The Tops of Clastic Dikes
The tops of most clastic dikes in the megaflood region are truncated by erosion (erosive contacts between flood beds), but not all. Sometimes you get lucky. In certain places dikes connect to their source beds, in others a few fill bands (sheets) connect smoothly with bedding. The following are a few examples of both truncated-top dikes and connected-top dikes. All photos are mine.
A dike that originates at the base of the uppermost rhythmite descends for several meters through six underlying rhythmites, branching and tapering along the way. Burlingame Canyon, Walla Walla Valley.
An unsheeted, single-fill dike intrudes downward into coarse, laminated sands at an angle oblique to the cut face. It is filled by the same material it intrudes. Its top is truncated by coarser gravel deposited by a subsequent flood. We cannot see how the dike connected to its source bed above due to erosion, but clearly the dike has no connection to a source bed below. The base of the structure is well exposed. South-flowing, high energy flood channel and eddy bar setting. Smith Coulee, Channeled Scabland.
A sheeted dike, containing 10 sheets, intrudes the Touchet Bed and is filled by silty, sandy Touchet Bed sediment. Look at the left side of the dike. Follow the three left-most sheets up from the bottom of the photo to where the make a sharp leftward bend and merge with bedding. This relationship is subtle, but crucially important to understanding how these dike formed. It is the top of a clastic dike, the place where sheeting begins and a the place a dike begins to descend. This relationship applies to nearly all Pleistocene-age clastic dikes in the megaflood region of Eastern Washington. Slackwater setting. Pataha Creek Valley.
A sheeted dike, composed of 11 sheets, intrudes Touchet Bed rhythmites and is filled by Touchet Bed sediment. The three sheets along the right margin are truncated along a subhorizontal surface. The eight sheets to the left represent a second, younger intrusion (possibly more than one). Slackwater setting. Pine Creek, Walla Walla Valley.
A large sheeted dike intrudes Touchet Beds. Look at its left margin left of the hoe. The left-most sheet is about 15cm wide. It begins as subhorizontal bedding, then abruptly descends, becoming a dike that cuts across the light-colored rhythmite immediately below. Slackwater setting. Lowden, Walla Walla Valley.
A dike descends from its silty source bed. Tucannon River Valley.
A dike descends from its gravelly source bed, the gravelly base of a rhythmite. Tucannon River Valley.
A sand dike descends from its source bed. We can clearly see that the erosional base of the dark-colored sandy unit (basal portion of a rhythmite) is incised into the light-colored silt beneath (slackwater top of the previous rhythmite). The top of the dike is not truncated by the sandy unit, but is continuous with it. Tucannon River Valley.
Light-colored slackwater upper portions (U) of four flood rhythmites (A,B,C,D) are delineated from their coarser grained lower portions (L). The dike which cuts across A and B can be interpreted in two ways. Either it formed prior to deposition of rhythmite C and its top is truncated by C, or it formed during deposition of rhythmite C and is continuous with the lower portion of C. Tucannon River Valley.
Convoluted bedding gives way to a descending dike. Slackwater setting. Confluence of Walla Walla River and Columbia River at Wallula.
Light-colored Touchet Beds unconformably overly Neogene gravels. The light-colored dike, descending through the gravel, is filled with Touchet Bed sediment. There are no light colored sediments beneath the gravel deposit, only basalt bedrock. Tule Road, Yakima River Valley.
Several sheeted dikes descend from light-colored Touchet Bed sediment that unconformably overlies oxidized Neogene fluvial sandstone. No sediment resembling that filling the dikes is not present in strata beneath the oxide sandstone. Emerald Road, Snipes Mountain, Yakima River Valley.
Several generations of clastic dikes are truncated by erosional surfaces (a,b,c,d) in a complex deposit consisting of loess units, reworked loess (diamict), pedogenic carbonate paleosols, flood deposits, and older stuff. Note the dikes never crosscut younger horizons, only older ones. Rulo Site, Walla Walla Valley. Redrawn from Bader et al. (2016).
Three dike generations documented in three flood rhythmites (R1,R2, R3) at Moxee Mammoth Site. Redrawn from Lillquist et al. (2005).
A sheeted dike with two branches (lower left) cuts silty, oxidized sediment and has its top truncated by an erosional surface, above which lie a stack of calcic paleosols developed in loess parent materials (mostly). Elevation of the site is 285m, well below the level of Lake Lewis (~370m). Rulo Site, Walla Walla Valley. Pencil for scale.
The top of this dike fades out below the contact with the overlying rhythmite due to bioturbation and soil processes.
Sags in bedding at Walla Walla River bluffs near Touchet, WA.
A dike descends through several rhythmites at Wallula. Cannot recall if its truncated or connected (I took this photo in 1995).
Dikes descend from and are truncated dark-colored flood channel gravels cutting silty, light-colored Touchet Beds at Smith Hollow Rd, Tucannon Valley.
A sheeted dike, sourced in Pleistocene sediments overlying Miocene basalt descends several meters into bedrock. Walla Walla Valley.
An incipient dike begins to form and descend.