Page 122 - Petrology of Sedimentary Rocks

wedges), while tensional spasms cause Dv (block-faulting and arkoses). These two
phases often alternate and may be interspersed with restful phases, forming quartz-
arenites or carbonates. Minor volcanism and intrusion may occur. The Amadeus Basin
of central Australia appears to be a good example of incipient plate separation with
later compression, collapse and healing. Continental plates may also slide laterally,
K/K (read K slide K), producing Dv uplifts and basins. See Lowell 1972 GSA.

Aulacogenes are incipient KAK.

IIB. PLATES SEPARATING, Ensimatic rift Kl\sK, read K split-s K). Further
cracking apart of continental plates exposes simitic basement. A very important, deep
I i near “eugeosync I i ne” is formed; usually vast outpourings of basic to intermediate
volcanics, often with deep-water sediments (shales, cherts) and turbidites; sandstones
usually phyllarenites, volcanic- or plagioclase-arenites. Some material (of ten quartzose)
may spill in from the stable Kraton (Qk) during phases of relaxation, especially in the
“miogeosynclinal” belts that flank the medial “eugeosyncline.” Alternating spasms of
compression (causing Dh and normally phyllarenites) and tension (causing Dv) are the
rule, with abundant volcanism (X). Examples are the Appalachian geosyncline during
much of the Paleozoic when it had a kraton-ward miogeosynclinal belt and medial
eugeosynclinal belt between the osculating American and European plates. The Ural
geosyncline is also probably an example of this.

IIC. PLATES SEPARATED, Oceanic rift produced (KOA). Plates have finally
separated, osculating ceases, and a substantial oceanic zone lies between the two
fragments. Continents continue to drift apart and the ocean widens, the mid-ocean
ridge is active, and mantle currents remain in their positions. Sediments are provided by
the kraton and the now-rigidified neokratonic frame and tectonized source lands that
had been active highs during phase II (KhK). Detritus spills off the continental margin
into the newly-formed oceanic basin, forming a “geocline,” or Atlantide geosyncline.
Tectonics mostly R or Q, some Dv, little or no Dh, l volcanism not generally important.
Example: Atlantic coast of North America during Mesozoic and later time. Sandstones
not distinctive; dirty quartzarenites common, and some rejuvenation phyllarenites.
IIIA. PLATES COLLIDING, Ocean/Continent (KG 0, read K crash 0). Basic
oceanic crust is thrust under the edge of the continent in a trench and subduction zone.
Trench sediments, turbidites, volcanics horribly mingled; intrusion of granites, ophiol-
ites, etc. Common volcanic island arcs, (classic “eugeosyncline”) with a kraton-ward
miogeosynclinal belt. Pacific type geosynclines of Crook. Mostly continuous intense
DhX (thrusting and folding with volcanism; phyllarenites and volcanic- or plagioclase-
arenites), but with occasional resting or even tensional phases. “Miogeosyncline” can be
supplied mainly frorn the kraton side; “eugeosyncline” from the volcanic arc. If
volcanics break through on the continent (not a separated island arc), a huge terrestrial
volcanic pile may result instead of a geosyncline (e.g., W. Mexico).

Dickinson has proposed that plates may shear (slip sideways) instead of collide,
but sediment types are probably not much different; there may be less volcanism. This
shear situation may be symbolized K/O (read K slide 0).

IIIB. PLATES COLLIDING, Continent/Continent (K * K). The two continental
plates were once separated by a substantial ocean. Conditions appear to be not much
different than IIIA (K 0) except less volcanism; usually a tectonic land is lifted up at
the line of collision. The Himalayas are an example of K SK. Intense Dh caused by

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