Near and Far Field Hydrofracture in the Formation of Sheeted Clastic Dikes

Near field (blue) and far field (red) hydrofracture curves shown. During the hydrofracture period, fluid pressure drops as fractures propagate (crack volumes increase as they propagate). At the near field scale, pressure rises and falls with each new crack and fill. The far field curve represents mean fluid pressure during the same time, the curve shows a gentle decline towards O3, when fractures can no longer open.

Near field and far field are concepts that I use to describe the fracture corridor (several centimeters wide) and the surrounding body of sediment (several meters wide). They differentiate two scales of fluid pressure response.

Development of a sheeted clastic dike over four time steps. Top row: Far and near field regions shown (arrows). Middle row: Geology. New dikes form with each new flood and their tops are truncated by the subsequent flood. Bottom row: Injection of new sheets of sediment correspond with pressure pulses that appear as spikes in near field hydrofracture curves.

Typical downward-intruded, sheeted dike at Latah Creek near Spokane, WA. A single dike, composed of a set of sheets. The set was clearly created during a single flood and a brief period of hydrofracture.

In my model, hydrofracture is triggered by rapid loading of silty-sandy sediment by a megaflood. A subvertical fracture opens in the O1-O2 plane in response to the vertical load (extension in O3 plane) and is immediately filled with wet sediment (propped). Because drainage is slow through bedrock gaps, substantial flood loads are sustained over minutes to days. Hydrofracture likely continues intermittently over that period of time.

Flood loads cause new fractures to open. A new fracture opens adjacent to those formed earlier. The new fracture fills immediately. Then another opens and is filled, and another, until the load subsides and pore fluid pressure within the sediment body (far field region) falls below the least principle stress value O3 (e.g., fluid pressure equilibrates to background "reservoir" levels).

There is a subtle difference between the near field response - that within narrow fracture corridors, each a few centimeters wide - and the far field response, that of the larger body of sediment that encompasses several fracture corridors. The blue curve in the figure above shows the fluctuating fluid pressure corresponding to repeated cracking and filling during the hydrofracture period.

The red curve shows the broader scale, or far field, pressure drop during the hydrofracture period, a mean value felt by the body of sediment. In both the near and fair field, fluid pressure remains between O1 and O3, essentially the upper and lower limits of the sediment's resistance to fracture. Silt skins form during the leak-off period for each propped fracture (drainage to formation, drainage to adjacent fills). Resistance values of O2 and O3 are nearly identical, thus fracture orientations are random; dikes form coalesced polygonal networks in plan view.

The model shows how each megaflood loading event produces:

a.) Sheeted dikes of nearly identical character everywhere they occur.

b.) Crosscutting, sheeted dikes can form with seconds of each one another.

c.) Both sheeted and single-fill dikes at the same time in the same sediment.

d.) Vertical compression in O1 and horizontal extension in O3. Since O2 probably equals O3, dikes show random orientations in plan view.

e.) Dikes across a broad region, but only within the Ice Age floodway.

Sheets filled with two distinct grainsizes (tan silt and gray sand). Formed by two floods or just one? Either fractures tapped two different flood beds in the stack, one silty, one sandy (requiring two floods separated in time - decades or longer) or fractures opened to the surface in two pulses during a single megaflood, tapping circulating, sediment-laden currents at the base of the flow; cracks were supplied different material at two different times over a period of minutes to days. Pine Creek, southern Walla Walla Valley, OR.

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