ANISOTROPY, IMPORTANCE OF DIP WITHIN CARBONATE SEQUENCES

Significance of dip within sequences of carbonate rocks must be evaluated when investigating mine and quarry dewatering projects, evaluating sustained yield of wells and well fields and when developing numerical flow and transport models within the Valley and Ridge. Thin shale partings, metabentonites even stylolites contribute to anisotropy and support perched water where dips are small <10 to 15° and topography favors deep water tables. If overlooked during hydrogeologic site characterization efforts, hydraulic gradients, and directions of groundwater flow can be seriously mischaracterized.

Anisotropy tends to increase as dips exceed 15 to 30°. Pumping cones of depression become elongated parallel to stratigraphic strike. Width to length ratios of 1:7 to 1:23 have been documented. Drawdown is restricted across bedding even where lineament-and fracture trace-related structures and faults cross-cut stratigraphic sequences. “Cliff-like” gradients have been documented immediately adjacent to the 960-foot deep former Bell Mine, Centre County, PA and deep Central Appalachian quarries with nearly vertical dips. Groundwater flow tends to be perpendicular to bedding along mountain flanks that lack deeply eroded transverse valleys. Conduits are poorly developed in this setting. Stratigraphic strike dominates groundwater flow approaching transverse valleys and karst drainage nets are well developed. Other interactive geologic variable in addition to dip and stratigraphic sequence must be evaluated for site characterization efforts to be effective, e.g. petrographic properties, topography, karstic and tectonic base levels, water table and paleo-water table configurations, presence of paleokarst, climate among more than 36 variables routinely considered by the author when working in karstic terrains.