Microfolding in the Permian Castile formation: An example of geometric systems in multilayer folding, Texas and New Mexico J.I.D. ALEXANDER Center for Microgravity and Materials Research, University of Alabama, Huntsville, Alabama 35899 A. J. WATKINSON Washington State University, Department of Geology, Pullman, Washington 99164-2812 ABSTRACT Outstanding examples of small-scale folds in the Castile forma- systems. Even more striking is that competent layers both thicker and tion are of considerable interest to structural geologists because, thinner than folded layers remain planar. We believe that current fold owing to the low-strain condition of the rock (-25% shortening), they theories for the onset of folding, which are based on the premise of afford a view of folds that have not yet formed pervasively throughout folding perturbations being identically unstable, cannot adequately the multilayer. Some folds have long, along-the-layer continuity in- explain this observation. A published theory is discussed that incorpo- volving few of the layers of the multilayer sequence. Others, predomi- rates both gravity and surface-tension effects and offers an explana- nantly in the hinge zone of larger folds (wavelength >1 m), have more tion for this observation. It suggests that folding in the Castile was predominant components of across-the-layer folding. Combinations likely a phenomenon akin to a Kelvin-Helmholtz instability, that is, a of these components result in dendriform patterns of distribution of critical rate of strain must be exceeded before folding occurs. folds and possible zones of interference between systems. One of the most striking features of the Castile folding is the INTRODUCTION presence of planar layers between folded layers, creating internal fold One of the outstanding descriptions of small-scale folds in the geo- logic literature is that of Kirkland and Anderson (1970). They described small-scale folds in a multilayered anhydrite-calcite formation, the Castile, in Texas and New Mexico. We believe that these folds are of considerable interest to structural geologists because, owing to the low-strain condition of the rocks, they afford a view of folds that have not yet formed perva- sively throughout the multilayer. This gives us an unusual opportunity to examine the geometry of such isolated fold systems. The field observations stimulate potential ideas as to how such systems may interact with one another, leading to the complex geometries that are so typically observed in pervasively folded systems. Whereas the main analyses by Biot (1957, 1961,1964,1965a, 1965b), Ramberg (1963,1970a, 1970b, 1979), Smith (1975, 1977, 1979), Fletcher (1974, 1977, 1979), and Johnson (1977) of the onset of geologic folding may broadly explain some of the observed structures, certain other features in the folded Castile are difficult to ex- plain by these analyses. We would like to bring to the attention of geolo- gists an analysis with both gravity and layer-interfacial effects (surface tension) included, which has the advantage of giving an explanation of certain key observations (Wollkind and Alexander, 1982). The first part of this paper describes in more detail than in Kirkland and Anderson (1970) some of the fold geometries observed in the Castile formation. It promotes the idea of describing multilayer folds in terms of geometric systems (Watkinson and Alexander, 1979); that is, within the Figure 1. Locality of Castile outcrop, on the state line between total multilayer fold system, there are sets or groups of folds with geometric Texas and New Mexico. See Anderson and Kirkland (1987) for more features ( wavelengths and/or amplitudes and/or profile shapes) distinctive details. from other sets. Geological Society of America Bulletin, v. 101, p. 742-750, 12 figs., 1 table, May 1989. 742 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/742/3380736/i0016-7606-101-5-742.pdf by guest on 26 September 2021 MICROFOLDING IN PERMIAN CASTILE FORMATION 743 Figure 2a. Example of an isolated fold system involving one ob- Figure 2b. Example of multilayer folds having predominant com- vious predominant anhydrite layer. ponents of along-the-layer folding, with a small component of across- the-Iayer folding in the top right-hand corner. The second part draws attention to one or two specific fold geome- pecially when compared to those observed in modern-day sabkha envi- tries and outlines the elements of Wollkind and Alexander's analysis, in ronments (for example, Shinn, 1983, p. 195-196). In fact, these folds are the context of a discussion of previous analyses, which provide an attractive some of the most periodic and regular that we have observed anywhere in explanation for such features. We argue that the folding in the Castile the world. formation has occurred because a critical rate of strain has been locally A key observation made by Kirkland and Anderson was that the exceeded. The planar layers were thus no longer stable to small perturba- tions of a critical wavelength (defined by the critical rate of strain), and so these perturbations grew. The onset of folding in the Castile thus can be viewed as a stability problem in the same class as the Kelvin-Helmholtz instability (Chandrasekhar, 1961; Yih, 1965). The Kelvin-Helmholtz in- stability is a shear instability which arises in parallel flows of stratified fluids. The essential mechanism of the instability can be described as follows. The available kinetic energy of the relative motion of layers of the basic flow is converted kinetic energy of the basic disturbance. For layers separated by an interface, instability will occur, provided the kinetic energy of the disturbance is sufficient to overcome both the potential energy needed to raise and lower the fluid (whenever the density decreases with height) and the increase in surface free energy consequent to a defor- mation of the initially planar interface. ORIGIN OF THE CASTILE MICROFOLDING— "SEDIMENTARY" OR TECTONIC? We preface our discussion of the observations on the folding with a brief review of ideas concerning the origin of the folds. There is still lively debate, concerning both the origin of the forces that formed the Castile folds and the exact state of the layers at the time of folding. Kirkland and Anderson (1970) carefully considered and recently reiterated (Anderson and Kirkland, 1987) the various ideas concerning the origin of the folds, that is, slump triggered by earthquake activity, ripple marks, crystal growth, or tectonics. They considered that because the folds are polyharmonically associated with larger-wavelength folds, and have reasonably consistent fold axial directions parallel to regional structural trends (but oblique to the projected basin paleoslope), the folding was most likely of tectonic origin. As we shall illustrate further, these Castile folds are strikingly periodic and regular (although locally developed), es- Figure 3. An example of one of the few chevron/kink-like folds observed in the Castile outcrop. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/742/3380736/i0016-7606-101-5-742.pdf by guest on 26 September 2021 744 ALEXANDER AND WATKINSON unfolded portions of the layers have thickened in comparison to the folded portions, consistent with the idea that lateral compression of the layers led to either layer thickening or buckling. Also, the field evidence for a con- trast in layer behavior between the anhydrite and the calcite layers seems consistent and unequivocal. It is, however, a notoriously difficult task to ascertain the exact state of the layers at the time of folding, that is, whether the folds are "soft sediment" or tectonic (see detailed discussions in Elliott and Williams, 1988; Maltman, 1984). In terms of the mathematical modeling, the field observations are the basis for establishing two of the major boundary conditions or constraints for the model—layer-parallel shortening and "viscosity" contrast between the anhydrite and calcite-rich layers. Although we cannot be dogmatic about the exact rheological state of the layers at the time of folding, we consider the modeling to be appropriate as long as the layers have short- ened and were in a "viscous" state at the time of folding. Furthermore, as we shall illustrate, we believe that the modeling can explain the intermit- tently developed nature of the folds. As structural geologists, we would challenge the sedimentologists to prove that the layers were unconsoli- dated at the time of folding! As modelers, it is not crucial to the model. Indeed, if the system was "fluidized" at the time of folding, the inclusion of surface tension may be particularly appropriate for modeling this particu- lar fold system (see later discussion also). FIELD OBSERVATIONS The field observations of the folding of the Castile formation are based on our own observations of the Eddy County state-line outcrop, New Mexico (Fig. 1), coupled with observations on core samples very Figure 5. Dendriform patterns of combinations of across-the-layer and along-the-layer compo- nents. These patterns typ- ically occur in the hinge zone of larger-scale folds. The insert (b) shows Figure 4. An exam- blocked-out areas of ple of multilayer folding. folds. The folds tend toward a similar fold geometry. Note the change in sym- metry of the folds from the lower to the upper folds. kindly made available to us by Anderson, and on observations made by Kirkland and Anderson (1970). The formation consists of alternating laminae of brown calcite and gray-white anhydrite. The anhydrite layers are as much as 5 mm thick with a mean thickness of 1.1 mm. The brown calcite-rich layers, containing some organic material plus some intermit- tently distributed anhydrite crystals, are on average 0.4 mm thick and as Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/5/742/3380736/i0016-7606-101-5-742.pdf by guest on 26 September 2021 MICROFOLDING IN PERMIAN CASTILE FORMATION 745 Figure 6.