^PSSynsedimentary Deformation of the Upper Permian Capitan Reef Carbonate Platform, New Mexico*

By

D.W. Hunt¹, E. Kosa², A.J. Simo³, and L. Piccoli³

Search and Discovery Article #30036 (2005)

Posted September 7, 2005

*Poster presentation at the AAPG Annual Convention, Calgary, Alberta, June 19-22, 2005

Click to view posters in PDF format.

       Poster 1       Poster 2       Poster 3

¹Norsk Hydro Research Centre, Sandsli, Norway

²Shell UK, Ltd, Aberdeen, United Kingdom

³University of Wisconsin-Madison, Department of Geology & Geophysics, Madison, Wisconsin ([email protected])

Abstract 

The basinward thickening and steepening of the ‘fall-in' beds, back reef strata equivalent to the Upper Permian Capitan Reef, has long proven the subject of debate. Recent integrated structural and stratigraphic studies, show that these back reef strata are cut by syndepositional dip slip faults in the outer 5-6 km of the platform. The fault systems are known to extend 33 km along strike parallel to the platform margin and have displacement of up to 30 m. Most tip-out upward within the platform succession and localized the development of penetrative karst systems. Here the architecture of the faults and their impact on back reef sedimentation, stratal geometries and correlation is illustrated using field and GPR data.

Most of the syndepositional faults are steep, with dips more than 80°, and a normal and/or reverse sense of movement. Four different morphologies represent the relationships between the upper tips of the faults and the platform top (or free surface) during deposition; (I) faults that broke the platform top, (II) buried faults that tipped-out upwards below a growth monocline, (III) buried faults that tipped upward into a fault splay and a growth monocline and (IV) buried fault tips with no expression at the free surface. The distribution, thickness, geometries and changes in facies of growth strata allow for precise reconstruction of the timing and rates of fault growth.

Understanding the role of syndepositional faulting adds a new and dynamic variable to the evolving picture of the Capitan depositional system.

       Location and structure map of the Guadalupe Mountains.

       Stratigraphic cross section through the Guadalupe Mountains (Kerans et al., 1992).

       Line drawing of east wall of North Slaughter Canyon, showing syndepositional faults and fractures.

Rationale and Aims 

In many carbonate platforms and buildups, fault and fracture systems are formed during deposition, as result of both tectonic and gravitational processes. Such faults and fractures tend to strike parallel to the platform margin, and have vertical extent of 10's-100's of metres.

In hydrocarbon reservoirs understanding the distribution of such early formed faults is important because they can either act as fluid flow conduits, baffles or barriers depending on their diagenetic alteration and fill composition. To date, relatively little work has been done to document the vertical and lateral distribution and variability of such early-formed fault/fracture systems.

Here we report on the results of an integrated structural, sedimentological, and diagenetic study that sets out to investigate the spatial and temporal variability of syndepositional fault and fracture systems found within the Upper Permian Capitan platform, Guadalupe Mountains, USA.

The aims of this poster are to illustrate: the spatial distribution and variability in the syndepositional faults and related structures, that the fault and fracture systems parallel the platform margin and grew as the platform was tilted down-to-the-basin, how the fault systems grew through fault segment linkage, the impact of the faulting and related deformation on depositional patterns and stratal architecture as a result of variations in rates of fault displacement, fault tip propagation, and platform aggradation. Details of the fill heterogeneity within the solution-modified fault and fracture systems have been described in detail by Kosa et al. (2004) and are not considered here.

Geologic Setting 

The Upper Permian Capitan platform is superbly exposed in the southeastern Guadalupe Mountains. The range is uplifted in the footwall of N-S trending Tertiary basin and range faults, and has an overall gentle eastward dip.

The Upper Permian strata lack significant structural complexity. The most important structures are: 1) platform margin parallel syndepositional faults, fractures and related monoclinal folds (this trend controls extensive Permian-aged karst systems), 2) N-S trending normal faults and joints attributed to and the Tertiary uplift of the range. These two important trends form a primary structural heterogeneity that acts to control the distribution of Tertiary karst systems and also the geomorphology of the Guadalupe Mountains. The Cretaceous to early Tertiary Laramide deformation is thought responsible for tightening of syndepositional folds such as the Walnut syncline and Guadalupe ridge anticline.

Syndepositional Fault Systems 

The syndepositional faults and fracture systems are readily differentiated from younger structures because they 1) are normally associated with growth strata, 2) tip out upward within the platform succession, and 3) are normally karst-modified and contain fills with Permian-aged sediments, cements, and fauna.

As is apparent in map pattern, cross section and stereographic projection, the faults have a very consistent orientation and parallel the platform margin. The faults have dips in excess of 75° and a dip-slip sense of displacement (apparent from both offset data, mesoscale structures, and slickensides). In Slaughter Canyon, the maximum offset measurable in backreef strata is 24 m. The faults are vertically and laterally segmented.

In map-view the fault zones are laterally segmented across a range of scales. For example, km-scale fault systems Z1-Z2 and Y1-Y5 in North Slaughter Canyon are offset laterally across a 200 m wide transfer zone that is extensively faulted and fractured. In the transfer zone, faults and fractures I-X deflect by c. 10°. In detail, both the Z1-Z2 and Y1-Y2 fault systems consist of en-echelon fault segments, 10 m to 600 m long, that are laterally offset by 2 m to 30 m.

Vertical segmentation of the syndepositional faults is apparent in cross-section. Individual segments have a length scale of 10-80 m and are normally arranged en-echelon. Linkage of individual fault segments is mostly across restraining bends. Location of the fault segment boundaries is closely related to the platform stratigraphy. The Yates 2, 3, and 4 HSF's are separated by thick incompetent sandstone packages and consequently form distinctive mechanical units across which there are major changes in fault-zone architecture. Fault segments within the Yates 3 and 4 HFS's are interpreted to have initiated as dilational fractures in the outer arc of fault-tip propagation folds. Further hangingwall subsidence resulted in the accumulation of displacement on these fractures and the eventual downward propagation of separate fault segments to link with the main fault tip. Thus, vertical propagation of the faults occurred through both 1) the upward growth of the main fault tip and/or 2) the downward growth of previously isolated fault segments to link with pre-existing faults or fractures.

Faulting and sedimentation 

Comparison of cycle and HFS thicknesses between sections measured in the foot- and hangingwall of syndepositional faults clearly demonstrates that subsidence on the Capitan platform was highly variable. Differential subsidence related to fault movement varied both temporally and spatially across the platform, across different faults. It also varied through time and along the trace of individual faults. Further complication arises as the sense of movement on some faults was reversed as they were rotated past the vertical by overall down-to-the-basin differential subsidence.

It is due to such temporal and spatial changes in the rates of fault movement, coupled with variability in rates of platform aggradation and fault-tip location, that the impact of faulting on sedimentation is itself very variable. However, 4 main fault tip types are readily differentiated. Several different stages of fault tip arrangement tend to characterize the growth of most faults.

On many of the faults, significant changes in thickness are observed. However, the impact on facies changes is only really apparent in the Yates 1 and 2 HFS. For example, significant facies changes occur across faults A, C, and E. It is unfortunate that within the Yates 3-4 HFS it is generally impossible to differentiate the effects of fault-induced changes in facies from the inherent variability within the tepee-pisolite facies. In the Yates 3 & 4 HFS's the most obvious changes are in the distribution of the sandstones, as is apparent in the Walnut syncline growth fold, for example.

Conclusions 

The Capitan platform is cut by closely-spaced syndepositional faults that parallel the platform margin. The faults have a maximum measurable displacement of 24 m in back reef strata. They are laterally and vertically segmented, with vertical segmentation controlled by the stratigraphic hierarchy of the platform succession. Faults grew through segment linkage. The rates of fault-related subsidence vary along and through time on individual faults, and between faults. Faults throwing down-to-the basin were steepened as they grew by differential subsidence. Movement on such faults was normally reversed as the fault was steepened past the vertical.

The impact of faulting on facies patterns varied as a function of the interplay between rates of fault-induced subsidence, platform aggradation, and fault tip propagation. The impact of faulting on sedimentation is most apparent in the Yates 1 and 2 HFS's but is unclear in the Yates 3-4 HFS carbonates. This is due, in part, to the inherent variability within tepee-pisolite facies that dominate these HFS's where the faults have been studied in detail.

References 

Kerans, C.K., Fitchen, W.M., Gardner, M.H., Sonnenfeld, M.D., Tinker, S.W., and Wardlaw, B.R., 1992, Styles of sequence development within uppermost Leonardian through Guadalupian strata of the Guadalupe Mountains, Texas and New Mexico, in Murk, D.H. and Curan, B.C., eds., Permian Basin Exploration and Production Strategies: Applications and Sequence Stratigraphic and Reservoir Characterization Concepts: West Texas Geological Society, Symposium 92-91, p. 1-7.

Kosa, E., Hunt, D., Fitchen, W.M., Bockel-Rebelle, M.-O., and Roberts, G., 2004, The heterogeneity of paleocavern systems developed along syndepositional faults, Upper Permian Capitan platform, New Mexico, USA, in Permo-Carboniferous Carbonate Platforms and Reefs: Society of Economic Paleontologists and Mineralogists, Special Publication 78 (AAPG Memoir 83), p. 291-322.

Palmer, A.N. and Palmer, M.V., 2000, Hydrochemical interpretation of cave patterns in the Guadalupe Mountains, New Mexico and West Texas: Journal of Cave and Karst Studies, v. 62, no. 2, p. 91-108.