In this note I want to describe the Avalon Formation. Deposition of this unit was mainly controlled by tectonism.
The Avalon
Formation consist of cycles, which are characterized by siliciclastic dominance (Fig. 1). The formation begins
with calcareous limestones and sandstones which
are assigned to the ‘A’ Marker member. The ‘A’ Marker member at the bottom contact
with the Whiterose Formation, which is represented by silty shales. The thickness
of the member regionally changes. Sandstones are developed as quartzose, very fine
to fine-grained. Cement of this sandstones is mainly calcitic. Bioclastic
debris is present. Limestones which also are assigned to the ‘A’ Marker member are
bioclastic, peloidal, oolitic and contain high amounts of sand grains. Thin interbeds of red, green and
grey shales occur locally. (Sinclair, 1992)
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| Fig. 1. Correlation of the particular units. South Mara C-13 to North Ben Navis M-61. It can be observed shoaling -upward cycles of the Avalon Formation. (after: Sinclair, 1992). |
Deposits of this member in
the southern part of the Jeanne d’Arc Basin, contrary to the northern part are
more abundant in sand grains, which indicates that clastics derived from the
south. Oolites formed on sand grains and have calcite coats. Oolites are
indicative of wave-dominated, shallow-marine environment. In the southern part
developed more thin beds of oolitic, bioclastic limestones, which make known
more distal environment. Bioclatic debris are also characteristic of this kind
of environment. It was interpreted that deposits formed in shoals, lagoons and
barriers. (Sinclair, 1992).
Green, grey, red shales were deposited in marsh environment which
occured at the end of the deposition of the regressive 'A' Marker member.
The top of
the ‘A’ Marker member is sharp and is overlied by coarsening-upward sandstones
cycles Sandstones are silty and composed of very fine grains, cemented by
silica. Sandstones are generally bioturbated. At the base of this sandstones occur intercalations of grey shales, which are
also bioturbated. Upward sandstone change gradually into more clean without
silt, cross-beded, fine to medium grained
also bioturbated and cemented by silica. This cycle represents
transition from offshore to shorface environment. (Fig. 1). Clean sandstones represent
lower to middle shorface. Upward changing to coarser grain is a record of coastline
progradation.
This
coarsening-upward, shoaling sandstones are repetitive and was interpreted as
record of episodic progradation or shifting of point sources and delta lobes connected with avulsion.
The Avalon
Formation is ended by angular mid-Aptian unconformity. This unconformity is
observed on the seismic sections and is reflected as truncation of reflectors (Fig. 2)(Sinclair, 1992).
 |
Fig. 2. Seismic Section. The southern part of the Jeanne d'Arc Basin (after: Sinclair, 1992).
|
Above the
unconformity lie rocks, which are assigned to the Ben Navis Formation. The
Ben Navis Formation is mainly represented by sandstones. This unit is divided
into the Gambo member constituting the bottom of the formation and
fining-upward sandstone sequence.
 |
| Fig. 3. Carbonaceous conglomarates assigned to the Gambo member overlie truncated section of sandstones of the Avalon formation. (from Sinclair, 1992). |
The main
factor which controlled sedimentation of the Avalon Formation was tectonism.
The Avalon sequence was deposited when thermal subsidence took place, which
occured after Late Cimmerian rifting. Barremian to Aptian time was characterized
by huge detritus input from the south. The progradation of coastline was into
the north. During this time, the Avalon Uplift was rejunvenated and it was the
main alimentation area. The 'A' Marker member has similar thickness, the same litology and the paleoenvironmental
facies, and it was interpretated as lack of active faulting during mid-Baremian
deposition. Late Baremian-early Aptian deposites, which lie above the 'A' Marker member has different thickness. It was caused by subsidence increasing to the
north and progressing uplift and erosion of the Avalon Uplift. During deposition
of the Avalon Formation only the Murre fault was active. It was reflected by
higher thickness off the Mure Fault (Fig.4).
 |
| Fig. 4. Izopach map of sediments of the Avalon Formation without the Egret Member (from Sinclair, 1992). |
In
mid-Aptian conditions, which controlled deposition in the Jeanne d’Arc Basin changed and was
created unconformity, which defines the top of the Whiterose/Avalon sequence. Such
conditions lasted till late Albian. This period is characterized by active
faulting. North-east and south-east faults was active and distinctive changes
in deposits thickness occur across this zone. All sediments of the Ben Navis
and the Nautilus Formation deposited under active-faulting conditions and it
has an refelection on difference of thickness. The sedimentary package and underlying
basmenent was fractured in mid-Aptian.
To sum up, main factors which controlled depostion of the Avalon Formation was subsidence and sediment input.
Wish you a Happy New Year, by the way. :)
Cheers,
~ Weronika
Sinclair K., 1992. Tectonism: The dominant factor in mid-Creataceous deposition in the Jeanne d'Arc Basin, Grand Banks, Marine and Petroleum Geology, Vol. 10, No. 6, 530-549.
