Skip to Main Content

Roadcut Section B

There are three readily recognizable, multistory channel-fills present in Section B: A, the largest, with a minimum of 7 meters of erosional relief, B, the smallest, with 1.25 meters, and C with 2.25 meters.  Channel-fill A contains crinoid grainstone and siliciclastic mudstone fills, B only crinoid grainstone fill, and C crinoid pack-/wackestone fill.  The erosional channel bases are readily apparent where different lithologies are juxtaposed, however, where siliciclastic mudstone comprises both the fill and the channel- bank material, the erosional margin can be inferred by different bedding geometries, horizontal in the bank and inclined in the fill.  Channel-fills comprise approximately 45% of the material exposed at this roadcut.

The prominent Channel A can be divided into two separate units, 1) a thicker, lower unit primarily composed of siliciclastic mudstone interbedded with crinoid grainstone and pack/wackestone interbeds organized into channel-fills that pinch-out both up- and down-dip, and 2) a thinner, upper crinoid grainstone unit (1 on Section B).  The bedding in the lower heterolithic unit is essentially parallel to the channel base, very gently inclined in the center becoming more steeply inclined over the NE margin; with one exception, the much smaller, interbedded grain-/pack-/wackestone channel-fills do not onlap the main channel base but also are concordant or parallel to it.  Farther to the NE of the steeply inclined margin the bedding again becomes basically horizontal, see yellow dashed box 1, an indication that here Channel A has a nearly horizontal base.  This base concordant geometry indicates simultaneous deposition over the entire channel and out onto the adjacent interfluve, to borrow a fluvial geomorphology term.  The bedding in the upper crinoid grainstone onlaps the erosion surface separating this grainstone from the underlying heterolithic unit.  Both of these units are formed primarily by vertical accretion, however, the grain-/pack-/wackestone interbeds of the lower indicate deposition from currents flowing parallel to the strike of the inclined channel margin and hence down the axis of the channel itself.

The bedding in both Channels B and C onlap their margins, again an indication of vertical accretion.  It is tempting to combine B and C into one larger channel-fill, however, the steep, northeast margins of B and C could not be connected because where this connection would occur only siliciclastic mud is present and thus any eroded margin would be very difficult to identify.  It should be noted that both Channels B and C have broad, flat bases and geographically restricted, steep margins, a geometry indicated for the much, much larger Channel A, thus all three channels may have very similar aspect ratios.

The uppermost pack-/wackestone unit (2 in Section B) has a sheet-like geometry and perhaps marks the change from more channelized or confined deposits to unconfined units, a change seen in many of the other sections.

section-b