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Day 4, Stop 1

Curtis Formation Stop


West flank San Rafael Swell on I-70, north side of interstate.



Upper Jurassic


Rock Units:

Curtis Formation, upper San Rafael Group


Features Present:

Curtis Formation tidal bundlesoccur in exposures along I-70.  These are sigmoid-shaped sets of cross-strata, enclosed within mud drapes of fine-grained sediment and organic debris (Figure 1A).  Fully developed, sigmoidally shaped tidal bundles are well developed at essentially one stratigraphic horizon, but all megaripple cross-bedding in the sequence contain well-developed tidal features.  Look for the associated flaser bedding and herringbone cross-bedding. The sigmoidal sets are as much as 80 cm thick and extend as much as 11 m laterally.  The enclosing beds are from 0 to 8 cm thick and thicken toward the sigmoid toeset.  Locally, within the toeset, two mud beds may occur, separated by thin sandstone beds.  The upper surface of some sigmoidal cross-bed sets is truncated by a planar to undulatory scour surface (Kreisa & Moiola, 1986).    The Curtis Formation contains a broad suite of tidally generated sedimentary structures, including flat-bedded sandstones and packages of organized tidal rhythmites (east side of the long I-70).  Figures 1A, 1B and 2 illustrate the internal sedimentary structures of Curtis tidal bundles.  Overall, the Curtis Formation consists of approximately 55 meters of green-gray, fine- to very fine-grained, moderately- to well sorted and weakly dolomite cemented sandstone.


Depositional Environment:

Tidal, nearshore

The Curtis Formation was deposited in the Jurassic shallow marine seaway that transgressed southward.  Transgressive events and embayed shallow water are factors in development of shallow-marine tidal features.  Curtis Formation sigmoid tidal bundles are probably formed within channels or at the margin of bars in an estuarine environment during relatively high sea level in a high-stand systems tract (Wilcox, 2007).   The tidal bundles reflect increasing then waning flow velocity during a tidal episode.  For instance, incoming flood tide and outgoing ebb tide are reflected by higher flow velocity while high slack water and low slack water represent low flow velocity.  Pause plane surfaces marked by mud drapes of fine grained sediment represent standstill phases during the subordinate tide and slackwater (Kreisa & Moiola, 1986).  A tidal origin for these sandstones is indicated by their bimodal to polymodal paleocurrent pattern.



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




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.



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.

Last Updated: 1/13/23