Wednesday, July 4, 2007
Thrusting on Whidbey I. west beaches, cont'd
E-W fractures indicate orthogonality with strong N-S stressing of mid-Whidbey Island
A Close-up of Thrusting on Whidbey Island
There are thrust planes found in the cliffs on the west side of W.I.; these are undeniable, as analyzed in previous reports in earlier submissions. However, whether they are predominantly the work of glaciers, to the exclusion of other tectonics, must be demonstrated.
The photos above show that there are a fair number of small thrusts near the Hancock Lake, which is within two kilometers of the well-documented Fort Casey-Ledgewood-Baby Island NW-SE still-active fault zone. Since these change their appearance with the annual slumping and retreat of the cliffs near Hancock Lake, this raises some doubt about their incipience and magnitude.
One large thrust plane has not ameliorated with the years (shown in photos), even though several other large sheets noticed nearby have disappeared. This is shown in the cliffs just south of Hancock, and I have explained the entire phenomenon as due to the significant scarp there. Driving on the hiway 525 roadway near Greenbank, one can see the scarp, as the automobile chugs down, climbing it going south. This scarp is on the narrowest part of the mid-island, springs occur there, and the thrust plane is on the immediate south side of this scarp in cliffs.
I have suggested that one of the glaciers pushing south, rammed against this scarp, forcing the top layers to slide to the south- giving the appearance of a thrust sheet. Since the thrust plane is almost horizontal, this conjecture still seems reasonable. The fact that some of the associated thrust planes have disappeared over the years introduces some doubt. This disappearance indicates that whatever occurred was not of large lateral extent (except for the one plane remaining).
One confusing occurrence is that of cross-bedding in dunal sands, which sometimes appears in cliffs near by. Inter-dunal zones, which may occur above and preceding later dunes look like thrusts. One may sort this out, by looking at fractures- and not the cross-bedding. I take it that parallel fractures (three or more) are not inter-dunal intervals, and these were noted in the report for Ebby’s Landing. Coastal dunes are voluminous in the cliffs, and even occur in the island interior; one such is on the east side of East Harbor drive, near Sands Lane- falling continuously on the shoulder. Some of the photos above may appear as cross-bedding glimpses. But the buckles are undeniably due to compression and thrusting. The question remaining is: since these buckles are very near the bottom of Till, this should strongly suggest that glacial sliding from the north has produced them.
Notice the one layer which is buckled, with the bed riding over itself to the north (all of these photos and comments are for beaches running north to south, on the west side of the island). This is opposite what should occur, for a glacial sliding over the upper layer causing thrusting toward the south. Rather, it appears that the force has come from below, forcing the bed to ride over itself.
Adding to all this mix, one photo shows that there are orthogonal fractures, running E-W in the cliffs. This would occur with breakage perpendicular to the dominant stress- which was N-S for the mid-glacial period (not the latest). Although not conclusive, this throws the argument to the side of glaciers being the dominant entity in this 50k ybp time.