Unconformities, Paleokarst Facies, and Porosity Evolution: ABSTRACT

Abstract

The study of unconformities and paleokarsts from the perspective of modern facies analysis and modeling offers potential advantages in terms of organizing and guiding observations, comparisons, interpretations, predictions, and hydrodynamic considerations. Karsts that developed at major unconformities may result in karst facies and profiles different in many respects from those of meteoric diagenesis in Quaternary carbonates in tropical areas (e.g., the Caribbean). Many paleokarsts developed on mud-supported carbonates after mineralogical stabilization, deep burial, and tectonic deformation, but without the diffuse recharge and flow characteristics of the Caribbean model or the influence of a coastal marine mixing zone. The general concepts of water table and vadose and phreatic regimes need careful review when applied to heterogeneous permeability networks. Karst facies and profiles are controlled by (1) previous permeability networks of the affected formation, (2) balance and interaction of climatic, biologic, and hydrologic environments that enhance or reduce these permeability networks, and (3) timing, rate, and succession of environments, and stages of evolution. Karst facies, facies associations, and their profile arrangements generally vary, and may be complicated by relict and rejuvenated features common in evolved karst profiles. In detail, karst facies are defined in terms of (1) corrosion-erosion morphologies, (2) diagenetic overprints, (3) karst sediments and cements (speleothems), and (4) biologic associations. A common, mature, authigenic karst profile consists of the following zones. (1) Soil, infiltration zone--down to the limit of root penetration. (2) Percolation zone--with vertical passages and abundant sedimentation, collapse, and cementation, commonly containing relict features (cave levels) from deeper horizons or local saturation zones. (3) Oscillation zone--characterized by periodic water saturation and, in terms of lithofacies, commonly indistinguishable from the permanent lenticular zone (shallow phreatic); predominantly horizontal passages with bedding-plane control and erosional features are the key characteristics of this zone; cave sediments show evidence for reducing depositional environments; many well logs show a characteristic kick in the gamma ray (B marker), together w th a decrease in the sonic activity. And (4) deep phreatic zone--characterized by incipient, slow corrosion and/or cementation and grading into the unaffected formation. In most places, a rock formation is first exposed to the deep phreatic zone and evolves through the shallow phreatic into the vadose as a result of the dismantling of the upper part of the profile. In this way, the classic concepts of youth, maturity, and senility can apply to parts of the karst profile or to the entire profile, and can provide a basis for comparing other profiles of the same karst system. Base level changes are commonly sharp and produce repeated horizontal cave levels that are abandoned in the vadose part of the profile. In many paleokarsts, those relict cave levels have been confused with repeated surfaces of subaerial exposure. Correlating different karst profiles and the structural-lithologic patterns of the affected formation offer the possibility of reconstructing the evolution of drainage patterns during major unconformities. This karst facies modeling can also provide a basic tool for reservoir evaluation in exploration and production. End_of_Article - Last_Page 1440------------