless, the overall appearance of each photo is suggestive of in-situ brecciation of bedrock, rather than the transport and scattering of rock fragments from a distant location. The severity of the brecciation shown above is within the range of severity of brecciation seen at glacially brecciated sites on the Avalon Peninsula.
At each of the locations shown above, bedrock brecciation originated with subglacial bedrock frost heave. Some upward-displaced blocks were ejected from substrate and subsequently displaced horizontally by creep in overlying cold glacial ice. Horizontal displacement of blocks by moving ice tends to break the geometric correlations between blocks and their neighbors.
Two more views showing disorganized patterns of brecciated bedrock are shown above. The sedimentary bedrock seen in the second photo was brecciated by subglacial frost heave.
Brecciation with sloping beds
The above photo shows beds in sedimentary rock that appear to dip downward away from the camera. The severe brecciation of the rock could be the result of undermining and collapse, or the photo might show the edge of an impact crater. If the site was glaciated, then the brecciation might be the result of ice moving down-slope toward the camera, creating a zone of reduced ice pressure. Lee side brecciation is commonly seen on the Avalon peninsula and can be the result of ice pressure reduction beneath either warm-based or cold-based glaciers. Beneath cold glacial ice, lee side brecciation can be enhanced by concurrent subglacial frost heave when pressurized groundwater migrates upward along permeable boundaries between sloping sedimentary beds.
The above two photos show sedimentary beds that appear to dip comparably to those shown in the preceding photo from Gale crater. Bedrock brecciation resulted during deglaciation of this site when pressurized groundwater moved toward the base of an overlying cold glacier. The groundwater moved upward from depth, travelling between sloping bedding layers toward a region of reduced ice pressure. Elongated blocks seen in the second photo, above and to the left of the hammer, have been displaced parallel to upward-sloping bedding by pressurized ice that accumulated behind the blocks.
Sporadic brecciation
The above photo from Gale crater shows a region of relatively intact bedrock with sporadic areas of brecciation. On the Avalon Peninsula, isolated brecciated areas signify areas where subglacial groundwater ascent was concentrated.
A localized area of ice-disrupted bedrock is visible in the foreground (just left of center) of the above photo. The terrain in the background shows varying degrees of bedrock brecciation. The thickly bedded sedimentary bedrock underlying the entire area is approximately homogeneous. The variability in the degree of brecciation is a probable result of uneven rates of groundwater reaching the rock-ice interface beneath cold glacial ice cover.
The photos above show localized occurrences of subglacial frost-heaved bedrock set off against a background of intact bedrock. These occurrences exemplify the uneven action of subglacial groundwater as it migrated upward along preferred channels, froze and fractured bedrock. Subaerial bedrock frost heave could also, in principle, form localized areas of shifted/brecciated bedrock as seen in the photos. However, a wide range of observations on the Avalon Peninsula points to the conclusion that subaerial bedrock frost heave has not occurred to a significant extent anywhere on the Avalon. All bedrock disruption by ice has occurred in a subglacial environment.
A significant degree of variability in the intensity of brecciation is apparent in each of the above four images of bedrock at Gale crater. Several possible non-subglacial causes of bedrock brecciation (for example, landslides, floods, impact) would be less likely to generate the localized effects seen in the above photos.
Brecciation with fissures
A common mode of bedrock brecciation seen at Gale crater is illustrated in the above photo. Slabs of rock that appear thin relative to their horizontal dimensions have separated from one another enough to form fissures, but otherwise have translated or rotated only by small amounts. There is some evidence of vertical shifting of rock. The resulting geometric pattern resembles ice pans on a partially frozen river during spring breakup. One potential cause for this pattern of brecciation would be the contraction of a water-saturated, partially lithified layer of sediment as it became dehydrated (mud cracks). However, while the fit between adjacent blocks is good in many instances, there are enough irregularities in the fit to suggest that dehydration shrinkage alone is not adequate to explain the complete picture.
In both the photos above, the illustrated bedrock has been fractured and separated in a subglacial process. The fractured layer is a single bed of sedimentary rock that detached from the underlying bed when pressurized groundwater intruded between the beds during deglaciation. Groundwater further intruded vertically and froze between individual blocks, forcing them apart. The process was aided by overlying cold glacial ice which deformed via creep, transferring disruptive shear stress to the bedrock.
Four localized occurrences of bedrock brecciation through the formation of fissures are shown above. In all four instances, an upper sedimentary bed has become loosened from underlying bedrock, allowing blocks from the top bed to fracture and slide horizontally. Basal shear stress imparted by cold glacial ice, in combination with intruding subglacial groundwater, accounted for the brecciation seen in the two Avalon Peninsula photos.
Four more examples of bedrock brecciation with fissures separating polygonal blocks are shown in the above photos from Gale crater. Each of the photos also show indications of upward displacement of blocks occurring in association with horizontal displacements.
The system of subglacial-ice-generated fissures seen above is accompanied by an instance of upward block displacement (lower right of frame). The orange strap is 1 m long.
Several significant fissures were observed at the above-pictured location where bedrock brecciation is dominated by upward displacement. Pressurized subglacial groundwater intruding into and freezing within bedrock causes fracturing and block displacement. Block displacement can be either horizontal or upward, presumably following the path of least resistance. Disruptive basal shear stress imparted by deforming glacial ice favors horizontal block displacement (fissure formation) over upward displacement.
At each of the sites shown above, block displacement was primarily horizontal. Basal shear stress likely affected the brecciation in both instances, although intruding pressurized groundwater was integral in driving the overall process. An ice flow pattern might be discernible in the first photo, while the brecciation through fissure formation seen in the second photo was guided by a lack of bedrock confinement on one side of the feature.
Rafting of blocks
All eight of the above images from Gale crater show instances of brecciated bedrock where blocks have been rafted up onto other blocks. Rafting of blocks suggests that an area was subjected to lateral compression forcing an adjacent blocks to ride up over their neighbors. Alternatively, rafting could signify that some blocks were pushed upward from below, before settling back on top of neighboring blocks. Glacial ice loading could account for lateral compression of a region of bedrock. Up-welling and freezing of groundwater is one possible explanation for upward displacement of blocks.
At each of the sites shown above, large ice-fractured blocks have been pushed upward into glacial ice by pressurized groundwater that intruded into bedrock and froze during deglaciation at the conclusion of the Younger Dryas cold period. After glacial ice departed, the ice-lifted blocks settled back, often resting on top of neighboring blocks.
Transport of rafted blocks
The above eight photos from Gale crater show blocks that have been rafted and transported, ending up resting on rocks that might not originally have been their immediate neighbors. Transporting of rafted blocks would be expected if blocks became embedded in moving glacial ice.
Two views of the same site, taken in opposite directions, are shown above. Sedimentary bedrock at this location was brecciated by intruding ice originating from pressurized subglacial groundwater. The two blocks lying roughly horizontally are fragments of a single block that has split into two pieces. The pieces were ejected from substrate by subglacial bedrock frost heave and then transported a few meters down slope by glacial ice deforming in creep.
Each of the above four photos shows blocks that have been ejected from bedrock substrate in a subglacial frost-heave process and then transported by deforming glacial ice.
Locally intensified brecciation
The first photo above shows a severely brecciated bedrock area at Gale crater. A mound of intensely brecciated rock can be seen at the left of the frame just above center. The second photo above provides a closeup view of the mound of more finely brecciated rock. This type of feature is common in Avalon Peninsula bedrock that has been brecciated by the freezing of pressurized subglacial groundwater. Variations in the geometry of bedrock confinement and in the geometry of groundwater intrusion, combined with variations in groundwater flow rates, can account for abrupt spatial variations in fineness of brecciation.
Coarser bedrock brecciation is seen giving way to finer brecciation in the above photos. The second photo shows a closeup of the more intensely brecciated bedrock. A locally elevated groundwater intrusion rate could account for the area of more severe brecciation. The sedimentary bedrock at the illustrated location was deemed to be approximately homogeneous in composition.
The above three photos show an area of concentrated brecciation occurring within a group of large blocks displaced upward by subglacial frost heave action.
Sometimes, areas of concentrated bedrock brecciation can resemble deposits of glacial till. Close examination of fractured rock shown within the white rectangle in the first photo above indicated that the rock (second photo) was broken in situ and not transported.
Another area of localized concentrated subglacial bedrock brecciation is shown above.
Tilted upward-displaced blocks
The above five photos show blocks that have been displaced upward and tilted relative to adjacent bedrock. This mode of block displacement is seen in subglacially brecciated bedrock on the Avalon Peninsula. The usual cause is a small horizontal displacement in an upward-displaced block that causes one end of the block to jam on adjacent bedrock while the other end settles back into the cavity underlying the displaced block.
Five examples of blocks that were shifted upward and tilted by subglacial frost heave action are shown above. In all cases, it appears likely that the blocks were first displaced upward without tilting, and then translated a short distance horizontally by movement in overlaying glacial ice. The horizontal translation resulted in jamming on the down-ice side while the up-ice side was free to settle back into the underlying cavity. The tilted blocks indicate ice flow direction, with ice having moved in the direction from the low side of tilted blocks toward the high side. At most sites, this implied ice flow direction was found to be consistent with other independent indicators of ice flow direction.
Small-scale bedrock brecciation
The photos above show six instances of finely brecciated bedrock at Gale crater. This type of brecciation is commonly seen interspersed with areas of larger-scale bedrock brecciation on the Avalon Peninsula. The more finely brecciated bedrock seen on the Avalon could represent areas where groundwater came close (within a few cm) to the base of a cold glacier before freezing.
Six instances of finely brecciated bedrock are shown above. In all cases the brecciation occurred beneath glacial ice and was accompanied by nearby occurrences of upward-displaced bedrock blocks. The upward-displaced blocks imply the freezing of pressurized subglacial groundwater within local bedrock.
Linear features
Brecciation that shows linear features, as seen in the above image, suggests ice flow as a factor controlling the shape and orientation of blocks and fissures. In the above image, ice flowing from left to right could account for the apparent streamlining, along with the elongated shapes of multiple blocks sharing a common orientation. Some blocks are tilted, with the high side pointing down-ice, as would be expected based on the discussion presented previously (see Tilted upward-displaced blocks).
The alignment of frost-heaved blocks seen in the above photo resulted from a rough coincidence between the strike of planes of vertically-dipping bedrock foliation and ice flow direction. The presumed ice flow direction was toward the upper right corner of the frame.
Lee-side brecciation
Intense bedrock brecciation on sloping surfaces can result when glacial ice moves down-slope. Lee-side brecciation is a potential key indicator of bedrock brecciation occurring in a subglacial environment. This topic will be discussed further in its own section. Examples of lee-side brecciation caused by deforming cold glacial ice and augmented by intruding subglacial groundwater are abundant on the Avalon Peninsula.
An example of lee-side brecciation is shown above. The brecciation resulted from a combination of shear/tensile stress exerted by cold glacial ice moving toward the camera and groundwater intruding into an area of diminished ice pressure.
Return to page Mars or view Martian Analogue - Section 1 (Gale crater: Features resembling subglacial bedrock frost heave).
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