The heavy mineralogy of the sand fraction for beach samples reported by Sayles (1966) and 10 new offshore samples from South Monterey Bay was determined optically. For each sample the percentage of the more diagnostic transparent minerals is plotted graphically in order of persistence: zircon, garnet, biotite, apatite, clinozoisite and epidote, lawsonite, green hornblende, oxy-hornblende, glaucophane, sphene, zoisite, augite, jadeite, hypersthene, enstatite, and tremolite & actinolite. Additional data on accessory transparent minerals, composite grains (rock fragments) and opaque minerals are listed with each graph. An updated bibliography is presented to include all new work on the geology and sediment of Monterey Bay.

Sediments of Monterey Bay are divisible into five heavy mineral provinces. Two of the provinces are from the Salinas and Pa,iaro rivers, the other three are not traceable to any known source. Sediments of the Salinas River have high garnet content, and the minerals glaucophane and lawsonite distinguish the Pajaro River sediments. A mineral province is restricted to beach sands along the north shore of the bay, and is carried into the bay by longshore drift from the northwest. The heavy mineral provinces do not coincide with the age differences of the sediment cover. The San Lorenzo River does not produce a detectable mineral province.

The grain size of the sediment cover decreases uniformly with water depth from the shoreline to a depth of 300 feet, then becomes coarser in a band along the edge of the continental shelf. Grain size modes correspond to conditions of wave agitation over most of the bay. Polymodal samples and samples not in agreement with this relationship can be shown to contain relict Pleistocene components. Sediment is being transported seaward from the shorelines by simple wave agitation, and deposited at various water depths according to grain size.

The sediment cover occurs in bands that are aligned subparallel to the depth contours. The outermost band on the continental shelf contains coarse sediment of Pleistocene age. The middle band is of very fine grained sediment of Holocene age. The nearshore band is of medium and coarse grained sediment probably of both Holocene and Pleistocene age. Pleistocene influences are found in this band at depths of 100 feet in the northeast corner of the bay.

The following work is part of a continuing study of the sediments and sedimentary processes of the continental shelf of central Califor nia done in cooperation between the University of California, Berkeley, and the Coastal Engineering Research Center, U. S. Army Corps of Engineers. Sediment analyses of the samples were done at the Univer sity of California, Berkeley, utilizing the facilities of the Depart ments of Civil Engineering and Geology and the Institute of Marine Resources. The results of this study will be presented in three separate reports:

Part A Introduction and Grain Size Data (this volume) Part B Mineralogical Data Part C Interpretation and Summary of Results

The first two reports, Parts A and B, will be presented with little or no interpretation. In Part C the authors' interpretation of the data plus background information and previous work in the study area will be given.

The area covered by this report extends from Pigeon Point in the north to Sand Hill Point in the south. With the completion of this report a complete section of the continental shelf of California from Russian River to Monterey Bay will have been studied. The methods of sediment analysis employed in the overall study are grain size analysis followed by heavy mineral analysis and interpretation.

Grain s i z e , heavy mineral, and o r g a n i c c o n t e n t a n a l y s e s of 43 marine and 9 i n t e r t i d a l and f l u v i a l samples p l u s d a t a from 28 marine samples from a previous study by Sayles (1965) form the d a t a i n t e r p r e t a t e d in t h i s r e p o r t f o r the area landward of 90 meters depth (50 fathoms) from P i l l a r Point t o Pigeon P o i n t , C a l i f o r n i a , between t h e Golden Gate and Monterey Bay.

This study i n d i c a t e s : ( 1 ) ' The grain s i z e of t h e sediment decrease off shore except f o r a nose of c o a r s e r sediment from -36 t o -50 meters extending from the north t o the l a t i t u d e of Half Moon Bay. (2) The heavy mineralogy is dominated by hornblende w i t h v a r y i n g amounts of a u g i t e and hypersthene. F r a n c i s c i a n m i n e r a l s a r e found only i n t r a c e amounts. (3) Three major source areas f o r t h e s u r f a c e sediment a r e apparent (a) a great v a l l e y- Sierran source r e l a t e d t o the sediments of t h e present San Francisco o f f s h o r e b a r ; (b) a l o c a l source r e l a t e d t o the drainage basins now emptying d i r e c t l y i n t o the area off s h o r e ; (c) a q u a r t z d i o r i t e source presumably montara t o t h e north. (4) The major d i s t r i b u t i o n p a t t e r n of the sediment may be explained (A) from -90 t o -20 meters by the superimposition o f l o c a l stream drainage on the s h e l f on r e l i c t sediment from the north during lower stands of sea l e v e l w i t h l i m i t e d reworking and mixing of sediments a t province boundaries during t h e l a s t rapid r i s e of sea l e v e l ; and from -20 meter t o present sea l e v e l by p a r t i a l homogenation of l o c a l sediment during a slow rise i n sea l e v e l .

The heavy mineralogy of the sand fraction for 44 offshore, 9 beach, and 3 stream samples for this region is determined optically. For each sample the percentage of the more abundant or more diagnostic transparent minerals is plotted graphically in order of persistence and additional data on accessory transparent minerals, opaques, and composite grains (rock fragments) are listed.

This section of the study of Bolinas Bay deals with the heavy mineralogy of the 0.175 - 0.124; 0.124 - 0.088; and 0.088 - 0.061 mm size fractions of 49 sediment samples (Fig. 1) [see Isselhardt and others (1968) for grain size and sizing procedure] plus the 0.124 - 0.088 and 0.088 - 0.061 mm size fractions of one disaggregated cliff sample 2030. Offshore rock samples 1974, 1975, 1976, 1977, 1998 and 2000 and cliff and shore samples 2027, 2028, 2029, 2031, and 2032 were not dis aggregated for mineralogical analyses. These rock samples will be discussed in Part C. Sample 1988 was lost and 2007 was too small for analysis.

The treated size fractions were divided further by separation in the heavy liquid tetrabrom-ethane (Krumbein and Pettijohn, 1938, p. 325) with a density of 2.95 gms/cc. Particles with a density greater than 2.95 gms/cc were called heavy. Particles with a density equal or less than 2.95 gms/cc were designated light. Grain mounts were made of both the heavy and light particles of each size fraction on glass slides with Cadex mounting media (index of refraction = 1.55).

Grain size and heavy mineral analysis of 6 cliff, 12 beach, and 44 marine sediment and rock samples from Bolinas Bay were done as part of a study of sediment transport on the continental shelf of California.

The following work is part of a continuing study of the sediments and sedimentary processes of the continental shelf of central California done in cooperation between the University of California, Berkeley and the Coastal Engineering Research Center, U.S. Army Corps of Engineers. Sediment analyses of the samples were done at the University of California, Berkeley, utilizing the facilities of the Departments of Civil Engineering, and Geology, and the Institute of Marine Resources. The results of this study will be presented in three separate reports:

Part A Part B Part C Introduction and Grain Size Data (this volume) Mineralogical Data Interpretation and Summary of Results.

The first two reports, Parts A and B, raw data will be presented with little or no interpretation. In Part C the authors' interpretation of the data plus background information and previous work in the study area will be given.