Water Resources Collections and Archives

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 .