10.8.2.4: The sedimentary journey

On top of the oceanic crust is where sediments can be deposited.

ABYSSAL DEPTHS: Chert & shale

In the offshore realm, far out in the Pacific Ocean (but atop the Farallon Plate), deep sea sediments of chert and clay shale accumulated. The source of the material for the chert was the dead bodies (“tests”) of radiolaria, which rained down through kilometers of sea water as “marine snow.” Unlike calcite, which tends to dissolve in the deep sea below the carbonate compensation depth, the silica that makes up the radiolarian skeletal material tends to be stable.

Figure \(\PageIndex{1}\): Chert of the Marin Headlands Terrane crops out on Marshall’s Beach, south of the Golden Gate Bridge in San Francisco.

Figure \(\PageIndex{2}\): Chert and shale, rhythmically interbedded, as exposed at Marin Headlands, California.

We don’t fully understand the alternation between chert and shale that characterizes the Marin Headlands Terrane, but it may be due to annual or seasonal cycles, or Milankovitch cycles in Earth’s orbit. Some workers have suggested it’s a diagenetic separation of the two sediment types: that is to say, the radiolaria skeletal remains and the clay are deposited at the same time, and only later separated once buried, by diagenetic processes.

Because chert is made of silica, it is both chemically stable and rather hard, so it stands up well to weathering and erosion. As a result, chert can be found underlying some of the highest landforms in the Coast Ranges.

Figure \(\PageIndex{3}\): Graded bed in turbidite, Usal Beach, Mendocino County, California. Bedding is vertical, and fines (gets younger) to the right.

TURBIDITES: sand & mud via underwater avalanches

Closer to shore, the weathering and erosion of the continent brings a lot of sediment into the ocean. Along the continental slope, turbidity currents deliver slurries of sand and mud in suspension into the deep water. Though they come from surface waters, they are dense due to all their entrained sediment, and they tend to hug the bottom as they advance. Occasionally, they will travel along submarine channels, and spread out into complexes of abyssal fans. As these turbulent currents slow down, they drop their sedimentary load, with larger (heavier) particles dropping out first, and finer (lighter-weight) particles staying suspended in the water column for longer, eventually dropping out last. The resulting graded beds are signatures of this mode of deposition. We call the overall packages of graywacke sandstone and black shale deposited by turbidity currents “turbidites.”

Figure \(\PageIndex{4}\): This conglomerate bears a diverse assemblage of clasts, including blueschist and sandstone. Overall, the clasts do not appear to record much deformation. In other words, this appears to be a primary sedimentary conglomerate. Kayak Beach, Angel Island State Park, California.

NEARSHORE FACIES: River-borne sand & gravel

Closer to the coast, and in shallower waters of the continental shelf, rivers debouching into the sea drop deltaic deposits. For the most part, this is sand and mud, but in many places pebbles and cobbles (gravel) make it all the way to the shore too. These sedimentary deposits result in rocks such as shale (former mud), sandstone (former sand), and conglomerate (former gravel). Sometimes plant fragments are preserved in these rocks as little scraps of coal.

The ideal rock sequence for the rocks making up the subducted seafloor is therefore something like this:

Figure \(\PageIndex{5}\): A cartoon summary of the original relationship between various rock units of the oceanic sequence, prior to being fed into the subduction zone (with accompanying metamorphism and deformation).

Figure \(\PageIndex{6}\): An example of a schisty texture, developed in a blueschist-grade metasandstone on Angel Island, California. A small kink fold emphasizes the trend of the shistocity (foliation).

METASEDIMENTS: schist and metaconglomerate

When they metamorphose, these sedimentary rocks transform into phyllite, schist or metaconglomerate. Mudrocks like shale can recrystallize to make shiny, foliated phyllite, which will turn to schist with higher grades of metamorphism. Sandstones turn into “metasandstone,” which looks much like sandstone does (but contains key metamorphic minerals when examined in thin section). Some metasandstones can also develop a schistocity (scaly foliation, as seen in the example here).

Conglomerates made of a variety of clasts can demonstrate an interesting response to shearing stresses: the weaker rock types will smear out into ribbon-like shapes, while the stronger rock types will resist deformation as rigid chunks or else break into pieces. These “stretched pebble conglomerates” record some deformation, but not so much as to qualify the metaconglomerate as mélange.

Figure \(\PageIndex{7}\): Coastal outcrop on Angel Island, showing a brown and green conglomerate, with pebbles of various sizes showing different degrees of shearing. Some are apparently intact and retain something close to their original shape, while others have been broken by (vertical) brittle fractures. Still others have been smeared out making a moderately strong (horizontal) foliation. These are continental margin deposits that have been fed into the subduction zone.