The Avalon Formation

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)

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.

The Egret Member

In this note I want to focus on the Egret Member (Fig. 1). This unit belongs to the Rankin Formation and is interesting from economical point of view.

The Egret Member contains the Upper Jurassic (Kimmeridgian) rocks.

It is easily to detect this member both downholes and geochemical logs. It is because the Egret Member rocks content higher amount of organic carbon (TOC total organic carbon) in comparison  with another deposits. Average TOC content is up to 12 % (Fowler and McAlpine, 1993).

Fig. 1 Litostratigraphic column and geochemical log of Jeanne d'Arc Basin (from Fowler and McAlpine, 1993).

The thickness of the Egret Member is laterally changing and range from 55 m (Rankin M-36) to 226 m (Fortune G-57)(Fig. 2).

According to McAlpine (1990) who subdivided sedimentary fill in the Jeanne d’Arc Basin into six main depositional sequences, the Egret Member belongs to epeiric basin (in my last note it is described as the First Thermal Sag).  

Fig. 2 The Jeanne d'Arc Basin. Grey dots mark wells which reached or penetrated the Egret Member (from Huang et al., 1996).

To the southern boundary the Egret Member is predominantly represented by laminated brown marls intercalated with calcareous mudstones (Rankian-M36) and  to the northwest gradually become dominant slightly calcareous shale, siltstone and minor sandstone (Trave E-87).

Examinations based on cutting samples coming from Trave E-87 and Fortune G-57 let to determine 4 types of lithology, which represent all the Egret Member:

1. Dark brown laminated shale (the richest in organic matter)
2. Light brown to grey shale
3. Marlstone/limestone and claystone
4. Sandstone (subordinate)

Lithologies alternate with each other rhythmically. TOC also varies. Diagrams of TOC show organic rich layers alternated with organic poor layers. Lithology and its thickness is vertically changing. Examination of samples under the microscope showed that the most of organic matter is amorphous (Fig. 3). 

Fig. 3 Percentenge of litologies from cutting samples examnations. A- carbonate, B- shale/claystone, C- siltstone, D- sandstone. TOC diagram. Measurements of Archer K-19 well. The broken lines indicate boundaries od the Egret Member (after Huang et al., 1996).

Depositional Environment of  the Egret Member

The Egret Member rock were deposited in low energy marine environment. It is suggested by fine-grained and laminated deposits and high organic content.

According to isopach maps Egret Member deposits are the thickest in the central part and on the Outer Ridge (eastern part of the basin)(Fig. 4). Between this two zones occurs zone with thin deposits. This suggests that it was sill there. The sill probably acted like a barrier and circulation was curbed. The highest value of isoliths of carbonates occurs in the southern part which suggests a closeness of carbonate shelf or bank. Occurrence on the south of oolitic and skeletal packstones below and above member also suggest carbonate shelf. The Rankian Formation, excluding the Egret Member was deposited in normal marine conditions (based on examination of microfossils). The Egret Member was formed in shallow waters with depth about 25-50 m. It was probably anoxic basin, which is suggested by occurrence of ostracods, which can live in extreme environment, and lack of foraminifera.

The content of terrestrial organic matter is low and it indicates that delivery from continent was minor. Both factor: restricted circulation and high plankctonic productivity (especially by dinoflagellates) led to creating suboxic and anoxic conditions in the bottom waters. High amounts of organic matter were accumulated. As I mentioned before the amorphous organic matter is dominant it is because after accumulation during early diagenesis was reworked by anaerobic bacteria. 

Fig. 4 a) isopach map of the Egret Member B) isolith map of carbonate of the Egret Member (after Fowler and McAlpine, 1993).

Sedimentary cycles in the Egret Member

The Egret Member has a cyclic nature. Examination on this aspect was carried out by Huang et al. (1996).  They use variations of TOC and permeability which was calculated from well logs. Estimated permeabilities  obtained from different vertical distances was put on variograms. Results confirmed cyclicity of sedimentation (Fig. 5). 

Fig. 5 Semi-variogram of permeability.  Hibernia K-18 (from Huang et al., 1996).

With the use a mirror display of the gamma ray Huang et el. (1996) visually identified cycles. Minima of gamma ray defined cycle boundaries. It show the distinctive cyclic variations in the Egret Member which are probably related to alternating layers of different lithologies such as clay, silt and TOC content.

In thick source rock intervals (Archer K-19) can be determine three different distinctive cycles: large, medium and small scale. In another, smaller one (Rankin M-36) can be recognize two cycles: large and medium scale.

Description of cycles:

The large-scale cycles thickness varies from 16 to 60 m. Boundaries are determined by layers of low gamma ray. On the southern boundary of the basin low gamma layers are represented by marl/limestone, on the northern-east by siltstone and sandstone. The large-scale system are composed of medium-scale cycles with a thickness varies from 4 to 15 m and whose barriers are also determine low gamma ray layers. The medium-scale cycles can be divided into small-scale cycles. 

Examples of occurrence  of cycles in particular wells:

Rankian M-36 (Fig. 6)

At this well the Egret Member possess four large-scale cycles. Each of them consist of four medium-scale cycles.

Archer K-19(Fig. 6)

At Archer K-19 the Egret Member contain four large-scale cycles which in further division consist four medium-scale cycles. Each medium-scale cycle possesses four to six  small-scale cycles.

Fig. 6  Sedimentary cycles obtained by mirror dispal of the gamma ray log. Rankian M-36, Archer K-19. A, B, C, D define arge-scale cycles. A1, A2, A3, A4 define medium-scale cycles (from Huang et al., 1996).

The relationship among the large, medium and small-scale cycles observed in others wells is similar.

Generally the ratio of:

-  large-scale / medium-scale cycles is 1:4

-  medium- scale / small-scale cycles is 1:5

Correlation of cycles

Four large-scale cycles can be single out (A, B, C, D). Every large-scale cycle contain medium-scale cycle (1,2,3,4).

The large-scale cycles are laterally continuous and can be correlated between wells. However thickness of this type of cycles varies laterally. The changing thickness is related to different depositional environments of the Egret Member, compaction and probably local erosion.

The medium-scale cycles are different in thickness laterally and vertically, but can be also correlated with others wells.

The small-scale cycles cannot be correlated between particular wells.

Example of correlation of the Egret Member are shown in Figure 7. 

Fig. 7 Correlation of sedimentary cycles in the Egret Member in Egret K-36, Rankin M-36 and Hibernia K-18. (from Huang et al., 1996).

Estimation of the duration of cyclicity

Although the laterally and vertically thickness variability of cycles in wells, the structure of the cycles shows that they represent record of some periodic geological process.

The relationship between three orders of cyclicity and their time ranges suggest that the depositional cycles are related with climatic and oceanic changes caused by orbital forcing (the Milankowitch cycles).

The cyclicity of large-scale cycles was calculated to be about 413 ka and suggest eccentricity cycle. The medium-scale cycles is about 100 ka and suggest also eccentricity. The small-scale  is about 20 ka and can be interpreted to have precession cyclicity (Fig. 8). 

Fig. 8 Range of estimated cyclicity of the three orders in the Egret member. E 1, E2, E3 - eccentrity cycles. P 1, P 2 - precession cycles (after Huang et al., 1996).

To sum up the Egret Member rock can be explained by orders of sea-level and climate fluctuations which were orbitally forced.

Sedimentation rates in the Egret Member

 

Fig. 9 Sedimentaion rate in (cm/ka)  for the Egret Member in the Jeanne d'Arc Basin (from Huang et al., 1996).

The estimated sedimentation rate of the Early Kimeridgian time in the Jeanne d’Arc Basin varies from 3,8 cm/ka (Ratkian M-36) to 14,7 cm/ka (Fortune G-57). The deposite of the Egret Member in three wells have higher sedimentation rate- 10 cm/ka (Archer K-19, Fortune G-57, Terra Nova K-18) and it is probably evidence of deltaic condition in this zone (Fig. 9).

The sedimentation rate was changing during the time which shows figure 10. This variation can by probably caused by rapid transgressions followed by gradual regressions. It can be worth to observe C3 and C4 fractional thickness. Decreasing of sedimentation rate in this cycles suggest increasing sea level. It also indicates that subsidence rate was constant. The smallest fractional thickness occurs in C3 may represent condensed section which popular occurs in rapid transgressions. Higher sedimentation rates are connected with lower sea periods (decreased accommodation).

Fig. 10 Hibernia K-18 A vertical variations in sedimentation rate B vertical variations in medium-scale cycle's thickness (from Huang et al., 1996).

Relationship between sedimentation rate variation and TOC content is worthy of note and it is the last aspect which I want to bring up in this note.

Figure 11 shows interrelationship between sedimentation rate and amount of TOC.  Huang et al. (1994) interpreted this as amount of  TOC accumulation is connected with sea-level and climatic change which has a reflection in sedimentation rate.  High amount of organic matter is related with small sedimentation rate. Factors which caused enlargement of accumulation of organic matter during Kimmeridgian are: transgression, warm climate, inhibited oceanic convection and anoxic near the sea floor. The Egret Member rocks are developed as laminated, brown shales, which represent anoxic conditions. A rising of sea level and warm climate created favorable conditions to deposition and preservation of organic matter. Euhedral pyrite was detected in samples and it is next evidence of existence of anoxic water. Marlstone/limestone and unlaminated claystone which also belong to the Egret Member, represent more oxygenated water (connected with lower sea-level) and decreased organic matter accumulation. During a period of low sea level organic matter was scattered in thick sediments coming from lands. Periods of higher sedimentation rates caused quick burial of organic-rich beds. 

Fig. 11 Egret N-46 (see text for explanation)(from Huang et al., 1996).

Described factors are very important for understanding why the Egret Member is so interesting not only from economical point of view.

Bibliography

Fowler M. G. & McAlpine K. D., 1994. The Egret Member, a prolific Kimmeridgian source rock from offshore Eastern Canada. In: Petroleum Source Rock Case Studies (Ed. B. J.Katz), Springer, Berlin, 111-130.

Huang Z. et al., 1996. Cyclicity in the Egret Member (Kimmeridgian) oil source rock, Jeanne d'Arc Basin, offshore eastern Canada, Marine and Petroleum Geology, Vol. 13, No.1, 91-105.