"Fractures in detachment folded Mississippian-Pennsylvania Lisburne Group carbonates provide insight into the distribution and character of natural fractures as a function of folding and lithology. Data from five detachment folds suggest that hinges show a higher fracture density than limbs. This study also suggests that the amount of shortening does not play a significant role in determining fracture density or uniformity of fracture orientation. A mechanical classification based on lithologic homogeneity reflects natural fracture distribution as a function of lithology more accurately than conventional lithologic classifications. Two main fracture sets were observed, a N-S set, perpendicular to fold axes, and an E-W set, parallel to fold axes. Statistical analyses suggest that E-W fracturing occurred before and during folding and that N-S fracturing occurred both before and after folding"--Leaf iii.

"The distribution of fold related fractures and other mesoscopic structures in asymmetrically folded Mississippian to Pennsylvanian Lisburne Group carbonates gives clues concerning the mechanism of folding. Since fracture sets pre-date and post-date folding, it is important, but sometimes difficult, to determine which fracture sets are related to folding. Higher density of fold related fractures and dissolution cleavage in the hinges than limbs of two folds in the study area is evidence for fixed hinge detachment folding. However, geometric modeling of box shaped folds in the study area suggests that some folds may have formed by either detachment folding or trishear fault propagation folding. Formulaic modeling of fracture density in a stratigraphic section using stratigraphic attributes such as lithology, bed thickness, and chert content predicts general trends in fracture density, but other factors such as slip along bed contacts may obscure the relationship between fracture density, lithology and bed thickness"--Leaf iii.

In this volume, the geologic framework is established with review papers by experts in carbonate generation, rock properties, sequence and seismic stratigraphy, and structural deformation. Then seismic expression of carbonate terranes is explored in case studies showing the importance of integrating seismic and petrophysical control with geologic models.

The Carboniferous Lisburne Group is a major carbonate reservoir unit in northern Alaska. The Lisburne is detachment folded where it is exposed throughout the northeastern Brooks Range, but is relatively undeformed in areas of current production in the subsurface of the North Slope. The objectives of this study are to develop a better understanding of four major aspects of the Lisburne: (1) The geometry and kinematics of detachment folds and their truncation by thrust faults. (2) The influence of folding on fracture patterns. (3) The influence of deformation on fluid flow. (4) Lithostratigraphy and its influence on folding, faulting, fracturing, and reservoir characteristics. The Lisburne in the main axis of the Brooks Range is characteristically deformed into imbricate thrust sheets with asymmetrical hanging wall anticlines and footwall synclines. In contrast, the Lisburne in the northeastern Brooks Range is characterized by symmetrical detachment folds. The focus of our 2000 field studies was at the boundary between these structural styles in the vicinity of Porcupine Lake, in the Arctic National Wildlife Refuge. The northern edge of thrust-truncated folds in Lisburne is marked by a local range front that likely represents an eastward continuation of the central Brooks Range front. This is bounded to the north by a gently dipping panel of Lisburne with local asymmetrical folds. The leading edge of the flat panel is thrust over Permian to Cretaceous rocks in a synclinal depression. These younger rocks overlie symmetrically detachment-folded Lisburne, as is extensively exposed to the north. Six partial sections were measured in the Lisburne of the flat panel and local range front. The Lisburne here is about 700 m thick and is interpreted to consist primarily of the Wachsmuth and Alapah Limestones, with only a thin veneer of Wahoo Limestone. The Wachsmuth (200 m) is gradational between the underlying Missippian Kayak Shale and the overlying Mississippian Alapah, and increases in resistance upward. The Alapah consists of a lower resistant member (100 m) of alternating limestone and chert, a middle recessive member (100 m), and an upper resistant member (260 m) that is similar to Wahoo in the northeastern Brooks Range. The Wahoo is recessive and is thin (30 m) due either to non-deposition or erosion beneath the sub-Permian unconformity. The Lisburne of the area records two major episodes of transgression and shallowing-upward on a carbonate ramp. Thicknesses and facies vary along depositional strike. Asymmetrical folds, mostly truncated by thrust faults, were studied in and south of the local range front. Fold geometry was documented by surveys of four thrust-truncated folds and two folds not visibly cut by thrusts. A portion of the local range front was mapped to document changes in fold geometry along strike in three dimensions. The folds typically display a long, non-folded gently to moderately dipping backlimbs and steep to overturned forelimbs, commonly including parasitic anticline-syncline pairs. Thrusts commonly cut through the anticlinal forelimb or the forward synclinal hinge. These folds probably originated as detachment folds based on their mechanical stratigraphy and the transition to detachment folds to the north. Their geometry indicates that they were asymmetrical prior to thrust truncation. This asymmetry may have favored accommodation of increasing shortening by thrust breakthrough rather than continued folding. Fracture patterns were documented in the gently dipping panel of Lisburne and the asymmetrical folds within it. Four sets of steeply dipping extension fractures were identified, with strikes to the (1) N, (2) E, (3) N to NW, and (4) NE. The relative timing of these fracture sets is complex and unclear. En echelon sets of fractures are common, and display normal or strike-slip sense. Mesoscopic and penetrative structures are locally well developed, and indicate bed-parallel shear within the flat panel and strain within folds. Three sets of normal faults are well developed in the area, and are unusual for the Brooks Range. One set is parallel to and another is transverse to the strike of the folds. A single major normal fault has an intermediate orientation. The normal faults cut across folds, but may have been active late during folding because fold geometry differs across faults and some folding apparently continued after normal faulting.

"The mechanical properties of individual stratigraphic layers in a multi-layer sequence of sedimentary rock influence the deformational response before, during, and after fold development. To demonstrate this, the mechanical character of stratigraphic layers and mesoscopic deformational structures within individual stratigraphic layers were documented in two well-exposed outcrop-scale detachment folds in the Lisburne Group carbonates, northeastern Brooks Range, Alaska. Fold geometry and fold-related mesoscopic structures indicate that flexural slip and flexural flow are the operative fold mechanisms until a critical interlimb angle of 90° is reached, after which homogenous flattening occurs. Changes in bed thicknesses due to homogenous flattening alter the overall fold geometry. Lithostratigraphic unit boundaries do not always coincide with mechanical unit boundaries. Thin shale layers lower the bedding interface strength and commonly form flexural slip horizons that define mechanical unit boundaries. As fold shortening progresses, slip horizon spacing is interpreted to decrease, causing mechanical unit thickness to decrease. Newly forming mechanical boundaries alter the conditions of deformation, which change the overall fold dynamics. Surveyed fracture sets reveal the influence of lithology, mechanical unit thickness, anisotropy, and structural position on fracture distribution within individual mechanical units. Fracture densities vary from set to set and unit to unit due to structural and stratigraphic controls within these folds"--Leaf iii.