REEVALUATING CHERT-BEARING HORIZONS AS EVIDENCE OF UNCONFORMITIES IN THE MIDDLE JURASSIC ELLIS GROUP, WYOMING AND MONTANA

For at least 60 years, chert-bearing horizons have been recognized as potentially significant horizons in the Middle Jurassic units of Wyoming and Montana. Over the last three decades, chert-bearing horizons have been increasingly used as the basis for recognizing unconformities within and for dividing the Bajocian and Bathonian Gypsum Spring and Piper formations. Yet, concurrently, several separate studies have indicated the sporadic occurrence and lateral discontinuity of these chert horizons.

Outcrop measurements and T-R cycle analyses of the Gypsum Spring, Piper, and Sawtooth formations suggest that a number of the chert horizons do not coincide with significant cycle boundaries. The chert-bearing horizons are divided into three different types including chert-limestone breccia (Type 1), bedded chert (Type 2), and chert nodules (Type 3). The only type that coincides with a cycle break is the chert-limestone breccia that occurs at the erosional contact between the Middle Jurassic and post-Mississippian. The Type 2 bedded chert is found in the upper surface of the middle member of these formations, but does not coincide with a cycle boundary. This surface is considered a silcrete formed in an arid climate as the product of replacement from ground water or capillary rise of silica-rich water into the soil above the water table. Therefore, the Type 2 chert only indicates that an erosional surface occurred stratigraphically above the bed not necessarily at the chert horizon. The Type 3 chert nodules are interpreted as silicified gypsum nodules as they share similar external and internal structures as surrounding gypsum fabrics.