1 June 2016
Varves along Hawk Creek, Washington
Posted by Callan Bentley
The first stop on my Columbia Plateau / Channeled Scablands trip was to a place called Hawk Creek, that I found out about from the just-published (by Mountain Press) Washington Rocks! book, by Eugene Kiver, Chad Pritchard, and Richard L. Orndorff. I received a review copy of this slim, trim volume the week before I left for Spokane, so I was delighted to be able to put it to immediate good use. My colleague Bill Richards and I drove there, and found a it at tidy little valley on a tributary to Lake Roosevelt.
The lake level was low, but that was a blessing in disguise. It allowed us to see deposits from a previous lake, Glacial Lake Columbia, which existed here at the time of the Glacial Lake Missoula outwash floods (late Pleistocene). Rumor had it that there were varves hereabouts!
Varves are seasonal depositional couplets common in lacustrine settings that are situated in strongly seasonal climates – dark, fine grained clay-rich sediment during the winter (when the lake was frozen over during the winter, and therefore calm), and light-colored and coarser (silty) during the summer, when runoff brings bigger bits into the lake, tumbling along and mixing in and jacking up the oxygen levels en route.
We spied a promising outcrop on the other side of the creek, with finely layered sediments peppered with swallow burrows:
But the true varves were found as blocks in Hawk Creek itself, where we fished them out from amid their neighbors (SO much Columbia River Basalt!) and broke them open to spy their internal laminae:
Here’s a thin piece, showing the very fine grain size and very thin laminations:
The best ones I saw are shown here: that’s about a decade of sedimentation, with some years seeing more robust sedimentation than others.
These structures speak of very, very, very calm water conditions. But our real quarry for the day was the opposite: raging, almost unimaginably powerful floods.
…So on we drove…
Thin laminations, color, texture reminded me ‘homologously’ of Eocene Green River Fm.
Yep, although this has 50 million years less of lithification to hold it together… 🙂