Online Quiz

Vendian History

1Ga, a supercontinent “Rodinia”
–  formed at the end of the Grenville orogeny

550MY, a supercontinent “Pannotia”
– as a result of collisions & dismemberment of Proterozoic supercontinent

these events was indicated by:
1. The Vendian basaltic rocks around the margins of the continent
2. Fault-bounded troughs containing very thick Vendian strata within margins of craton

breakup of the supercontinents

Sea must have surrounded the continent at first & gradually spread over craton during Late Cambrian & Early Ordovician

Transgression caused by a rise of sea level due to breakup of the supercontinents

The breakup represents fundamental global reorganizations of plate configurations, that’s why we know N-America as it is now

structural modification of Cratons

The broad & gentle warpings of craton
– caused by small changes of isostatic equilibrium due to density changes in the underlying lithosphere
– have caused formation of:
1. Arches (Raised areas) with thin & less complete sequences of strata interrupted by unconformities
2. Basins (depressed areas) with thicker & more complete sequences, subsided rapidly & received greater & more continuous sedimentary accumulations than Arches
3. faults that may form aulacogens within the outer portion of cratons

Arches Vs Basins

Basins & Arches have greatly influenced the accumulation of oil, gas, salt, gypsum, pure limestone, & other economically important resources associated with sedimentary rock

Cratonic Basins & Arches are 2 types according to the warping time:
1. Warping during sedimentation caused:
– Thickening strata in basins
– Thinning strata in arches
2. Warping after sedimentation caused thickness patterns superficially

Warping favored accumulation of petroleum, which migrated up-dip from the basin where it was generated

contrast between basins & arches is due only to differential erosion after warping, this warping was too late to trap petroleum

How do we acquire the information for studying broad and & cratonic structures?

As we know much of the evidence for arches lies in the surface outcrops, but only the edges of the basins are exposed, so subsurface information is required to define basins & buried arches

our informational sources was from: Deep drilling for water & petroleum

Deep drilling for H₂O & petroleum provided 3D to our observations geophysical devices; such as magnetic & gravity surveys which revel information about special types & thicknesses of buried rocks

seismology provides the greatest insight into thickness, structure, & in some cases the lithology of buried strata

General features of the earliest Paleozoic sediments

1. The sandstone facies
2. The carbonate facies

The sandstone facies
1. Quartz-rich sand: dominant Cambrian sediment on the craton with glauconite- bearing fine sand, shale was minor
– Carbonate sediments replaced all of these during Ordovician times

2. Cambrian cratonic sandstones rank among the most mature in the world
– gains: well rounded, sorted, & consists 99% quartz with traces of other stable mineral

3. Ripple marks is a strike feature for the Cambrian sandstone, from ripples feature the origin of the current can be deduced


The carbonate facies
By the beginning of Ordovician time, so little land remained exposed that deposition of terrigenous clastic material nearly ceased

Clastic limestone was the main sedimentary deposits, these carbonate show ripple marks, oolites & trace fossils (burrowing action)

All the above features of carbonate indicate the shallow water or tidal origin

Early Paleozoic Paleoclimate

Climate evidence from organic reefs, land plants, coal, soil, & O₂ isotope data aren’t available in early Paleozoic strata

the abundance of oolite, some very shelly limestone, & local evaporite deposits do tend to support a warm climate

Presence of wind deposits & scattered evaporites →think of arid deserts

The End