Whidbey Pleistocene Stratigraphy, Part 1: Useless Bay and Double Bluff: Pleistocene strata & (maybe) earthquake-deformed interglacial sediment
By Dave Tucker
This trip visits the lowest and oldest Pleistocene sequence in the north-central portion of the Puget lowland. (See the table of Whidbey Island Pleistocene units on the main page.)Deposits of the Double Bluff glaciation are exposed at the end of this walk, but the main attraction is a wonderful and varied sequence of Whidbey Formation interglacial sediment along the beach bluff. The sediments are in places fantastically deformed into swirls and whorls, and invaded by clastic dikes. These are interpreted to form during earthquakes on the South Whidbey Fault (Johnson and others, 1996; full ref and links at the end of this article).
Getting there: Head toward Whidbey Island’s south end, either via the Mukilteo-Clinton ferry, or by heading south down the length of the island from Deception Pass. Either way, you’ll find yourself on Highway 525. From Highway 525, turn south on Double Bluff Road- this is about 200 feet east of milepost 17, 8.5 miles west of the ferry terminal in Clinton, or 19 miles south of Coupeville. Two miles down Double Bluff Road, there is a public beach access, restrooms, a picnic area, and nice sandy beaches on the shores of Useless Bay. Consult a tide table before trying to walk very far to the west; you’ll want a falling tide and plenty of time. It would be tough to escape a rising tide here! The nearest reference point in the tide tables is Bush Point, not too far north on the west side of Whidbey. The total distance along the beach to the southern point of Double Bluff is 1.95 miles. The hike is featured in Bob Carson and Scott Babcock’s Hiking Guide to Washington Geology (scroll down on that webpage).
From the end of the road at the County Park, head west along the beach; it is about 1/4 mile to reach the first bluffs, which then back the beach all the way to the southern point of well-named Double Bluff Point. As the tide recedes, it reveals sandy flats with great examples of sand ripples. I bet this would be a fine place for swimming! It is south-facing, with fine views over the water. You can see the Olympic Mountains, east to the Cascade Crest, and Mount Rainier on a good day.
When you reach the point where the bluffs first rise directly above the beach, (UTM 0E536021 N5313420) you will find a 10-foot-high erratic sandstone boulder that has eroded out of the bluff above stranded on the beach. It is of the Chuckanut Formation, so was transported over 40 miles (as the crow flies) from the nearest surface exposures in the Chuckanut Mountains, almost due north of here. The beach is littered with a wonderful variety of smaller boulders and plenty of beach pebbles and cobbles, all that remain of the bluffs when they extended further south. The sea is steadily eating away at them; Don Easterbrook has estimated that they have receded 100 feet to the north in the past 40 years or so. Active erosion and retreat is evidenced by the many slumps along the bluff’s length, the steepness of the bluffs, and the lack of vegetation- too much erosion, too fast, for trees to get a toehold. You might even find fossilized mammoth bones on the beach, weathered out of the Whidbey Formation.
The bluff is as much as 300 feet high. Most of the deposit exposed in the bluff along here is the interglacial Whidbey Formation, a sequence of alluvium, fine-grained lacustrine sediments (often finely laminated), and peat. The Whidbey was laid down in between the glacial deposits of the Double Bluff Drift (~ 175- 200,000 years old) and younger Possession Drift (around 70,000 years old). Don Easterbrook, who has studied this section for several decades (but did not actually see the glaciers), says that the Esperance Sand unit is found at the very top of the bluff, which was deposited as an advance outwash deposit as ice of the 15-25,000 year old Vashon Glaciation approached this area. If that is the case, then the intervening Possession Drift is missing, victim of erosion. For a full discussion of the stratigraphy in these bluffs, and plenty more on glaciomarine drift, find a copy of Don Easterbrook’s 1994 GSA field guide; the reference is below.
(Time to digress: ‘Drift’ is material deposited either directly from ice or from meltwater issuing directly from ice. Drift includes ’till’, which is laid down directly by ice without any intervening water transport, but drift also includes outwash gravel and sand deposited by streams flowing outward from the front of the glacier, and glaciomarine drift (gmd), which is sediment that rains down from floating ice to the floor of the sea or a lake. To be recognized unequivocally as gmd, fossil shells of clams or other molluscs in growth position, meaning intact with hinges closed, and not arguably remobilized from some other deposit, need to be present. They are often pretty scarce and require persistent, diligent searching. All till is drift; not all drift is till. Got it? End of digression. Let’s get back to the exposures!)
Continue walking west on the beach. Enjoy the variety of deposits within the Whidbey Formation; you can claw your way up through the sand pile at the base to see details in the beds- some are very finely laminated silt-clay lake deposits deposited in quiet waters, others are cross bedded coarse sand or gravel from streams. You will certainly be curious about the odd deposits about 30-40 feet up the bluff that cut through or invade other layers. Some of these are anastomozing, sending off convolute branches or looping back into themselves. In places these structures can be seen to originate from a lower layer, and intrude upper layers. These structures are generically referred to as ‘clastic dikes’, meaning that they are 1) composed of sedimentary grains and 2) have invaded overlying strata, analogous to an igneous dike. The generally accepted explanation for the clastic dikes here is seismic activity along the South Whidbey Fault (Johnson and others, 1996, linked below). There have been several historic earthquakes on this major northwest trending fault. When an earthquake shakes the ground sufficiently, watery sediment layers can be mobilized to rush upward to invade overlying layers. Because the ground is shaking, sediment grains in the invaded sediments may also be moving relative to each other, permitting water-transported grains from the lower layers to invade in convoluted patterns off the main dike. The dikes can even reach the surface and ‘erupt’ as sand volcanoes; during the 2001 Nisqually earthquake, clastic dikes reached the surface in many places to form mini ‘sand volcanoes’ a few inches to a foot high. Because the dikes are composed of water-mobilized sediments, they are also known as liquefaction structures. It would be interesting to notice if any of the clastic dikes in these bluffs appear to reach the top of an overlying layer to blow out onto the surface (as it existed at the time), then covered and preserved by later sedimentation. How much of the Whidbey Formation was laid down after the event or events that caused this deformation?
About half way to the southern point of Double Bluff, you will reach a prominent brushy gully reaching almost to the beach, which serves as a good landmark (UTM at this gully is E0536343 N5312982). Just beyond to the west are the coolest liquefaction structures I saw- two sets of them. A lower set of tan dikes invades lighter buff-colored dense sand; these look like the ones we’ve been seeing for most of the way along the beach. The upper ones, however, are different. These are clearly layered, though convoluted, and dark gray to buff. They rise upward several meters from a similar-looking sequence of beds only about a meter thick, which appear undisturbed, into another massive sand layer. It is not possible to say that these separate liquefaction structures represent two discrete earthquakes. Both could have arisen simultaneously out of different subsurface saturated layers and into separate massive sand layers above.
You will have noticed big chunks of black, flakey peat by now, lying on the beach. Eventually, a peat bed shows up in the Whidbey Formation, rapidly thickening from zero to around two meters. It is resistant to erosion, and holds moisture in the midst of the vertical sea of dry sediment, so forms a resistant ledge that bushes grow on. Examine a chunk of peat lying on the beach, you will see how dense it is, almost like wood. This organic stuff has been buried by over 100 feet of sand and mud, as well as having a few thousand feet of ice ride above it at least twice. You’d be flat, too! In a few hundred yards, the peat again pinches out, either marking the limit of the swamp it formed in, or the effect of later erosion.
The bluff begins to get lower here, and eventually a layer of drift comes into view, sloping down at an angle to the west, until it is at eye level. According to D. J. Easterbrook, this is the Everson glaciomarine drift, around 15,000 years old, deposited on the sea floor as the last great ice sheet melted back at the end of the Vashon glacial stade. If this is Everson, however, then once again there is an erosional disconformity, because here the Whidbey has been drastically eroded away, and there is none of the Esperance sand preserved beneath the Everson, the youngest gmd this far south. After a low, brush-covered interval, a different gmd comes into view, a dense hard clay studded with pebbles. This thickens and forms most of the low bluff when you reach the point at last (UTM E0533939 N5312580). Don Easterbrook’s research calls this glacial unit the Double Bluff Drift; he says it is glaciomarine drift, though shells are very sparse. This is the oldest glaciation preserved in the central and northern lowlands. You can turn the corner at the point and walk along to the north, but pay attention to the tide!
The bluffs along Useless Bay record a glacial advance and recession (Double Bluff), then the Whidbey interglacial period when mammoths grazed in peat bogs, and at the top, the Esperance sand, heralding the coming advance of the maximum ice advance of the Vashon glacial period. There is a small remnant of unconformable Everson glaciomarine drift, dropped off the underside of the last great ice advance as it melted. Missing, either never deposited or, more likely, eroded away during Esperance time, is any record of the Possession glacial advance. The bluffs themselves reflect persistent erosion by the implacable sea.
Easterbrook, D.J., 1994, Stratigraphy and chronology of early to late Pleistocene glacial and interglacial sediments in the Puget Lowland, Washington in Swanson and Haugerud, editors, Geologic Field Trips in the Pacific Northwest, volume 1: Geological Society of America, p. 1J 1- 38
Johnson, S.Y., Potter, C.J., Miller, J.J., Armentrout, J.M., Finn, C., and Weaver, C.S., 1996, The southern Whidbey Island fault: An active structure in the Puget Lowland, Washington; Geological Society of America Bulletin v. 108, p.334-354.
The abstract is at doi:10.1130/0016-7606(1996)108<0334:TSWIFA>2.3.CO;2. You can read the full text at this University of Washington website: