Clastic Dikes: The Tops of Clastic Dikes
The tops of clastic dikes in the megaflood region appear five ways.
1.) Truncated at a bedding contact or an erosional surface
2.) Fracture and fill
3.) Fade out, possibly by bioturbation
4.) Sag becomes dike
5.) False upward pinchout
Fracture and fill. Bedforms become infill. The quarter is located at a bedding contact between two flood rhythmites. A fracture opened in the top of the lower bed and filled with sediment. The sand was not vented upward from some layer below. See how the rippled sand grades smoothly upward into the overlying massive silty-sand? This is how it works.
Fracture and fill. A dike that originates in coarse sand at the base of the uppermost rhythmite (R7) descends several meters through six underlying rhythmites (R1-R6), branching and tapering along the way. The fracture opened in the substrate when an Ice Age flood inundated the landscape. The topo of the dike is buried by the same R7 sediment; fracturing and filling occurred early on in the flood episode. Touchet Beds are unusually thick here in Burlingame Canyon at Gardena near the center of the Walla Walla Valley.
Truncated entirely. An unsheeted, single-fill dike intrudes downward into coarse, laminated sands at an angle oblique to the cut face. It is filled by material matching the larger deposit. Clastic dikes in the megaflood region tend to have fills that look like the stuff they intrude. The dike's top is truncated by cobbly gravel deposited by a subsequent flood. Some amount of erosion (vertical loss) has taken place along that contact, but not a huge amount. The erosion prevents us from seeing how the dike connected to its source bed above, but clearly the dike has no connection to a source bed below. The flat base of the structure is well exposed. This is a high energy flood channel-eddy bar setting with southward flow indicators. Smith Coulee, Channeled Scabland.
Composite dike. A more subtle variant. 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.
Composite dike. 11 sheets compose the lower half of this dike. The 3 sheets along the right margin are from an older dike that enters from the left. It is oblique to the cut face there. Younger sheets descend from the top, out of the frame. The two sets of fill bands merge to become a composite dike. Two episodes of diking and exploitation of a pre-existing weakness (the older dike) by the younger dike. If you only observed the lower half of this dike, you would leave with one impression (i.e., 1 dike w/ ~10 fill bands). If you observed the upper half, you leave with a more complete understanding (i.e., 2 dikes, 1 with 6-7 bands, the other with 3). A certain amount of time passed between the injection of the first and second dikes. I would argue that period is the inter-flood hiatus. At minimum, the age of the two dikes differ by the time that passed between two successive floods, assuming the first and second dikes are sourced in the first and second flood rhythmites. Pine Creek, Walla Walla Valley.
Composite dike. Look at the left margin of this large dike, just left of the hoe. The left-most sheet is about 15cm wide. It appears to be bedded, non-dike sediment (subhorizontal bedding). The bedded sediment abruptly descends, becoming a wide fill band that merges alongside the other bands from a younger set sourced from above, out of the frame. Classic composite geometry in Touchet-type dikes. Slackwater setting at Lowden in the Walla Walla Valley.
Sag become dike. A dike descends from a silty source bed into sand below. The gray sand is the lower backflood portion of a flood rhythmite. The brown silt is the slackwater upper portion. Sagging occurred during slackwater sedimentation. Tucannon River Valley at Starbuck, WA.
Truncated or Fracture and Fill? An unsheeted dike filled with coarse sand descends from the base of one rhythmite into the silty-sandy upper portion of the next-older rhythmite. The dike appears to be sourced in the gravel and fills a fracture created during deposition (initial backflooding of valley). But the base of the gravel is erosional. If the gravel eroded away a few feet of sediment, creating a local unconformity at the contact, the dike could have been sourced in the lost sediment. Tucannon River Valley.
Fracture and fill. A sand dike descends from its gray, sandy source bed into brown, silty sand. The coarse gray sand incises into the light-colored silt beneath. This is a contact between two Touchet Bed rhythmites. The sandy unit provides sediment to the descending dike. Tucannon River Valley.
Fracture and Fill or Fracture, Truncation, and Sag? Light-colored slackwater sediments (upper portions of rhythmites, U) contrast with coarse grained lower portions (L) in four megaflood rhythmites (A,B,C,D). The dike that cuts across rhythmites 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 (filled by C). Tucannon River Valley.
Fracture and fill. Convoluted bedding gives way to a descending dike. This dike didn't form at the beginning of this flood event (i.e., at the onset of flooding). Rather, it formed a bit later in the event, during the slackwater period during deposition of the upper portion of rhythmite. Deformation in the upper portion either was syn-depositional, or occurred when the next flood overrode the still-wet sediment. Are those dish structures in the lower bed? Slackwater setting in "late" rhythmites near the confluence of the Walla Walla and Columbia Rivers at Wallula. You can see Wallula Gap (and Russia) from here.
Fracture and fill. Light-colored Touchet Beds unconformably overlie Neogene fluvial gravels of the Snipes Mountain Conglomerate. The light-colored sheeted dike descends through the gravel and was filled from above. There are no light colored sediments beneath the gravel deposit, only basalt bedrock. Tule Road, Yakima River Valley.
Fracture and fill. Several sheeted dikes descend from light-colored Touchet Bed sediment that unconformably overlie oxidized fluvial sandstone of the Snipe Mountain Conglomerate (Ellensburg Fm). Sediment contained in the dikes is sourced in the overlying Touchet Beds, not in strata beneath the sandstone. Megafloods overrode most of fault-bounded Snipes Mountain, leaving behind a body of Touchet Beds sediment inset into the older unit. Several outcrops along Emerald Road expose flood beds composed almost entirely of reworked Snipes Mountain material, so keep a sharp eye. Granger, WA in the Yakima Valley.
Multiple truncations. 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) of pre-late Wisconsin age, carbonate paleosols, uncemented Missoula flood deposits, and other stuff. Note the dikes never crosscut younger horizons, only older ones. Rulo Site, Walla Walla Valley. Redrawn from Bader et al. (2016).
Multiple truncations. Three generations of clastic dikes were documented in three flood rhythmites (R1,R2, R3) at the Moxee Mammoth Site by Lillquist et al. (2005). Note the dikes terminate downward and never crosscut younger horizons. Redrawn from his figure.
Deformed rhythmites obscure dike terminations. Many of the dikes (bold lines) in this roadcut appear to terminate upward Careful inspection reveals that the tops of all that do so occur in a package of Touchet Beds in which bedding contacts are obscured by deformation (right half of diagram). Some dikes terminate at or near the high-relief slide surface. The top of one dike is even dragged to the right by the sliding, but was not offset. Most likely all dikes here are fracture and fill type, sourced from above that pinchout at the bottom. Dry Hollow near Clyde, WA.
Truncation. A sheeted dike with two branches (lower left) cuts silty, oxidized sediment and is truncated by an erosional surface. Sediments below the surface are pre-late Wisconsin age (many tens to hundreds of the thousands of years old). Above the truncation surface lies a stack of calcic paleosols developed in loess (mostly). Elevation of the site is 285m, well below the level of Lake Lewis (~370m). Rulo Site, northern Walla Walla Valley. Pencil for scale.
Two truncations in a composite dike. Try your hand at interpreting the crosscutting and injection relationships. The prominent cobble is in place (not part of the dike fill).
Fade out. The top of this dike fades out below the contact with the overlying rhythmite probably to bioturbation worm and such. Rhytmites here are very silty. Silt does a poor job of preserving bedding, often appearing massive.
Sag? Sags in bedding crosscut lower beds at Walla Walla River bluffs near Touchet, WA.
Truncations in a composite dike. A dike descends through several rhythmites at Wallula. The top is truncated by a bedding contact between two Touchet Beds. Lower Walla Walla Valley at Wallula.
Multiple fracture and fill with truncation. Dikes descend from and are truncated by dark-colored flood channel gravels (up-valley backflood or down-valley spillover?) cutting silty, light-colored Touchet Beds at Smith Hollow Rd, Tucannon Valley.
Fracture and fill. Sheeted silt-sand dike intrudes Columbia River Basalt at Lewiston, ID. The fill is Touchet Bed material. There are no interbeds in the basalts that could have contributed sediment like this. Sheeted dikes intruding bedrock provide good evidence that a single injection event commonly creates a set of fill bands, typically fewer than 10.
Fracture and fill. Sheeted silt-sand dike intrudes a joint in Columbia River Basalt at Weaver Pit, Walla Walla Valley, WA. The joint only has to open a little bit each time to form a dike like this, apertures of ~1 cm. Note lack of shearing; joint opening is Mode I. Sediment-filled dikes intruding hard bedrock attest to a fracture and fill origin. Bedrock removes several variables that can complicate interpretations in the Touchet Beds, where the same sediment inside the dike also surrounds it. There is no possibility of sagging or fade out in bedrock.
Sag to dike. Dike begins to form in sandy sediment and descend. Doesn't some fracturing have to occur to create the pathway down?
False upward pinchout. If you only saw the left side of this dike, it would pinch upward. But a full excavation reveals two trunks are connected by thin stringers (sheets). The same number of sheets occur in both parts.
Truncation? Either the thin dike is sourced from the base of the little channel fill or the channel fill truncates the top of a dike formed earlier.
Top and bottom hidden. Most of the time, this is what you see. The top of the dike just isn't well exposed or accessible. The bottom of the dike is buried. Lewiston.
However, you can often find either the dike's top or bottom by doing a bit more snooping. This dike, the same one shown above, tapers dramatically and pinches out in a boulder gravel deposited by the Bonneville Flood. Lewiston, ID.