Examples in the northern Brooks Range suggest that some such folds may not have originated as a result of fault propagation but instead as detachment folds. The Brooks Range folds originated as buckles in a competent unit over an incompetent unit. Anticline growth was by rotation of the limbs about fixed hinges in the competent unit and by thickening of the incompetent unit, but without propagation of a fault up a ramp. Commonly, the steep forelimbs of these anticlines were subsequently truncated by thrust faults as shortening continued.
Folds formed as detachment folds differ fundamentally from fault-propagation folds.
A detachment anticline is cored by an incompetent unit and, if the anticline has been truncated by a thrust fault, a syncline is commonly present in the footwall ramp. Distinguishing between these two fold mechanisms thus has important implications for interpreting the amount and distribution of reservoir, seal, and trap, and the development and preservation of porosity and permeability. Basinal sequences were defined by similar criteria, excluding subaerial exposure surfaces, in conjunction with variations in the mineralogy and clast content of onlapping basinal facies. Variations in platform margin geometries and facies stacking patterns within the Miette and Ancient Wall buildups and surrounding basinal sediments reveal the effects of carbonate sediment production and clastic sediment supply within a hierarchy of relative sea-level change. Definitive subaerial exposure features occur only at the top of sequence 3.