West flank San Rafael Swell on I-70, north side of interstate.
Curtis Formation, upper San Rafael Group
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.
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