A POSSIBLE ORIGIN OF HYDRAULICALLY SIGNIFICANT BED PARALLEL MACROPORE NETWORKS IN SILICICLASTIC BEDROCK

Bed parallel macropores (BPMs), commonly referred to as bedding plane fractures, are key pathways for groundwater flow in sedimentary bedrock. Long recognized in carbonate rock, they are now recognized as common in siliciclastic strata. Despite increased recognition of BPMs as integral parts of hydrogeologic systems, their origin remains poorly understood.

Our research combines borehole and outcrop observations in an effort to understand the genesis of BPMs in Lower Paleozoic siliciclastic bedrock of the North American cratonic interior. In Minnesota and Wisconsin, BPMs are common in sandstone, siltstone and shale, to depths exceeding 200 meters. They are densely clustered along discrete (<2m) stratigraphic intervals. Individual macropores in two dimensions are elongate along bedding, with an irregular “pinch and swell” geometry. They can be traced laterally up to a few meters, and have an aperture height of less than 5 cm. Limited 3-dimensional exposure of BPMs, as well as hydraulic properties measured in boreholes and springs, are indicative of a larger scale geometry that includes connectivity as a network of channels along bedding.

BPMs are commonly associated with interbedded sandstone and mudstone displaying stratabound brittle to ductile deformation features that include breccia, boudin, contortions, flame structures, and mm- to cm-scale pipes, sills and clastic dikes. Aperture geometry of individual BPMs commonly mimics adjacent contorted beds. Deformation included significant lateral redistribution of sediment, typically most pronounced along thin beds of mud that were compacted and squeezed laterally into thick pods.

We propose an origin of BPMs via shallow stratabound redistribution of water and variably lithified sediment during deformation early in burial history. Water expelled by sediment compaction collected in macopore-scale voids connected in an anastomosing network. The macropores may be analogous to water voids produced in laboratory experiments simulating sedimentary deformation. Such voids are regarded as transient in experiments, but the presence of partially lithified and possibly thixotropic sediment, and the apparent confinement of deformation to discrete stratal intervals, may have caused the voids in our ancient examples to persist until further lithification.