Earth Movements on Whidbey Island: 03 Summary
FAULTING (see attachment on the website):
The Baby Island- Ledgewood- Admiralty Head fault is our most certain fault. It runs NW-SE, and can be seen and sampled just off the coast of Saratoga Road (adjacent to Baby I.). It protrudes into the air with a set of small hogbacks running 130- 135 degrees from north, and the rocks are obviously hardened in their exposure- appearing to be Pliocene in breaking strength. It is Pleistocene, since it cuts that age of sediments, and this accentuates the fact that compressive stresses may increase the strength and hardness of sediments. Although the fault is of lengthy expression (Admiralty to Baby Island), it is hard to see it in the interior of the island, north of Greenbank- it may be related to the occurrence of springs and seeps near Nettle Road, which has a drainage running NW-SE.
I believe that the fault is still active, based on the road breakage and slumping at Ledgewood; but there is only one concrete slab indication that it is left-lateral, and the main observations support the conclusion that it is compressive and causing uplift at selected locations like Admiralty Head (it could still be left lateral).
Other faulting on the main body of Whidbey Island can be called minor, and some of these are noticed in the cliffs. One of these was predicted based on changes in the water composition of water wells. This is the El Capitan beach fault, and it was noticed first, by the anomalously large amount of K compared to Chloride in the water. Its visual expression is the only set of clean cliffs south of the Borgman Road access. There is a seep, angular block insertion into fractures, and uplift in the sediments in the cliff.
The fault which has been most studied by our group is actually two faults or more, surrounding the Rocky Point exposure. There is a lesser observed fault on the north side of R.P., and the greenstones and metasediments are currently dragged between these two movements. The fault runs about 130 degrees from north, and has been exhumed by the USGS in a trench for measurement. It has a displacement of the main fault trace, to the north as shown by LIDAR, of about 330 feet- which would indicate that it is being sheared north-south at about 1 cm/year (since is it is seen in glacial till of less than 15,000 years age). Both of these faults surrounding the Rocky Point continue on to the SE, showing up in asphalt breakage near the Naval Commissary, in the formation of Tombolos or fossil Tombolos at Polnell Point and at the seaplane base south of the commissary. The Rocky Point exposure is uplifted in the middle causing fracturing- the greenstones appear to be detached from their original bedrock, since one end protrudes in the air and the other end is parallel to the faulting direction.
There is a general tendency for the faulting to become more eastward in expression as the faults are explored on the north end of the island. The Silver Lake and Creek run about 110 degrees from north, as does the El Capitan fault- compared to the SW-NE faulting at Baby Island and at SW Maxwelton Creek (toward upper Cultus Bay). This hints that the NW-SE fault trend is the result of the interference of the Pacific Plate moving northward relative to the North American plate running westward. The vector resolution of two stresses running perpendicular to each other is a diagonal between them, with exaggerated importance of each stress direction as the main thrust becomes nearer to the fault being observed. This diagonal trend would be the case when each stress is of similar effect (magnitude). However, the Pacific plate main thrust is west of the San Juan de Fuca expression, and there are spreading zones west of the Vancouver Island coast (locating the westward edge of the sjdf plate, by thermal vents), indicating that the Juan de F. plate is still moving east, far west of Whidbey Island. It is still possible that the Pacific plate stresses could lurch into the JdF. zone if the two movements are intermittent. However, the north movement has to be significant, compared to the east movement of the JdF plate, and one would not think that an occasional lurching would produce the strong thrusts noticed on the south end of Hancock Lake. My revised thinking (2003) is that those S. Hancock thrusts are a result of glacial sliding, and that the north thrusting caused by Pacific Plate lurching is overshadowed by the much larger movement during the Pleistocene (due to glaciers).
The only significant evidence of N-S thrusting due to occasional Pacific Plate lurching is that of the displacement of the Rocky Point fault as noticed by LIDAR. There are several N-S fractures which could add weight, including the one on the seaplane base Tombolo, and to the fractures which die out looking upward on the cliffs (as opposed to glacial-induced fractures which die out with depth).
My tentative conclusion about all this is that the N-S fractures are just now being formed, as the JdF plate moves under the continent and dies out (not yet), while the Pacific plate is getting closer to W.I. and beginning to assert itself.
FRACTURING (see 2nd attachment, in website):
Fractures may be seen at many orientations and locations in the cliffs, so that one cannot use them for specifics; however, the pattern which they make when seen for multiple entities is useful for determining the regional stresses causing them. For example, at Deception Pass area, they are mainly down to the north on the south side of the bridge, and oppositely on the north side of the bridge. This indicates a change of stress direction at the Pass.
Fractures may be seen to be related to Alkalinity in the wells which are near significant fractures (see attachment 3). This is the case with South Whidbey, where water wells show that there is a geochemical anomaly around Cultus Bay (see the alkalinity map on the website. I postulate that the connection is with HCO3, which is the most significant part of alkalinity in water wells. When rainfall finds open fractures, it develops a faster path of recharge to the water aquifer, and on the way creates the following reaction:
H2CO3 (carbonic acid in rain) + CaCO3 (calcareous cement in sediments) > 2HCO3 + dissolved Ca++.
Consequently, when recent fractures are created by stresses in the earth (by extensional forces), rain will increase the HCO3 in nearby water wells, and will give a signal that fractures and loose sediments are building up water pressure. This would be accompanied by a rise in water table, providing there is not excessive usage of well water. A result of this could be slumping, which causes large amounts of earth to spall off cliffs, lubricated by the rapid increase of subsurface water. Since this is the case with hiker Cronk's water well, we will use him as a test case to see how long this signal occurs before a new slump happens. The south coast has a history of 33 years/slump (somewhere, not necessarily in the same place repeatedly), so Don has a respite of up to 33 years before he loses his back lot.
2004 findings:
Fracturing, cont’d
Fractures should only be used to generalize about earth stresses- whether there are compressive or extensional local earth stresses. A fracture, as I use it, must be confirmed as part of a set of parallel cracks in the rock to be considered more than a random expression of exfoliation, jointing caused by differential thermal expansion, ancient weaknesses in the rock caused by fossil stresses, and gravity effects (incipient slumping, recent earthquake influences, and man-made cracks caused by blasting).
Fracturing should be used for stress information, only in conjunction with other stratigraphic or structural findings to confirm a pattern- e.g. compressive as seen about a fold or anticline, or extensional as seen near normal faulting, springs, and drainages.
Regional Fracturing, such as found in the surface of rock outcrops, may be used to determine local anomalies; e.g. whenever there is a well-defined regional trend, such as SW-NE over much of the Colorado Plateau, the local departure of parallel fractures from this orientation will indicate a local anomaly.
In the cliffs of Whidbey Island, a set of parallel vertical fractures may indicate the local stress state now. At Ebby’s landing, over a course of 5 years (observed by me), as erosion proceeded, small thrusts in the cliffs eroded away, leaving nothing in their place in the sediments previously hidden behind. These likely were caused by the recent glacial compressive forces as the ice moved downhill, causing local disruptions in the soft sediments.
A single fracture should not be used for any specific or generalized information. Further, fractures located vertically above others should be treated differently from those which are parallel horizontally (at roughly the same elevation); e.g., at the suspected location of the Baby Island fault in the cliffs across the Holmes Harbor (NW of the B.I.), there are two fractures parallel to each other in a vertical separation. The lower one has an X fracture, with a small lens of sand along the orthogonal part (down to the north), indicating secondary fracturing. The set of parallel fractures indicates the present stress state, but the down-to-the north has caused some relief to the stress. There is no displacement noticeable, but the parallel down-to-the south fractures are not what would be expected by ice moving south- which is what has transpired here. Rather, ice would be expected to cause the fractures to rise to meet the direction of ice movement (up-to-the-south). The conclusion in this case is that present stress may have caused the fractures, but the absence of displacement indicates that a fault is not present (but may be nearby).
FAULTING
Seismic may indicate that faulting occurs on a straight line trace at the subsurface, but we have not found this always to be the case at the surface.
At the NW-SE fault trace proceeding from Maxwelton Village to the “hook” at the north end of Cultus Bay, the trace proceeds SE from the village along the south Maxwelton Creek, but then it jogs to the north in the portion near Swede Hill, indicating that Swede Hill has uplifted since the original faulting. This is valuable information, since it indicates that the uplift can be timed- it is post-faulting.
At Cultus Bay, there is an anomaly in the faulting; although the NW trace can be seen on LIDAR and on the geographic TOPO, the perpendicular change in Cultus Bay indicates that there are two orientations- NE-SW as well as the orthogonal Creek orientation.
Lateral faulting on W.I. is sinusoidal in expression- that is, it is compressional at times and extensional at others. The fault expression at Baby Island, which is undisputable, is in the nature of small hogbacks just 100 meters distant from a normal dropping on the west side in the concrete barricade. This expression of down drop is disputed by the local inhabitants- who have a pecuniary interest in maintaining property values. They maintain that the down drop in the bulwark was put in the concrete originally, ignoring the fragmentation of the boundary concrete. However, they pointed out that there is a junction of two separately-poured concrete sections, and indeed, when one looks at the base of the bulwark, there is an old wooden separation junction. This uplift on one side of the fault appears to be the case for Admiralty Head also. It is possible that some of the suspected drop at B.I. (19 cm) is artificial, but there are other indications of down drop on the SW side, other than the concrete separation. There are sets of parallel fractures, which are wider at the top, than when they occur lower in the concrete- this indicates extension on the SW side, or settling as one approaches the concrete breakage.
The creek orientation seems to be a better method of finding linear faulting than does the LIDAR. Good examples include Silver Creek- running 110 degrees from north, S. Max Creek, running mostly NW-SE, the small drainage north of Loganberry Farm, running NW-SE, and the NW-SE drainage north of Mutiny Bay.
Ιgnore creeks or drainages running N-S, since these are expressions created by the scouring and carving made by the dominant glacial movements. Only when drainage crosses the N-S scrapings, can it be considered as happening since the glacier period.
