Curtis Formation Stop
Figure 1: Figure 1B, A model of internal sedimentary structures pictured in Figure 1A:
1 - A thin basal bed plus overlying, gently dipping laminations, interpreted as ACCELERATION PHASE.
2 - Topset laminations passing over a brinkpoint to foreset laminations, MAXIMUM VELOCITY.
3 - Sigmoidally curved parallel lamination, DECELERATION PHASE.
West flank San Rafael Swell on I-70, north side of interstate.
Curtis Formation, upper San Rafael Group
Sigmoidal bundles and the regular alternating rhythmites are attributed to tidal processes. Channelized tidal flow has distinct slack-water periods that result in mud drapes like these in the Curtis Formation. Curtis tidal bundles display cyclicity that is interpreted to be in response to the lunar month, neap/spring tide fluctuations. The most complete and well-defined bundles occur during the spring phase while neap-tide bundles are less well developed. Kreisa and Moiola (1986) reported on 28 bundles with cyclic variation in thickness and sedimentary structures. They noted that foresets developed during maximum flow of spring tides dip 25o to 28o, but neap-tide foresets are more gently inclined (12o to 25o). Accelerating tidal currents caused the foreset lamination of bundles to steepen. Curtis sigmoid and other laterally accreted tidal bundles are believed to have formed within channels or at the margins of bars (Kreisa & Moiola, 1986). Figure 3 is repeatedly alternating sand (some with ripple cross lamination) and mud rich beds on a scale of a few centimeters. Kreisa and Moiola (1986) interpret these features as having formed in very shallow water (probably intertidal); the flat bedded intervals formed in maximum tidal flow velocities, and the ripple, cross lamination developed during accelerating and/or waning flow conditions.
Figure 2: (T) = topset lamination, (F) = foreset lamination, (B) = brinkpoint, (S) = sigmoidal laminations, (P) = pause plane (Kreisa & Moiola, 1986).
Figure 3: Tidal rhythmites, alternating layers of mud vs. sand rich coupled sets.
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Disclaimer: The information is property of the University of Utah. Unless cited, images and files found on this site have been taken or created by the Geology and Geophysics Department at the University of Utah. Any use of these images should be cited appropriately. The stratigraphic column is from: Mathis, A. C. 2000. Capitol Reef National Park and Vicinity Geologic Road Logs, Utah, in: P.B. Anderson and D.A. Sprinkel (eds.) Geologic Road, Trail, and Lake Guides to Utah’s Parks and Monuments Utah Geological Association Publication 29. http://www.utahgeology.org/uga29Titles.htm
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