LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

88 3WJWEH CRETACEOUS DEPOSITS IN CALIFORNIA AND OREGON
may have been lost during, or subsequent to, its period of deposition.
This sediment is almost wholly detrital and has been derived from a commensurate
land area, which was then occupied by a river system of like
proportions, and which carried a volume of water corresponding to the
work accomplished. Such an area of basin-like form, adjacent to the
delta, is now occupied by the Trinity River system, which in Cretaceous
time had a possible drainage area of 5300 square miles, including a large
part of the present Klamath River drainage. The larger part of this
area comes within tbe limits of the Yolla Bolly basin, whose drainage
direction has been reversed since Cretaceous time. This reversal of
direction was due partly to faulting and uplift on its eastern side, and
partly to downwarping on its western border. Into this intermontane
basin, marine water entered from the east and received sediments from
the surrounding mountains, remnants of which, containing marine, partly
marine, and fresh-water Mollusca, still remain intact but widely scattered
within tbe basin.
It would require the removal of nearly one mile of surface material from
the entire drainage area here described to have supplied the sediment
found within the delta area in the Sacramento embayment. The accumulation
of detritus constituting this delta could have been built up only
under conditions of prolonged subsidence in the embayment, of which the
vertical measure would equal an abysmal depth scarcely credible, considering
the present restricted breadth of the trough; its breadth may have
once been greater.
Much of the coarser sediment brought into this area by the river was
dropped near its discharge point, whereas the finer sediment was carried
farther from it. The current from the river would be southward along
the west border of the trough, and this would automatically induce tidal
currents northward along its eastern border, producing segregated areas
of purely marine and partly freshened water on opposite sides of the embayment
and the setting up of rotating currents within it.
The axial part of the delta lies along the Shasta-Tehama County line,
dividing it into two unequal districts to the north and to the south of this
line. These districts would present in Cretaceous time considerable contrasts
in ecological conditions, which would react upon the faunas and
their development throughout the entire period.
At the north is the North Cottonwood district of restricted area, and
at the south tbe more extensive district crossed by the South fork, Redbank,
Elder, McCarthy, and Thomes creeks in Tehama County. Both
districts are rich in marine Mollusca, which constitute parallel and nearly
contemporary faunal successions which, when biotically compared, show
surprising contrasts. For these we must suppose either different sources of
origin, or different ecological conditions of life and development, or both.
In the southern district the lowest Cretaceous fauna is dominated by