Large-Scale Distributions of Sedimentary Stylolites in Carbonates: Field Observations and Emerging Insights Regarding Evolution, Strain, and Dissolution
Aharonov, Einat¹, Laronne Ben-Itzhak, Leehee¹, Karcz, Zvi², Toussaint, Renaud³, Kadori, Maor¹ and Sagy, Amir4
¹The Hebrew University of Jerusalem, The Institute of Earth Sciences, Jerusalem, Israel.
²Delek Drilling, Herzelia, Israel
³Institut de Physique du Globe de Strasbourg, CNRS, University of Strasbourg, Strasbourg Cedex, France
4Geological Survey of Israel, Jerusalem, Israel
Stylolites are rough surfaces formed by localized rock dissolution. They are prevalent in carbonates and other sedimentary rocks, and strongly impact the host rock porosity and permeability. Depsite their importance for the evolution of sedimentary basins and for accomodating compactive strain, the formation of styolites is somwhat of a mystery, since stylolites were never formed in the lab. In addition, their dissolution and strain accomodation, and interaction with neighboring features (such as other styolites and fractures) is not well understood.
We present a combined field and modeling study aiming to: 1. Characterize the large-scale behavior of sedimentary stylolites, and in particular define types of “stylolite populations” 2. propose a consistent physical model for stylolite and stylolite population evolution, 3. Propose an upscaling of styolite measures from the core to the field scale, 4. Provide an estimate of the large scale strain and dissolution/precipitation associated with stylolites.
Most previous studies have focused on relatively small-scale characteristics of single stylolites. Instead, we preformed a meso-scale field study on sedimentary stylolites in carbonates, characterizing large scale distributions of stylolites, including measurements conducted on more than 1 km long stylolites, the largest stylolite lateral distribution ever reported in the literature. Based on the field study, performed at 9 different field locations, we suggest to characterize the large scale distribution of stylolites via “stylolite populations”, which we divide into three end-member types: isolated stylolites (Type I), long-parallel stylolites (Type II), and interconnected stylolite networks (Type III). Type III can be further divided into anastamosing stylolite networks and stylolite-fracture networks. We characterize the different populations by different statistical parameters and measures, and discuss conclusions that may be drawn from small-scale observations regarding large-scale distributions.
Following the characterization stage, a mechanistic model is proposed to explain the evolution of the different population types. The model suggests that stylolites grow via interplay between localization by mineralogically-enhanced pressure solution [Aharonov and Katsman, 2009], and stress-driven interaction with other geological structures (such as other styolites or cracks):
Type I. Isolated stylolites grow from lenses of compositional heterogeneities on which pressure solution is enhanced.
Type II. Long-parallel stylolites grow by roughening of compositionally distinct bedding planes that undergo enhanced pressure solution.
Type III. Interconnected stylolite networks grow when stylolites of Type I or Type II interact among themselves or interact and connect with cracks.
We present field evidence that the proposed mechanistic model describes well the evolution of stylolites and their interaction with surrounding structures.
An important measure to obtain regarding stylolites is the amount of dissolution on them and the amount and type of strain they induce, where strain is a combination of compactive strain via dissolution on stylolites, extension in veins, and shear between interacting stylolites. The strain and dissolution on each of the different population types is estimated:
Type I, Isolated styolites: assuming some spatial distribution of isolated styolites, simple calculations show that dissolution on each styolite affects the region around it via shear and normal stresses. Dissolution will encourage distanced stylolites to connect. A population of isolated styolites will quickly mature to Type III populations.
Type II, Long parallel styolites: Here we measure stylolite surfaces at a scale larger than ever measured before (10-2-10 meters), using ground-based-LIDAR, producing a topographic map of the surfaces. Derived roughness characteristics show the styolites are self-affine with an upper cutoff at 10-100cms. This observed upper-bound of self-affine roughness is measured here for the first time, but has been previously predicted by theory [Ebner et al., 2009a,b; Koehn et al., 2007]. Our measurements support these theoretical models and together with them present a scenario in which stylolites evolve from preferential dissolution along an existing surface that was initially smooth and progressively roughened with time. Based on the theoretical roughening model that we adopted, the upper limit to fractality for this class of stylolites may be used as a measure of the amount of dissolution on stylolites. Indeed, the amount of dissolution on the stylolites calculated from the upper limit to fractality, is comparable to our estimates of dissolution from two additional independent techniques [Laronne Ben-Itzhak et al., 2012].
Type III, interconnected networks: This population type is the most complex to understand, and strain/dissolution on it is non-trivial. The amount of dissolution depends on how the network actually evolved, whether by connecting isolated stylolites via dissolution or fractures, or by long parallel stylolites connecting by canabelizing each other during dissolution. We propose an estimate of strain for the two cases.
The 3 population types also affect fluid flow differently, by producing a different spatial distribution of porosity. We discuss briefly the implications for flow.
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Ebner, M., S. Piazolo, F. Renard, and D. Koehn, Stylolite interfaces and surrounding matrix material: Nature and role of heterogeneities in roughness and microstructural development, Journal of Structural Geology, 32 (8), 1070-1084, 2010.
Koehn, D., F. Renard, R. Toussaint, and C.W. Passchier, Growth of stylolite teeth patterns depending on normal stress and finite compaction, Earth and Planetary Science Letters, 257 (3-4), 582-595, 2007.
Laronne Ben-Itzhak L., E. Aharonov, R. Toussaint, and A. Sagy, Upper bound on stylolite roughness as indicator for amount of dissolution, in revision for Earth and Planetary Science Letters, 2012.
AAPG Search and Discovery Article #120034©2012 AAPG Hedberg Conference Fundamental Controls on Flow in Carbonates, Saint-Cyr Sur Mer, Provence, France, July 8-13, 2012