SIGNIFICANCE OF STRATA-BOUND VEIN ARRAYS IN THE OLIGOCENE BRULE FORMATION, SCOTTS BLUFF NATIONAL MONUMENT, NEBRASKA

At multiple localities the White River Group of the High Plains Tertiary sequence contains strata-bound vein systems. The distinct strata-bound character suggests a diagenetic origin. The lower Brule Formation at Scotts Bluff, in Western Nebraska, contains two strata-bound calcite vein arrays within fine-grained volcaniclastic sediments. Modeling of vein strike data (n=275) indicates that the orientation of the vein arrays cannot be effectively described without a significant uniform component (>50%). This is consistent with an explanation invoking vein formation through diagenetically-driven syneresis. A vein strike preferred orientation occurs at 175°, which coincides with other structural trends in the region, including the Black Hills Uplift. Veins within both horizons are commonly arranged in localized radial patterns around fossil burrows in which one dominant vein orientation of ~175° extends beyond the others. Joint orientation data taken from the approximate stratigraphic interval display a different preferred orientation centered at 87°. A secondary peak in the joint data at 137° parallels the trend of Scotts Bluff and the North Platte River Valley across Nebraska.

Thin section analysis indicates that the vein fill is presently dominated by calcite. Rare chalcedony inclusions within the calcite fill resemble the crystal form of gypsum. These inclusions suggest a multiple replacement history, with the original gypsum material being replaced by chalcedony, followed by an almost complete replacement by calcite. Veins within the lower horizon are also concentrated within multiple gypsiferous paleosols representing ancient playa surfaces. This vein association, with the abundance of gypsum rosettes in these intervals, attests to the original gypsum composition of the vein fill.

While a strong uniform component to the strike distribution of the vein systems can be related to diagenesis, the existence of a preferred orientation reflects a regional stress field. Different joint preferred orientations record a change in this stress field. Reactivation along the Chadron Arch and far-field Basin and Range influences are possible sources.