II. Physiography of the MBNMS seafloor
The MBNMS seafloor can be divided into three segments (northern, central,
and southern) based on physiography and geology. The northern segment is
composed of a relatively broad-shelfed, smooth and undissected seafloor,
the central segment has a narrower shelf and highly dissected floor, and
the southern segment is composed of a very narrow-shelfed, moderately dissected
A. Northern segment
The northern segment lies between the southern Farallon Islands-Tomales
Bay area (the Gulf of the Farallones National Marine Sanctuary) and Point
Año Nuevo. The most prominent features here are the headward parts
of Pioneer Canyon, which continue from within the MBNMS down the continental
slope and out onto the abyssal plain west of the sanctuary boundary (Figure
1). Here the continental shelf is quite wide, smooth and undissected
by submarine canyons compared with the continental margin to the south.
However, beneath this wide continental platform lies the Outer Santa Cruz
Basin, a buried sedimentary basin filled with hydrocarbon-bearing Miocene
rocks of the Monterey Formation (Hoskins and Griffiths 1971; Heck et al.
1990). This is the most likely petroleum site in the MBNMS.
Some of the more spectacular and unusual geology of the central California
continental margin exists west of the northern segment boundary, located
at the base of the continental slope (Figure 1).
Pioneer, Gumdrop, and Guide Seamounts represent Miocene volcanoes that were
carried on the Pacific Plate to the now-extinct Miocene subduction zone
and are perched to start their slide beneath the North American continent
(McCulloch and Greene 1990). Pioneer Canyon cuts between Pioneer and Guide
Seamounts, exposing older rocks along its wall. A large mega-sediment wave
field at the base of the slope (Figure 1) is composed
of sediments and erosional detritus that the canyon funnels from the continental
shelf and erodes from the slope.
B. Central segment
The central segment extends from the Point Año Nuevo area to
south of Point Sur. It contains the most geologically diverse and physiographically
interesting seafloor within the MBNMS. The most dramatic features are the
Ascension-Monterey Canyon system, which has extensively dissected the continental
shelf and slope in the Monterey Bay area, and the many heads of Sur Canyon,
which have cut the continental slope just south of Point Sur (Figure
In addition to the extensive amount of erosional dissection occurring on
the margin in this segment, evidence of active faulting is quite evident.
Large gashes across the seafloor result from tectonic movement along offshore
faults. The most prominent feature resulting from faulting is Carmel Canyon
(Greene et al. 1973; Greene 1977; McCulloch and Greene 1990; Greene 1990)
where a segment of the Palo Colorado-San Gregorio fault zone extends northwestward
from Garrapata Beach, then across outer Monterey Bay to Point Año
Nuevo (Figure 1). At this junction the Monterey
Canyon is distorted, and the canyon axis meanders along the faults in this
area. Another fault gash (Figure 1) is found west
of Point Sur where a north-south trending fault offsets the lower part of
Sur Canyon and forms the eastern scarp of Sur Knoll (Greene et al. 1989;
Greene and Hicks 1990).
West of the central segment boundary, the Ascension system of canyons and
Monterey Canyon system meet. Downcanyon, a sea valley meander has formed
(Shepard and Dill 1966), named the Shepard Meander, and is thought to be
the result of a breached levee after the last sea level rise (Normark 1969,
1970) or formed from a previous large mass wasting event that clogged Monterey
Sea Valley (Greene et al. 1989; Greene and Hicks 1990). Mega-sediment wave
fields in this area are from sediment transported down the Ascension and
Monterey Canyon systems (Figure 1).
C. Southern segment
The southern segment extends from south of Point Sur to Morro Bay. Here
the MBNMS averages only 25 km wide, and contains a very narrow, moderately
dissected continental shelf and (Figure 1) Its
geology is not well understood compared to the other segments.
Lucia Canyon is the major submarine canyon in this segment, and its many
heads act as funnels for the rapid transport of detritus eroded from the
Santa Lucia Mountains to the deep abyssal plain (Greene et al. 1989). Lucia
Canyon, like Sur Canyon, debauches at the base of the continental rise;
sediment carried down these canyons covers the base of Davidson Seamount
(McCulloch 1989a). Similar to the seamounts found west of the northern segment,
Davidson Seamount is a Miocene volcanic edifice carried by the Pacific
Plate to the brink of subduction.
Next - Section III.
Geology and Tectonics
Geology Table of Contents