Geology of the north end of the Ruby Range, southwestern Montana

Abstract

This study consists of two parts: stratigraphy and sedimentation, and structure of rocks in the northern one-third of the Ruby Range of southwestern Montana. Detailed studies of Cambrian marine dolomite rocks in the Red Lion Formation and in the upper part of the Pilgrim Limestone resulted in their division into distinct rock units, termed lithofacies. These lithofacies contain features suggestive of subtidal, intertidal, and supratidal environments similar to those presently forming in the Persian Gulf. Stromatolltic structures occurring in the uppermost part of the Red Lion Formation are similar to those presently forming in Shark Bay, Australia. The Ruby Range within the map area is broken into a series of northwest-plunging basement (Precambrian metamorphic rock) blocks, differentially uplifted during the Cretaceous-Tertiary orogenic period. These blocks are bordered by upthrust faults, which are nearly vertical in their lower segments and are .low-angle in their uppermost parts. Asymmetrical folds in Paleozoic sedimentary rocks formed in response to the differential uplift of the blocks; thus they too plunge to the northwest. Displaced masses of rock border the range on the three sides within the map area and are interpreted as gravity-slide features resulting from uplift of the range. Normal faulting began blocking out the present range margins by Oligocene time.