ML20133G114

Trip Rept on 841128-29 Visit to Charleston,Sc Area to Inspect Apparent Seismically Induced liquefaction-flowage Features Near Middleton & Hollywood,Sc

ML20133G114
Person / Time
Issue date:	04/25/1985
From:	Alterman I, Cardone A Office of Nuclear Reactor Regulation
To:	Brocoum S Office of Nuclear Reactor Regulation
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FOIA-85-363 NUDOCS 8508080560
Download: ML20133G114 (14)

Text

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UNITED STATES E',,([g NUCLEAR REGULATORY COMMISSION y

W ASHINGTON, D. C. 20555 ytt 2 51985 MEMORANDUM FOR:

Stephan J. Brocoum, Acting Chief Geosciences Branch Division of Engineering FROM:

A. T. Cardone, Geologist Geology Section Geosciences Cranch, DE Ina B. Altennan, Geologist Geology Section Geosciences Branch, DE

SUBJECT:

VISIT TO CHARLESTON, SOUTH CAROLINA AREA TO INSPECT APPARENT LIQUEFACTION - FLOWAGE FEATURES On November 28 and 29,1984 we examined exposures of apparent seismically inducad liquefaction-flowage features near the towns of Middleton and Hollywood, South Carolina. We were accompanied by Prof. Pradeep Talwani and John Cox of the University of South Carolina; Greg Gohn, Rob Weems, and Steve Obermeier of the USGS; and Lyman Heller and Tom Schmitt of the NRC.

The recognition by the USGS staff and Prof. Taiwani of geologic evidence for paleo-liquefaction features in the Charleston area is a direct outcome of the NRC funded deterministic program of assessment of seismic hazard in the eastern U. S.

The object of our visit was to obtain first hand knowledge of the evidence for paleo-liquefaction near Charleston to better understand and evaluate the information resulting from the NRC sponsored research program and to help us assess the potential risk for Charleston 1886 type earthquakes in the eastern U. S.

Some of the exposures that we examined in the ditch near Hollywood, S.C. are discussed in a preliminary report by Gohn, Weems, Obermeier and Gelinas entitled " Field Studies of Earthquake-Induced Liquefaction-Flowage Features in the Charleston, South Carclina Area," Open File Report 84-670, USGS.

The geology of the areas near Charleston that we visited consists of unconsolidated Quaternary marine and fluvial sediments which thinly blanket Tertiary sediments in the Coastal Plain physiographic province.

The Hollywood ditch in which most of our time was spent is cut into one of six Pleistocene barrier beach deposits which parallel the coastal Plain shoreline. The depositional facies along the coast range in age from modern at the coast to over 1,000,000 years approximately 30 miles inland.

Our first stop was near Middleton, S.C.

In an old tidal creek depression bulldozing uncovered a gray-white, quartzose sand lens which apparently 8508080560 850624 PDR FOIA BELL 85-363 PDR

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intruded a reddish-brown clayey sand. The water level in the dammed creek was too high curing our visit to see the feeder conduits observed by Talwani and Cox at either side of the sand lens. Greg Gohn stated that to help verify if this is a liquefaction-flowage phenomenon, a comparison

,l should be made of the heavy mineral (dark colored) content in the gray-white sand vs that in the surrounding reddish-brown sand.

If the gray-white sand came from below it will show a significantly higher heavy mir.eral content since its time of exposure to the weathering process would be relatively brief.

Our next stop was at the eastern end of the Hollywood ditch, a soil conservation drainage ditch about 0.3 mi east of Towles Road, where our USGS guides established the following undisturbed soil profile beginning at ground surface.

Horizon Description A

0-15 cm thick friable, black soil, rich in organics and plant roots E

0-15 cm thick, very loose, medium gray, leached soil Bh 0.5-1.0 m thick, brownish black massive dense, hummate rich soil C

more than 175 cm yellowish-gray, friable, weathered, compact sand.

The bowl shaped infilled craters observed in the ditch profile (Photo 1 and 2) are somewhat characteristic of the general character of disturbance. However, it should be noted that while similarities exist, no two infilled craters or disturbance features were alike, nor did any feature persist in the third dimension, as the sand was scraped off and new planes of exposure were produces.hile we watched.

A common characteristic of the crater, which is assumed to have been produced by seismically induced liquefaction, is the somewnat rounded clasts of Bh soil of various sizes within a matrix of disturbed sand at the base of the crater. Above the Bh clasts are laminated layers of light and dark soil, slightly upturned at the boundaries, which in turn are overlain by a hummate rich Bh horizon of lesser thickness than that of the bounding undisturbed soil profile adjacent to the crater.

14 According to G. Gohn, approximate C dates on the various hummate occurrences are:

1.

Hummate overlying the laminated strata in the bowl shaped crater:

1500 YBP (years before the present).

2.

Hummus of undisturbed section of exposure, juxtaposed with laminated layers: 3000 YBP.

3.

Incorporated hummate clasts: 4500 YBP The explanation given for this is that the 4500 year date represents, the ageofthehummatewheni}4 dropped out of the zone of weathering, therefore maintaining a C count similar to its original. The 3000 YBP date for the undisturbed hummus is interpreted as the same stratum and unit as the incorporated 4500 YBP blocks but having been contaminated with younger organics leached.from above. The 1500 year old date represents carbon deposited after sedimentary infilling by the laminated beds in the crater. This hummate has also been contaminated. The USGS investigators believe these dates establish the upper and lower limits on the timing of the event. Assuming that the laminated beds infilled after the event which caused the subsidence of the section, the event would in our opinion

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have had to occur between 1500 and 3000 YBP. The 3000 year maximum is arrived at considering that the current age of the clasts is 4500 years but that the liquefaction occurred at least 1500 years ago, based on the age of the hummate at the top of the crater.

Several other cuts along the ditch showed similar craters with thin laminated beds overlying sand with incorporated hummate biccks. All of these features appeared to bottom out in the same sand as in the undisturbed portions of the cuts.

In a few places two or more liquefaction events could be discerned based on cross-cutting relations of structures, such as truncations of one form by another. One crater in particular (photos 3, 4 and 5) apparently exhibits 2 stages of liquefaction. The disturbed sand with Bh clasts and laminated sand layers on the right, formed by liquefaction, was later intruded by a pipe-like disturbed zone on the left, apaarently as a result of a later episode of liquefaction. The pipe-like feature flares at the top and truncates the laminated sand layers to the right.

In scoe places, old still-upright cycress logs, exhumed from the black hummate in the ditch excavation or by recent investigators are rooted at the bottom of the ditch, about 8-10 ft below present ground surface.

Trees of the area are mostly Southern Pine at the present surface (photo 6). The USGS investigator suggested that the cypresses indicate that the area was once a swamp, and would have been an excellent site for liquefaction phenomena during an earthquake.

On the second day of our visit, we returned to a part of the east-west ditch that turns north-south for a short distance exposing discontinuous horizontal white sand " sills" that fed upward to the surface (photo 7).

These were the only features indicating a possible 1886 age that appeared so directly connected with the surface. The sand in the " sills" was pure white, and appeared to come from below.

It was sandwiched between yellow l

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and buff sands above and below. Below the lower buff sand more white sand, possibly the source bed, was exposed in a few places (photo 8).

Along this stretch of the ditch there is evidence of lateral compression.

The lower black hummate is thrust upward close to the surface (photos 9 and 10). There are several apparent northerly thrusts along this 150-200 meter north-south trending ditch. Seismic reflection profiles have been taken along and perpendicular to the ditch by USGS to check for underlying structures.

While not all features seen could be explained, the unusual array of forms, structures and discontinuities, not common in ordinary undisturbed sediments, suggested to the investigators that these phenomena were liquefaction features.

At present, samples for dating in various places have been taken, and are awaiting results. Observational details are not definitive enough to determine ages of events.

p Tasks that remain are to determine how far from Charleston liquefaction-flowage features can be found, and how frequently do large earthquakes occur in the region. Can the observed limits of occurrence of these features provide convincing enough evidence that the Charleston seismic zone is unique to the Charleston area and may not be expected elsewhere in the coastal plain? The researchers hope that further investigations of liquefaction phenomena will eventually provide enough information to determine a return period.for Charleston 1886 type earthquakes. These are crucial questions that remain to be answered in this series of studies.

4.f&,&

A. T. Cardone, Geologist Geology Section, GSB, DE s.

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Ina B. Alterman, Geologist Geology Section, GSB, DE

Enclosure:

As stated cc: w/o photos J. Knight L. Beratan L. Reiter L. Heller L. Heller A. Murphy GSB T. Schmitt w

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