Structureless Sandstone Beds - White Bluffs, WA
Structureless sand is fairly unusual in the geologic record. Sandy sediment deposited by flowing water typically preserves bedforms. Bedform geometry contains tons of information geologists use to interpret flow conditions and geologic setting when the sediment was deposited. Massive sands come in two flavors: Sheet-like deposits (braided fluvial) or channel-like deposits (braided fluvial, fluvial lacustrine, marginal marine, glaciofluvial) and are interpreted as either a.) laminar concentrated currents in the upper flow regime or b.) gravity flows associated with subaqueous collapses. The absence of bedding (primary stratification) is due either to very rapid deposition or post-depositional disturbance that wipes out primary bedding.
Massive sands are known worldwide, but their formal recognition - especially by certain gurus of sedimentology - has come late in the game. For example, Miall (1977) omitted massive sands entirely from his comprehensive lithofacies scheme and basic classification systems for massive sands were in-progress as recently as the mid-1990s (i.e., Durham, 1995). The main reason a geological consensus on massive sands has lagged is that most beds described as "massive" are actually bedded and contain sufficient, if subtle, information to be classified into existing schema. This view suggests massive sands are faintly-bedded end members, not truly structureless deposits or products of some unusual process. The implication is that the geologist who determines a sand bed is massive just hasn't looked closely enough.
Photo: A "massive" sand bed in lacustrine strata at White Bluffs Overlook.
At White Bluffs Overlook, a few conspicuous massive sand beds punctuate lacustrine sediments of the Savage Island Member (upper portion of the Pliocene Ringold Formation). The Savage Island is composed primarily of lacustrine siltstones with thin fluvial sands and paleosol intervals developed during low water periods. It represents a fluctuating lake system and a broad, sluggish fluvial floodplain formed at the confluence of the south-flowing ancestral Columbia River and west-flowing Snake-Clearwater-Salmon Rivers. Correlative deposits of the Ringold basin are preserved in a set of smaller basins separated by fault-bounded ridges of the Yakima Fold Belt (i.e., Pasco Basin, Yakima Valley, Umatilla Basin).
The sands are thick and light-colored, unlike typical thin, gray fluvial sand beds found throughout the lacustrine section. They appear to represent infrequent interruptions to steady-state sedimentation on the nearly flat lake bottom.
Central questions that can be addressed at the outcrop:
a.) Are they truly structureless or just faintly-bedded?
b.) Are they deformed?
c.) What was the flow regime and depositional setting?
d.) Are characteristics of the massive beds shared by other parts of the stratigraphy?
e.) Do the beds laterally thin and thicken?
f.) What is the nature of upper and lower contacts?
g.) How deep was the lake?
h.) How do they compare to gray fluvial sand beds?
With observations in hand, take a crack at interpretation. Here are a few options:
1.) The sand beds are distal portions of subaqueous sediment plumes downslope of large landslides. If so, what collapsed?
2.) They are homogenized quick sands created by pore fluid escape (fluidization) triggered by seismic shaking. If so, what additional evidence would be needed to confirm timing of a shaking event?
3.) They are hyperconcentrated flow deposits. If so, describe the depositional setting.
4.) They are muddy sand beds with faint bedding. If so, why do they look so different than everything else in the exposure?
5.) They are shoreface storm deposits. If so, do they contain hummocky cross stratification?
Photo: Massive bed seen in upper portion of outcrop.
Photo: Massive bed with graffiti contrasts with "background" sediments: laminated lacustrine siltstones and gray fluvial sands of the Savage Island Member.
Photo: Interesting pattern at base of a massive sand bed (orange line).
Photo: Curious stratified inclusions in the massive sand bed.
Photo: A stratified rip-up clast or post-depositional deformation near the top of the bed?
Directions. White Bluffs Overlook is located along the Hanford Reach of the Columbia River 50km north of TriCities, WA. From Othello, drive Hwy 26 west for 17.5 miles to the White Bluffs Area gate. Turn L (south) and follow the gravel road for 8 miles to its end at the White Bluffs Overlook parking lot. A continuous outcrop lines the closed Old Ringold Road a short walk from the parking area.