Sedimentological conclusion of the GdA*

*mainly based on the GdA Fm. of the Lauzanier area - see map

The siliciclastic sediments of the Grès d'Annot Formation have predominantly been interpreted as the deposits of sediment gravity flows (eg. Fauret-Muret, 1955; Kuenen et al., 1957; Bouma and Coleman, 1985; Ghibaudo; 1985). Three facies associations are applied for separating the turbidites in the succession (Apps et al., 1987): GdAI (Picture 6.2.1), thinly bedded sandstones with siltstones and shales; GdAII (Picture 6.2.2), sequences of medium bedded (30cm-2m), rarely sub-amalgamated, sandstones separated by thin siltstones and shales, occasionally with conglomerates; GdAIII (Picture 6.2.3), comprises from few meters up to 100m thick sandstone beds which are sub-amalgamated to amalgamated, often separated by conglomerates and rarely by heterolithics (Apps et al., 1987).

GdAI-interpretation:

If applying the Lowe's (1982) summary figure (Figure 6.2.1.1a), which display sediment gravity flow type indicators, the deposited GdAI facies association must be classified as derived from low density turbidity currents. This conclusion is provided due to the sedimentary textures and structures of the cycles observed in the Grès d'Annot outcrops, like: Bouma base (ta) absence; if sandstone is present at base, grain sizes below medium grained sandstone are dominating; the cycles are well graded, whereby plane bedding and lamination are common, the lack of amalgamation and very well preserved, and mostly undisturbed, sequences of the Bouma divisions which are even emphasising the tet divisions. Further application of the Siemers- and Tillman's (1981) idealised sequences of sedimentary textures and structures (Figure 6.1.1c), which also includes base structures, implies that the gravity flow type of the GdAI facies association also must be concluded as a turbidity current. Based on the same features as mentioned (details, chapter 6.2.1 and figure 6.2.1.1a: application of Lowe's (1982) summary upon the GdAI facies association) and the additional sediment structures at base such as flute marks and tool marks. The upper contacts of the sandstone beds, marked with a sharp grain size break with either graded siltstones or graded shales and the preserved ripple current forms are either indicating a bypass of finer sediments or subsequent erosion.

The appearance of small-scale slumps (Picture 6.2.1.1b) and slide marks, mostly occurring in the fine grained sandstones of all facies associations, and the presence of micro-faults, which show close spacing, suggest that they were formed syn-sedimental. Most likely, these features formed, because the surface of the basin was irregular and initiated slight sedment failure down slope. However, no certain evidence was found regarding the down slope dipping direction, because the direction of the slumps and the slide marks are internally divagating strongly. The micro faulting is clearly visible in the well-defined laminae, particularly in the sandstones but also in finer grained beds.

Flute casts occur in swarms at the underside of beds, preferably at the uncommon ta-, and at the common tb division. Sometimes, flutes are associated with grooves, whereas the grooves rather tend to appear in the GdAII- and GdAIII facies association. The flute marks range from 5 - 30cm and are sub-conical structures with a rounded or bulbous up-current nose, the other end flaring out and merging with the bedding plane. They become their forms because current scour is burying holes in the unconsolidated surface which then are filled with coarser grained sediment.

The groove casts are raised, rectilinear, rounded to sharp-crested features (Picture 6.2.1.1c). Like the flutes, they seldom appear single but in sets. The observed grooves were mostly parallel, sometimes as two sets intersecting at an acute angle on the same surface. Normally, their relief is only a few millimetres, occasionally some centimetres. Mostly, they do not display their beginning or end. If they do, the beginning is abrupt and perpendicular to the surface, while the end is merging very smoothly with the surface. Their origin is the result of tools swept away by current, which engrave the surface of a relatively firm mud bottom (Pettijohn, Potter and Siever, 1987). Since both flutes and grooves are the product of current they are excellent paleo-current direction indicators.

Another current produced sedimentary structure is the rare featured parting lineation. However, it was only found at unconsolidated rock surfaces, except at one occasion, in the field. Therefore, this sediment structure could not serve as a paleo-current direction indicator. The reason for this is, they are dominantly present on fine-grained sandstone bed surfaces, which are not accessible in outcrops. They display sands that separate along bedding planes forming very regular flags, which are related to grain alignment (Bates & Jackson, 1984).

The ripple current marks are sited regularly throughout the Grès d'Annot Formation in the tc division (Picture 6.2.1.1d) . They form when sand is transported up the stoss side and avalanches down the lee side, causing the ripple to migrate downstream without any change in the level of sedimentation. However when sand is added to the system, the rate of accumulation of the lee side increases and each ripple climbs the backslope of the one in front of it to form climbing ripples. These are not as frequent as the simple marks, but are still often preserved. The sited low angles of the climbing ripples and the fact that the normal ripples are dominant, indicates a high flow regime. Since the sediment is swept to the lee side, not letting the ripples climb and mostly they were hindered. According to Pettijohn, Potter and Siever (1987), there might be a link between deformed ripples and convolute bedding, when the latter is perhaps an extreme end-product of ripple deformation.

Convoluted lamination is a common sediment structure in the upper divisions of the Bouma sequence of GdAI- and GdAII facies association, though preferably in the silty dominant td division. However, these folds or convolutions affect the laminations within a bed but not the bed itself (Pettijohn, Potter and Siever, 1987). Numerous authors suggest this appearance to have formed because of vertical transfer of material, which indicates some kind of internal readjustment of sediment in a quick or near-quick condition (Potter and Pettijohn, 1977).

Since the GdAI facies association is showing constant lateral continuity of units as well as of individual beds, it is only disrupted by one single truncated and channelised exception. It surely was not deposited on channel levees or between major distributary channels or canyons. Definitely, this part of the period during the Grès d'Annot deposition, reflects the lowest rates of sediment influx into the basin. This fact is confirmed by Mutti and Nordmark et al. (1987), which describe that the finer grained sediments are deposited much slower than the coarser grained sediments. Still the siliciclastic supply was relatively high, imaginable as a result of the low amount of shales and few detritus of the partly eroded underlying calcareous formations which were dissolved and flushed away. Additionally, the average sediment fill of the basin was very rapid (Chapter 4.2. and Figure 4.2b).

GdAII-interpretation:

If applying the Lowe's (1982) summary figure (Figure 6.2.1.1a), which display sediment gravity flow type indicators, the deposited GdAII facies association (Picture 6.2.2) must be classified as derived from rather moderate-, accompanied with high, density sandy turbidity currents. This interpretation is based on coarser grain size than in the low density turbidity currents, the lack of clay matrix, the dominance of massive sandstone, the common presence of normal- and occasional inverse grading and the undulated bedding that probably is caused by the dish structures. Further application of the Siemers- and Tillman's (1981) idealised sequences of sedimentary textures and structures (Figure 6.1.1c), which also includes base structures, show that the gravity flow type of the GdAII facies association must be concluded as a mixture between turbidity current and fluidised flow. Based on the same features as mentioned (details, chapter 6.2.2 and figure 6.2.1.1a: application of Lowe's (1982) summary upon the GdAII facies association) and the additional sediment structures observed at base such as, groove casts, load- and flute marks, flame structures and tool marks.

Cross bedding is a rare feature in the GdAII facies association. If they are present in the Lauzanier area, they are mostly found in the tb division and in upper parts of the ta division, as single set 10 - 25cm set height. Here they are however not found in the lower super-cycle of the succession. In the upper super-cycle, they are dipping with a maximum of 25°, normally comprised by medium grained sandstone. At Peira Cava, 20cm set height cross bedding is preserved beneath a sediment drape of shale and thin sandstones. The sandstones forming the cross bedding are well sorted and medium grained, dipping eastwards with a maximum of 30° and are asymptotic towards the base of their beginning. Former authors are deviating strongly regarding the interpretation of the cross bedding appearance in the Grès d'Annot Formation. Bouma and Coleman (1985) interpret this bedding as lateral accretion surfaces forming point bars within small-scale channels. Contrary, Allen (1970) assigns forsets of mega-ripples, preserving the morphology of the bedform displaying both the stoss and lee surfaces. According to Lowe (1982) and Apps (1987) both theories are disproved. Allen's theory is deniable because the foresets mega-ripples features storm waves or tidal current bedforms, which are not present. Bouma- and Coleman's (1985) hypothesis is dismissed since two foresets mega-ripples are present in the Peira Cava area. Lowe (1982) and Apps (1987) implies, therefore, that the gravity flows were responsible for all the cross bedding observed, supported through divergent paleo-flows which suggests opposite secondary currents to develop while the primary turbidity current encountered the slope. These entire hypothesis are either supported- or disproved of more recent research, such as Ravenne (1987), Sinclair (1993 and 1994), Midtun (pers. comm.).

According to Stanley (1975) and Nagahama et al. (1975) dish structures are most common in the sandstones of turbidites (Picture 6.2.2.1a). They also propose that the escaping water from rapidly deposited sands must be the origin of their existence (Tucker, 1985). Hereby, either the moderate- or high density sandy current are accompanied by finer grained sediments which are not separated before deposition. Under ideal circumstances, like deposition of a water rich fluidised flow, the captured water is forced to percolate upwards, driven by gravity of the density differences between sediment and water, in coherence with the overburden. During this syn-sedimentary phase, the finer grained sediments are able to be displaced and transported by initial laterally percolating water and assemble relatively impermeable laminations in water drainage pathways. This fine-grained lamination breaches when the escaping water has a sufficient strong vertical component to percolate upwards, enabling the dish structures to form their characteristic shape (Picture 6.2.2.1b).

Load casts are frequently observed sediment structure at base of beds in all the studied areas. They are bulbous, mammillary or papilliform downward protrusions filled with sand, produced by convective-like pattern of motion, which result in a vertical transfer of material. Such material is initiated by an unstable density stratification, such as occurs where a bed of sand is deposited on a less dense, water-saturated finer silt or clay (Pettijohn, Potter and Siever, 1987). The load casts may also become sack-like, then they are called pouches, if these become detached and sink downward, they are called load balls. All these structures are found in the area of Peira Cava.

The presence of sub-amalgamated beds become valid if the sandstones of the GdAII facies association are thicker than approximately 1.5m, which is often seen in the field (Appendix 2: Log Correlation). Thus the thicker sandstone beds do not comprise a constant individual upward fining but several upward fining beds, obvious through the abrupt grain size breaks at varying intervals. Therefore, this characteristic is called sub-amalgamation and not amalgamation. The sub-amalgamation took place because a sediment gravity flow eroded the upper divisions of the Bouma sequences of an already deposited and unconsolidated gravity flow. In most cases, resultants of only the ta and tb remained deposited, providing thicker sandstone beds to generate, which therefore are multi-cycle depositions.

Near the base of an individual cycle, or widely spread in a multi-cycle, there is a frequent presence of rip up clasts in all the studied areas but especially in the area of Lac du Lauzanier. Their length varies from millimetre - centimetre scale, whereas the relief is up to 50% less than the length but mostly smaller. They are rectangular, sometimes rounded at the ends. Generally they comprise heterolithics, occasionally a content of fine-grained sand and calcareous rip up clasts are found. These rip up clasts indicate erosion, whereas the lack of their roundness implies a rather short transportation distance and therefore is interpreted as derived from a local origin. Their existing is explained through an encountering gravity flow, which erodes its substrate that must have been relatively well consolidated. These ripped off clasts, predominantly from thin-bedded GdAI facies association, are then transported shortly by the gravity current and deposited as one of the first fragments of the current. Sometimes these rip up clasts are observed as being layered higher as in other cycles, which in this case must reflect a relatively stronger current as the lower layered rip up clasts. Another explanation for the existing rip up clasts may be the upward escaping water which percolate the layers of the siltstones and shales (details, chapter 6.2.1.1.), these may generate a disruption of these layers, causing their rip off without being influenced by an eroding gravity flow.

Locally, in upper parts of a cycle, concentrations of plant debris are very high. According to Apps (1987) Individual fragments may be 10s of centimetre long, with well preserved biological structures. They are have a bright surface sheen and are very brittle and fragile. Their excellent preservation indicates that the plant remains were transported and deposited rapidly, without being repeatedly reworked in well-oxygenated waters. Further the organic matter implies that the Grès d'Annot Formation were neither affected by deformation nor igneous activity or mineralisation, since the maximum temperatures almost certainly reflect the burial history of the formation (Elliott, 1985).

GdAIII-interpretation:

Former authors have described the GdAIII facies association, which is emphasising conglomerate beds and composite beds of sandstones exceeding thicknesses of 50m, with very diverse interpretations. Mostly the authors concentrate their work on the southern region, whereas the northern region is studied more recently. Some of these authors are: Stanley (1982); Cremer (1983); Jean (1985); Elliott (1985); Ravenne (1985 and 1987); Apps (1987); Sinclair (1993 and 1994); Midtun (not finished PhD thesis). Stanley (1982) interpreted examples at Contes and Peira Cava as welded slumps and turbidites. He implies that turbidity currents generated from submarine slides which evolved first into slumps and then into turbidity currents. Cremer (1983) interpreted them as deposits of high-density turbidity currents, using as a basis the model of Lowe (1982) (Figure 6.2.1.1a). The other authors predominantly approve of the interpretation of high density turbidity currents, except a hypothesis of Sinclair, who is implying a much shallower facies of a delta-near deposition.

If applying the Lowe's (1982) summary figure (Figure 6.2.1.1a), which displays sediment gravity flow type indicators, the deposited GdAIII facies associations (Picture 6.2.3a) must be classified as derived from high density currents. The sandstones scoured tops and imbricated clasts near the bases indicate that the base of the flow was turbulent. The conglomerates imply probably deposition from poorly disorganised bed load high-density turbidity currents. Further application of the Siemers- and Tillman's (1981) idealised sequences of sedimentary textures and structures (Figure 6.1.1c), which also includes base structures, show that the gravity flow type of the GdAIII facies association must be concluded as being deposited by fluidised flow and even by grain flow, considering the present conglomerates. Predominantly, this is based on the emphasised scours at base of beds, which suggest traction current to erode substrate. Additionally the base is normally poor graded, if at all. Relatively frequent inverse grading is present, compared to the other facies associations. The grains are often imbricated in massive sandstone beds. As in the GdAII facies association a repeating of sediment features are observed like: dish structures at higher parts, load casts of the underside of beds, flame structures and convolute bedding adjacent bed top.

The scours become more frequently in the upper super-cycle (Picture 6.2.3.1a), where the presence of the sand-rich GdAIII facies association is dominant. They are never seen in the GdAI facies association but appear rather sporadically in the GdAII facies association. At some levels in the upper part of the Grès d'Annot succession in the area of Lac du Lauzanier, they are represented as numerous representatives on an individual bed, mostly sandstone, surface (Picture 6.2.3.1b). Each representative on an individual bed surface is strictly aligned, without exceptions. Sometimes, they do not seem to occur in such numerous examples in other levels (Picture 6.2.3.1a).
In both cases, their shape is rather uniform. Formed as small-scale channel-like features. On the bed surfaces in the field, the outset is abrupt, mostly very steep, and sometimes even perpendicular. Their trough are observed as of minor extension but may exceed 5m. Though, the trough are believed to extend much more, since their appearance in outcrops does not show their whole structure. Especially their abrupt outset, which rather seems to be an erosional- than a traction caused outset. At the end, they merge smoothly with the bed surface. In sections, the relief of small scale channel-like through may reach up to 4m. Their fill is characterised by imbricated fragments, such as pebbles and more rare rip up clasts. Further, compared to the neighbouring rock, they distinguish through their well sorting and different grain sizes with low content of pebbles. Obviously, these small-scale channel-like features must be interpreted as traction process resultants of turbulent currents (Picture 6.2.3.1b).
This interpretation may be extended further, and explain the up to 38m of composite sandstone units. As mentioned, they are common in the upper super-cycle in the area of Lac du Lauzanier. Their existence and extension observed in outcrops implicates that high density turbidity currents must have had significant turbulence forces to erode small scale channel-like features over substantial extensions (Picture 6.2.3.1b). If such strong currents are encountering the bottom surface of the basin at regular intervals, heterolithics may certainly not settle long enough to deposit and consolidate, or if they may deposit at all. Even if the currents are not strong enough to generate small-scale channel-like features, they certainly still are capable of eroding and hindering the layering of finer grained sediments. Therefore, composite sandstone units record periods of deposition with either: close interval encountering high density turbidity currents, which prevent heterolithics to settle, or strong enough turbulence to erode finer grained sediments, or possibly a mixture of both influences. These currents must also be interpreted as the origin for sub-amalgamation and amalgamation. In cases were heterolithics are observed between the GdAIII facies association, they were able to deposit and consolidate over a sufficient period of time, avoiding terminable encountering currents. Thus, these appearing heterolithics, neighbouring composite units, often seem to be discontinuous or occurring with variable thicknesses in the outcrops (Appendix 2: Log Correlation). This implies that the turbulent currents are locally eroding the heterolithics. Nevertheless, considering the overall appearance of the Grès d'Annot composite units, individual pulse of a current eroding finer grained sediments must be interpreted as, not local, but extensive.

When interpreting the conglomerate beds, it is important to keep their significant contrasts of characteristics in mind. These beds may comprise pebbles up to 8cm, which are pebble supported and occupy approximately 50% of the total rock content. Gradually, their fining increase to the transition zone between conglomerate and pebble-rich sandstone. Within this range, many types of conglomerates are represented in the Lauzanier outcrops. The finer conglomerate beds are well sorted, often pebble alignment occurs and the base is predominantly scoured. Obviously characteristics are similar to the sandstones in the GdAIII facies association and therefore also must be interpreted as deposits of traction currents, derived from high-density turbidity currents (Figure 6.1.1a, figure 6.1.1c and figure 6.2.1.1a). The coarse conglomerate beds, however, show poorly- to moderately sorting and normally absence of parallel- or cross bedding and diverting erosive or non-erosive base. Thus, no evidences provide distinct interpretation of gravity flow type advancing the deposition of coarse conglomerate beds in the Lac du Lauzanier succession. Since the diverse characteristics are present, two flow regimes must have controlled the sediment distribution. Definitely high-density currents contributed to the partly erosive bases and light grading. Concerning the relatively coarse conglomerate beds, emphasising non-erosive bases and more poorly sorted conglomerates and total absence of sediment structures, implicate a more cohesive type of gravity flow. If the characteristics of southern regions sited conglomerate beds in outcrops are included, the hypothesis of the cohesive gravity flow existence is further supported. Here, a high percentage of conglomerates show very poor sorting with rare examples of boulders (Picture 6.2.3d) and no grading or sediment structures. These deposit features certainly demonstrate that they derived from a strong cohesive gravity flow. Authors imply, based on the respective characteristics, that these flows were debris flows (Cremer et al., 1983). At some sites, the break of the conglomerate bed top is abrupt and overlain by much finer grained sediments without significant proportions of even small pebbles (Appendix 2: Log Correlation, Log1, Log 2 and Log 5). Another example even emphasises a thin conglomerate bed overlain by thin interbedding of heterolithics (Appendix 2: Log Correlation, Log 4). These features may suggest either an erosion of substrata or a bypass of sediments. Nevertheless, since no erosional structures were observed a sediment bypass indicates to be the nearest explanation.

Especially in the upper super-cycle, near its base, metre scale slides seldom appear between composite sandstone units of the GdAIII facies association (Picture 6.2.3b). These bodies are determined as GdAI facies association heterolithics, which have been displaced from their former depositional locality. Because of remnants significant size, the sliding apparently started when bodies already obtained a sufficient consolidation and could not to be dissolved or substantially damaged while moving. Slides observed in the field do, in fact, not show any damaging at all, implicating displacements of rather short distances. The timing of this strong sliding phase is certainly connected with the sudden change of facies association of the lower- and upper super-cycles. The base of the upper super-cycles is eroding its substratum heavily (Appendix 2: Log Correlation). During this most considerable erosion period of the Lauzanier succession, severe erosion may have caused local small-scale slope failures and initiated bodies in substratum to tear off and slide. It is certain that tectonic activity is not the cause of slope failure at any stage during the sliding phase, considering the upper super-cycle being devoid of synsedimentary structures. Obvious characteristics of such slide bodies show slide marks at their underside. The marks are parallel aligned striations which form while slides are descending (Picture 6.2.3.1c).

On rare occasions flame structures are observed at bases of sandstones. These sandstones are strictly associated with an underlying interbedding of a heterolithic bed. The flame structures are consisting of flame-shaped plumes of mud, whereas the mud plumes colour darker than surrounding sandstones and therefore are easily recognisable in the field. Their scale reaches amplitudes of approximately 10cm. Their existence originate from substrate mud, which is squeezed upward, mostly, into sandstone, beds. These post-depositional structures indicate differential settling and compaction (Bates & Jackson, 1983).

Another, sometimes, present post-depositional feature of the GdAIII facies association is sandstone dykes. Their formes are sharp edged, straight-walled and the latter is contorted, whereas their width reach few centimetres and their length is observed as exceeding 50cm. They generate when water-rich sediment is deposited and pressure may turn the sand quick. These sands are capable of injection into fissures and produce dykes.