Dike-Sill-Dike Geometry in a Fluid-Driven, Sediment-filled Fracture
Coarse Grained Layers Behave as Efficient Pathways - Coarse grained layers appear to behave as efficient pathways during propagation of fluid-driven fractures that fill with sediment (clastic dikes). Under a vertical load (O1 is vertical), fluid pressure (Pf) opens a vertical dike against the minimum principal stress (O3 is horizontal), so Pf >> O3 at Time A. When the fracture encounters the coarse grained, low-resistance layer, further propagation becomes easier in the horizontal plane. The coarse grained layer provides an efficiency factor (R at Time B) that decreases as the fracture lengthens (r at Time D). For a brief period between Times B and C, fluid pressure can hold the fracture open against O1 because of the efficiency factor (Pf > O1-R). As the efficiency factor decreases to r, the vertical force begins to close the horizontal crack (Time D), but if Pf remains sufficiently high (>O3), it can continue to propagate by switching back to vertical and opening against O3. Without the efficiency factor provided by the coarse layer, the dike itself would not be able to open against the vertical load (O1). It would not become a sill. Sills are more difficult to open in fine grained sediment. The coarse layer cannot completely dissipate Pf because it is sandwiched between finer grained layers, which close the system and hold pressure.
Propagation from least to most resistant (easiest to hardest): (O1-R) < (O3) < (O1-r) < (O1).
The driving force of the fracture (Pf) and the resisting forces of the sediment (O1, O3) are not radically different. On average, fractures in Touchet-type dikes are only opening a centimeter; average fill band width is ~1cm. Fluid pressure that is allowed to build (tight sediment) will be released in the form of a sealed fracture. In coarse sediment, fluid pressure dissipates without causing hydrofracture (<O3).
The charts at right show relationships between fluid pressure, volume, length, and width. Pressure decreases over time, but at a slower rate through the sill segment due to the narrowing of the fracture. The sill segment is ~50% the width of the dike segments. Volume increases as the dike grows, but at a slower rate through the narrower sill segment. Length increases linearly. Width jumps from wide to narrow to wide in correspondence with the dike-sill-dike configuration. The two dike segments are the same width at this scale.
Real-world example of dike-sill-dike form in fine-coarse-fine sediment. Lewiston, ID.
Not faulted. Abrupt changes in the dike's path are governed by more or less efficient flow paths through the sediment. This dike pinches out at its bottom (below photo), but those little upward spurs are cool; vertical propagation right before the pressure fluid dropped. Pataha Creek, WA.
This dike is not offset by faults. Same relationships exist here as in the photo above.
This dike is offset numerous times along sub-horizontal faults. Last Chance Rd, Walla Walla Valley, WA.
Sheeted sills can extend for several meters, sometimes appearing as bedding if dike segments are not exposed. This is more common in sections containing a lot of coarse sand. Hwy 240.