Chance findings about early holocene tidal marshes of Grays Harbor, Washington, in relation to rapidly rising seas and great subduction earthquakes

Abstract

Tidal marshes commonly build upward apace with gradual rise in the level of the sea. It is expected, however, that few tidal marshes will keep up with accelerated sea-level rise later in this century. Tidal marshes have been drowned, moreover, after subsiding during earthquakes. This report tells of ancient marshes that endured rapid sea-level rise in a region that subsides during earthquakes. The soils of these marshes were unexpectedly encountered in borings for a public-works project at Grays Harbor, Washington. The borings were logged quickly and only a few of the core sections were conserved. The limited findings pose puzzles about how the ancient marshes endured and what their history implies for earthquake hazards. The borings establish that tidal marshes persisted during the early Holocene at Grays Harbor, an estuary along the Cascadia Subduction Zone of western North America. The persistent marshes are recorded by a unit of peaty mud up to 10 m thick and as much as 40 m below present sea level in the drowned valley of the Chehalis River. The unit was encountered in two areas 4 km apart that were tidal flats in the 19th century. The marshes originated less than 10,000 years ago and endured through most or all of an estimated 500–1,500 years. The borings further show that these persistent marshes eventually yielded to tidal flats, tidal channels, or both. The change is marked by sand and mud that overlie the peaty mud at a typically sharp contact. The marshes were drowned about 8,600–8,400 years ago if the sand and mud buried them promptly, or later if the sand and mud filled channels that migrated across the peaty mud. In one of the studied areas, tidal marshes became re-established locally in the early Holocene and widely in the middle Holocene, and deposits of middle Holocene marshes were overrun as recently as 1,000 years ago by a gravelly tidal channel. In the other area, tidal-flat and probably subtidal deposits make up all of the middle and late Holocene section below artificial fill; if marshes became re-established in this area after about 8,600–8,400 years ago, their deposits have been lost to erosion. The puzzles posed by these findings include: (1) How did the marshes manage to endure centuries of relative sea-level rise that likely approached 1 cm/yr on average? (2) Did the marshes also endure subsidence that accompanied great thrust earthquakes on the Cascadia Subduction Zone? (3) Was their eventual drowning triggered by a Cascadia earthquake of unusually large size, or can the drowning be explained by sea-level rise that included a jump from drainage of glacial Lake Agassiz?