An appreciation of the extent of syndepositional fracturing, faulting, and
cementation of carbonate platform margins is essential to understanding the role of early
diagenesis and compaction in margin deformation. This study uses integrated lidar and
outcrop data along the Capitan Reef from an area encompassing the mouths of both
Rattlesnake and Walnut Canyons. Mapping geomorphic expressions of syndepositional
faults and fractures at multiple scales of observation was the main approach to
delineating zones of syndepositional fractures. Ridge- groove couplets visible in
exposures of the Capitan Reef throughout the Guadalupe Mountains were targeted
because the ability to identify these as signs of syndepositional fracture development
would have implications for the entire reef complex. Results show that these ridgegroove
couplets are the product of differential weathering of syndepositional as well as
burial-related fractures. Recessive grooves have an average syndepositional fracture
spacing of ~13 m whereas ridges have a spacing of ~33 m.
vi
Smaller (~5-20 m-wide) scale erosional lineaments common in the study area and
mappable on airborne lidar are formed by differential erosion of planes of
syndepositional faults. Maps of these fault lineaments on the lidar show that
syndepositional faults extend laterally for 300 m - 2000 m and relay near the terminations
of the faults at each end. Faults can be further grouped into fault systems consisting of
sets of faults connected by fault relays that extend for at least the entire length (~12 km)
of the study area. Although vertical displacement along faults is typically less than 11 m,
syndepositional faults result in changes in structural dip domain of 1-6 degrees across an
individual fault.
Even smaller erosional lineaments (10 cm-1 m) are visible on the airborne lidar
that form as a result of differential erosion of individual fractures. Larger fractures (> 20
cm) can be reliably mapped on the lidar, but smaller features (< 20 cm) cannot be reliably
mapped with currently available data and can only be captured using field studies.
Fracture fill types are heterogeneous along strike as shown by comparisons of field study
locations. Siliciclastic-dominated fills are likely sourced from overlying siliciclastic units
of the shelf, which, in this area, were from the Ocotillo Siltstone. These silt-filled
fractures are broadly distributed, indicating preferential development and infill of
syndepositional fractures during the deposition of the Ocotillo Siltstone in the G27/28
high-frequency sequences. Development of early fractures is also shown to have been
influenced by mechanical stratigraphy with changes in fracture spacing between massive
to thick-bedded shelf-margin (~17 m fracture spacing) and outer-shelf facies tracts versus
thin-bedded outer-shelf and shelf-crest (~28 m fracture spacing).
Ultimately, this study demonstrated that the Capitan shelf margin was
ubiquitously overprinted by syndepositional fracturing and faulting and that this nearsurface
structural modification influenced early diagenetic patterns and internal
vii
sedimentation throughout the reef margin. Before this study, the extent and nature of
syndepositional fracture/fault development within the margin were largely unquantified.
Here, by integrating field observations and surface weathering reflections of these
fractures as observed in the lidar, we can demonstrate a widespread impact of early
fracturing more akin to analogous early-lithified margins such as the Devonian of the
Canning Basin of Australia. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/21892 |
| Date | 01 November 2013 |
| Creators | Jones, Nathaniel Baird |
| Source Sets | University of Texas |
| Language | en_US |
| Detected Language | English |
| Format | application/pdf |
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