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(415) 557-0413

Aprl 1 29, 1982

Todd Nelson, Senior Planning Geologist Contra Costa County Planning Department County Administration Building, Uorth \Jing P.O. Box 951 Martinez, CA 9'1553

Dear Todd:

We are placing on open file the following reports, revle11ed and approved by the County of Contra Coste for the City of Concord In compliance with the Alqulst·Prlolo Special Studies Zones Act:

Fault trace Investigation, proposed Subdlvlsl_on 5"8, Concord, Calif• ornla; by Geodata, Inc., May 11, 1979,

The reports for Minor Subdivisions l4S 31·73 and MS 124-78, Tass~jaros quadrangle, rr_edate the SSZ1 s and will be placed In our lnfonnal consul ting fl le,

Sincerely yours,

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EARL W. HART Office of the State Geologist CEG 935

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• ~Planning Department Contra

Costa County

County Administration Building, North Wing P.O. Box 951 Martinez, California 94553 -0095

Anthony A. Dehaesus Director of Planning

Phone: 372-2024

April 26, 1982

Mr. Earl Hart Office of the State Geologist Ferry Bldg., Room 1003 San Francisco, CA 94111

Dear Earl:

Enclosed are two fault hazard reports submitted to Contra Costa County for minor subdivisions in the Tassajara Quadrangle, Greenville-Marsh Creek fault zone. I find them interesting from an historical standpoint. Geologic studies on the Prentice property began before the January, 1980 earthquakes, both studies were concluded after the earthquakes, and neither report mentions any attempt to locate the fault trace from surface indicators other than bedrock attitudes.

I have plotted the center of the fault setback zone on the Narduchi Parcel. The eastern edge of the once-proposed setback on the Prentice Parcel very nearly coincides with your mapped trace of the fault, at the l"-2000' scale so it is not plotted.

Please enter both reports in the Alquist-Priolo report files.

Jim Baker's memo reviewing the reports for Prentice are enclosed. I am unable to locate any reference by Baker in any Planning Department files to the Narduchi Parcel report. The proposed minor subdivision, M.S. 31-78 was denied by the Plan­ning Commission, appealed to and approved by the Board of Supervisors in January, 1979 with conditions including a "soils reconnaissance" for Parcel B if asked for by the Planning or Building Department. (Parcel A was occupied by a residence prior to the subdivision). The report enclosed appears to be the result of that condition of approval. The map has not yet been recorded.

• (Please send me at your earliest f J/i"'/· Marsh Creek Fault, not included :.,,,_,,., were being revised .

convenience a copy of FER 117 on the Greenville­in the batch of FER's you sent while the SSZ's

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Sincerely yours,

Anthony A. Dehaesus Director of Planning

~~ Todd Nelson Senior Planning Geologist

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GEOLOGIC RECONNAISSANCE REFORT

for

TENTATIVE MINOR SUBDIVISION (MS )1-78)

MORGAN TERRITORY ROAD

CONTRA COSTA COUNTY, CALIFORNIA

INVESTIGATOR

JOHN D, TEWHEY, Reg. Geologist (No. 3349)

December 15, 1980

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1

LOCATION

The land referred to is situated on Morgan Territory Road,

County of Contra Costa in the State of California. The parcel is located

in the east-central portion of the 7,5 minute Tassajara Quadrangle (SE/4

Mt. Diablo ·15' quad) and in the NE t NE t, Section 31 and in the SE t SE t,

Section 30, T 1S, R 2E (Fig, 1),

OWNER AND APPLICANT

Thomas Narducci 10009 Morgan Territory Road Livermore, California 94550

GEOLOGIC SETTING AND GENERAL INFQRMATION

The 31.63 acre parcel is located nine miles from the summit of

Mt. Diablo on the southwest flank of the Diablo uplift, The entire area

is underlain by Cretaceous marine sediments, The area consists of hilly

rangeland (slopes to 70%) that is devoid of trees except along the sides

of the creek that.traverses the parcel in a NE-SW direction, entering

along the northwestern ·edge and exiting in the middle of the eastern edge.

The geologic data upon which this report is based is derived from

several sources1

1) Geologic map of California (San Jose Sheet) compiled by

T. H. Rogers, 1966,

2) The geologic map of the Tassajara Quadrangle, Alameda and

Contra Costa Counties, California compiled by Thomas W, Dibblee, Jr., 1980,

(Open File Report 80-544),

3) The preliminary geologic map of the Mt. Diablo-Byron area,

Contra Costa, Alameda and San Joaquin Counties, California compiled by

2

Earl E. Brabb, Howard S, Sonneman and John R. Switzer, Jr. in 1971. (U,S,G,S,

open file, Sheet 1 of 2, l" • 4000'),

4) Soil Survey of Contra Costa County, California, 1977, U, S,

Department of Agriculture Soil Conservation Service,

5) The 1957 (1112,000) and 1975 (1148,000) aerial photos were

examined,

6) Preliminary Photointerpretation Map of Landslide and other

Surficial Deposits of the Tassajara 7!' Quadrangle, Contra Costa and.

Alameda Counties, California by Tor H. Nilsen 1975, U. S. Geological Survey

Open File Map 75-277-53,

7) Geologic reconnaissance and mapping within the parcel by

the compiler of this report,

GEOLOGIC DESCRIPI'IONS

The Cretaceous marine sediments within the area of interest

consist of interbedded sandstones am shales, The sandstones are gray­

green when fresh, light to dark brown when weathered, Shales are gray to

black when fresh, light to dark brown· when weathered, Sandstones have a

blocky character and are more resistant to weathering than the shales,

therefore are more prominent in outcrop,

A geologic map of the Narducci parcel is presented in Fig, 2.

The parcel is located in an area in which there are high-angle faults

associated with the Diablo uplift, The faults in the Diablo system are

considered to be active, as evidenced by the 5,5 magnitude earthquake on

January 24, 1980, The epicenter of the January 24th earthquake has been

tentatively located on the Marsh Creek Fault (Dibblee, 1980) approximately

one mile north of the Narducci parcel,

3

The Marsh Creek Fault, as mapped by Dibblee (198()~ runs through

the southwestern portion of the Narducci parcel, As part of this investi­

gation, the geologic structure was examined closely in the creek which

traverses the parcel and in an erosion gully which is colinear with the

northwest boundary of the parcel between Morgan Territory Road and the ;'

Creek (Fig, 2), The 140 foot northeast-trending erosion gully is from five

to ten feet deep, The rocks exposed in the gully walls are weathered to

saprolite, but it has been possible to determine the original structure of

the sedimentary strata by clearing away the surface debris from the gully

walls, The strata within the gully have a general north-south strike and

dip gently to the east,

Good exposures of fresh rock occur in the creek, The creek flows

to the southeast in .Pa.reel B and then bends to the northeast near the south­

east boundary of parcel B. Sedimentary rocks exposed within parcel B are

gently dipping or horizontal (Fig, 2 and 4). At the bend in the creek

adjacent to the boundary between parcel B and parcel A, the sedimentary

strata have a NW-SE strike and are vertical, The transition from horizontal

to vertical orientations takes place over a distance of a few feet, The

sharp transition in orientation is interpreted here as being evidence for

a fault zone, The center of the fault zone in E-igures 2 and 3 has been

located midway between an outcrop of horizontal strata and vertical strata

which are located a distance of about ten feet apart within the creek bed,

The orientation of the fault zone has been determined by means of (1)

information on the location of the Marsh Creek Fault obtained in geologic

investigations in the Prentice Parcel (MS 124-78) which is adjacent to the

Narducci parcel to the south, (2) aerial photos and (3) orientation of the

near-vertical strata on the northeast side of the fault zone.

7

4

The limits of the fault zone are delineated within the Prentice

parcel (Fig. 9) as a result of exposures of strata in a creek bed on the

northeast side of the zone and exposures in an excavated trench on the

southwest side of the zone that was examined in detail by the author of

this report in September 1980. Strata in the creek bed are near-vertical,

similar to exposures on the northeast side of the fault in the Narducci

parcel. Strata in the trench were subhorizontal, similar to the orienta­

tions of rock on the southwest portion of the fault zone in the Narducci

parcel, The similarities in the orientations in the adjacent parcels

suggest that the fault is located in the 100 foot zone between the trench

and the creek in the Prentice parcel. A point in the middle of the 100 foot­

wide zone in the Prentice parcel was chosen arbitrarily and a straight line

was drawn between that point and the fault location described earlier in

the Narducci parcel, The orientation of the fault zone was thus determined

to be S 33° 13' E {Fig, 3), This orientation is compatible with {l) the

orientation of the vertical beds located in the stream bed in the Narducci

parcel and { 2) a faint linament observed in aerial photos {Fig, 7).

The fault zone in Figures 2 and J is defined as a 50 foot-wide

zone1 a 25 foot zone was demarcated on each side of the suspected fault trace.

The 50 foot fault zone is considered to be a "restricted zone" and it is

recommended that building sites not be located within the zone. The restricted

zone is defined precisely in Figure 3,

The field information obtained in this study basically supports

the location of the Marsh Creek Fault as determined by Dibblee (1980) (see

Fig. 5), This report places the fault slightly further to the northeast

than the Dibblee location, The orientation of the fault is essentially

the same in this report and in the Dibblee report,

'

5

SlJRFICIAL DERJSITS

In spite of the steep slopes within the parcel, no slumping

or landslide structures were observed in the parcel. This observation is

in accordance with the Nilsen surficial geology map of 1975 (Fig. 6).

An erosion gully up to ten feet deep is located along the north­

west boundary of parcel B, colinear with the boundary line. Maximum width ·

of the gully is approximately 12.feet. It is recommended that building be

restricted within 30 feet of the edge of the erosion gully mentioned earlier,

Building should also be restricted within JO feet of the edge of the creek

bed.

In order to minimize any potential for future hazard to structures

within parcel B, the following recommendations are made1

(1) Roof drains and downspouts should be positioned so that they

do not concentrate or augment stormwater flows into the gully or creekbed,

(2) Future owners of the parcel should monitor conditions in

the vicinity of the gully an:l. creekbed and take prompt corrective· action

1f significant·erosion occurs.

Rock riprap and prompt reseeding of exposed surfaces are corrective

measures recommended for repair of minor erosion.

Areas of ground slope of less than 20 percent and of adequate

size for installation of sewage disposal systems exist in parcel B.

The sewage disposal system in parcel B should be located down­

slope from the water supply well on that parcel and be laterally separated

from the well by at least 50 feet, There is ample room for a sewage disposal

system in parcel B which meets these recommendations.

6

ACCESS ROAD

The access to parcel B may be constructed along the Morgan

Territory Road frontage but should not be located within JO feet ofr,the

erosion gully located along the northwest boundary of the parcel. Road

construction should begin with the stripping of all organically contaminated

material. Road gravel may then be placed directly on the stripped natural

surface or on adequately compacted fill. In order to reduce the risk of

erosion damage, all exposed surfaces should be seeded with native grass

or other suitable groundcover prior to the start of the rainy season,

BEABING OF GEOLQGIC FACTORS ON INTENDED LAND USE

The effects of geologic factors upon construction and land use

of the proposed subdivision have been discussed, The information and

recommendations included herein'do not preclude the establishment of a

building site on parcel B. The area most suitable for a building site

location in parcel B is shown in Figure 8,

7

LIMITATIONS AND UNIFORMITY OF CONPITIONS

A) The recommendations contained in this report are based upon

the assumption that the geological conditions do not deviate from those

visible to us during our field reconnaissances, If variations or undesirable

conditions are encountered during construction, we should be notified so

that supplemental recommendations can be given,

B) This report is issued with the understanding that it is the

responsibility of the owner, or of his representative, to ensure that the

information and recommendations contained herein are called to the attention

of prospective buyers of parcels within this subdivision, Prospective

buyers should be advised that it is their responsibility to bring the

contents of this report to the attention of their architects and engineers

and to see that the recommendations contained in this report are incorpo­

rated in their construction plans and that all necessary steps are taken

to insure that the contractors and subcontractors carry out the recommendations

in the field,

c) The findings of this report are valid as of this date, However,

conditions of a property can change with time1 such changes may be the results

of natural processes or.the works of man on the subject property or on adjacent

properties, Also, changes in applicable or appropriate standards occur and

may result from legislation or the broadening of knowledge, Accordingly, the

findings of this report may be invalidated, wholly or partially, by changes

outside our control, This report should therefore be reviewed in the future

in the light of changes on the si~e, planned construction and the then

current applicable codes and ordinances, In any event, if construction

has not been completed within two (2) ~ears from the date of.issue of this

report, we should be afforded the opportunity to reinspect the property

and make any supplemental recommendations that we deem necessary or advisable.

8

D) We make no warranty, either expressed or implied, as to our

findings, recommerrlations, specifications or professional advice, except

that they are prepared and issued in accordance with generally accepted

professional geological practices,

~ Registered Geologist State of California

- Reg. No. 3349

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