Facies analysis and interpretation

In this section each of the major tectonic blocks will be discussed independently

from oldest to youngest before an attempt is made to relate and integrate their

depositional histories. Their relationship to the rest of the Southern Uplands will also

be briefly examined. The distribution of the facies is described in Chapter 2 and Table

2.1, the aim here is to provide a clear and succinct interpretation of the facies

associations and sequence trends, plus any other factors relevant to their depositon.

Portayew Block (extreme southern edge) - a progradational sequence exists at the base

of this block with the deposition of pelagic Moffat Shales in a basin plain environment

under anaerobic and then aerobic bottom conditions. These are overlain by a thin outer

fan sequence (Facies D3 and G) before the arrival of the suprafan as indicated by the

appearance of Cj turbidites in the succession.

Caimgarroch Block - metamorphism has masked much of the lithological variation in

this block with the result that the rather glassy, homfelsed turbidites have the uniform

middle fan lobes and the distal portions of middle fan channels. The absence of

identifiable vertical variation and true channelized deposits in the 650 m thick

succession is due to the homogenising effects of metamorphism and the deposits can

therefore only be loosely described as middle fan.

Money Head Block - the black pelagic and hemipelagic Moffat Shales were deposited in

a basin plain environment under anaerobic bottom conditions and were immediately

overlain with middle fan channel deposits, succeeded by inner fan channel deposits, in

what was a major and very rapid (catastrophic?) progradational event (see Fig 2.6).

There then followed a 600 m thick retrogradational megasequence (Ricci-Lucchi 1975)

in which massive, inner fan channel deposits (Facies A3) developed into bedded middle

fan channel deposits(Facies A4, B2, Ci and A3 (see Plates 2.2(A) and (B)) and thinned

upwards into interchannel depositis (Facies B2, E and Dj) (see Figs 2.6 and 3.19).

The latter occurred in five separate cycles, each progressively more distal than the last.

This retrogressive trend continued with the deposition of a very uniform 150 m thick

sequence of tractional B2 deposits characteristic of channel-mouth bar deposition at the

distal end of middle fan channels (Mutti and Ricci-Lucchi 1975). These deposits were

subsequently winnowed by probable contour currents (see Section3.4.1.4). There then

followed a major progradational event in the upper 150 m of the Money Head

Formation with facies associations, though disturbed by faulting, indicative of proximal

middle fan channel and channel levee deposition (Fig 2.6).

Float Bay Block - the basal 120 m of the Float Bay Formation was deposited at the

fringe of a middle fan lobe and outer fan followed by a rapid progradation into the

middle fan channel. The rest of the formation consists of a major retrogradational

megasequence from the middle fan channel into the outer fan and possibly basin plain.

Initially a thick sequence of Ci and B2 beds was deposited before developing gradually

into a series of thinning upward sequences as individual channels filled and were

Fig 3.19: One of a series of thinning- and fining-up sequences

characteristic of the Money Head Formation at Scarty Head (NX04494845). This sequence shows the infilling of a middle fan channel and switch to interchannel deposition.

characterised by regular interbedding of Ci turbidites and mudstones (Facies G) with a

low sand/shale ratio, becomes dominant. Within this 300 m sequence bands of Facies

G mustones and rare thin Di turbidites are indicative of abrupt switching of active

deposition between lobes and subsequent intermittent outer fan deposition. The spacing

between these bands decreases from 100 m at the base of the sequence to 12 m at the

top. Eventually middle fan deposition is replaced altogether by deposition of the 200 m

thick, G and D3 mudstones and siltstones of the Strandfoot Member in the outer fan

and/or basin plain.

Stinking Bight Block - the lower 60 m of these beds consist of a progradation from the

outer fan into a middle fan lobe followed by a return to the outer fan. The presence of a

4 m thick A4 greywacke within the lower outer fan sequence (typically Facies G and

D3) is anomalous, as is the rather frequent occurrence of Facies E turbidites within such

a sequence. This is not an association recognised by Mutti and Ricci-Lucchi and its

significance is discussed later. Above this there was a rapid, thickening upward

progradation into the middle fan channel, whose sandy Ci and A4 deposits have a high

sand/shale ratio and dominate the overlying 200 m succeession. After this the

depositional sequence is unclear due to tectonic disturbance, but consists exclusively

of

middle fan channel deposits, some of which are very proximal, and middle fan lobe

deposits, the former being the more dominant

Grennan Point Block - this block, consisting of the Moffat Shale Group overlain by the

Grennan Point Formation, contains a major progradational magasequence. At the base

the pelagic and hemipelagic shales of the Moffat Shale Group were deposited in a basin

plain environment in predominatly anaerobic bottom conditions, except during the

Ashgill when aerobic conditions prevailed. These are overlain by a 6 m sequence of

Di, D3 and G laminated siltstones and mudstones in an outer fan facies association that

progrades very rapidly into C2 and occasional Ci turbidites of the middle fan lobe (Fig

red and green mudstones was deposited indicating a brief return to an outer fan

environment during what was possibly a period of minor marine transgression (see

Section 3.4.1.1). The Moffat Shale Group and basal sequences of the Grennan Point

Formation including the red mudstones have been tectonically imbricated and are

exposed at three localities. Interestingly the red mudstones apparently occur at different

heights above the Moffat Shale Group at each of the localities, however this may be due

to tectonic faulting or folding and only the 80 m height is unequivocal. Thickening

upward sequences of between 5 and 25 m have been detected at each of these localities

which show no evidence of rapid lateral changes in the sedimentation pattern.

Throughout the rest of the Grennan Point Formation there is little change in the

character of deposition (Fig 2.9) apart from the gradual predominance of Ci turbidites

over C2 turbidites and an increase in the sand/shale ratio indicating progressive

movement into the middle fan channel.

Mull of Logan Block - the Cairnie Finnart Member is composed of middle fan lobe

deposits with occasional phases of outer fan deposition, however at Cairnie Finnart

itself there is a progradation into the middle fan channel with the deposition of a 70 m

sequence of Ci turbidites with a high sand/shale ratio (Fig 2.10). Mudstone interbeds

often contain red or purple layers indicating deposition of oxidised sediments. These

middle fan lobe deposits are overlain catastrophically though conformably by the

massive, disorganised pebbly sandstone deposits (Facies A3) of the Daw Point Member

(Fig 2.10) which are characteristic of deposition in the inner fan channel. The presence

of interbedded Facies E and G deposits towards the top of the succession are indicative

of channel levee deposition, while the rare occurrence of thin sequences of more

organised pebbly sandstones (Facies A4) suggest occasional downchannel retrograde

movement Overall though progradation continued into and throughout the Duniehinnie

Member with initial deposition of A2 organised conglomerates succeeded by Ai

(Fig 2.10). This facies sequence indicates progradation from an inner fan channel into

a slope channel. At the faulted boundary between the A2 and Ai conglomerates is an

anomalous 100 m thick progradational sequence from an outer fan association (Facies

Dj, D2 and G), through middle and inner fan associations (Facies Ci, A4 and A2) into

a slope channel association (Facies Ajand F) (Fig 3.20). The abrupt though

conformable transiton from the conglomeratic and slumped deposits at the top of the

Duniehinnie Member into the Ci turbidites at the base of the Chair Member marks a

retrogradational step into a proximal middle fan channel. There is no gradational

sequence development in this member, only rather rapid changes from middle fan

channel deposition (Facies A4, Ci and rare A2 and A3) to interchannel deposition

(Facies Dj, D3, G and E) (Fig 2.10).

The most striking lateral facies change is the complete absence of the Ai, A2 and

F deposits of the Duniehinnie Member 4.5 km to the NE along-strike, though this is

perhaps not surprising of deposition confined to a large, localised inner fan or slope

channel. The other major change is the northeastwards development of the Daw Point

Member from a disorganised, massive sand body (Facies A3) into a bedded succession

of organised pebbly sandstones (Facies A4), suggesting retrograde, downchannel

movement northeastwards.

Port Logan Block - the Port Logan Formation consists of a series of prograde and

retrograde megasequences that show no overall trend. Deposition took place mainly in

middle fan lobes characterised by Cj and C2 turbidites with a low sand/shale ratio (Fig

3.21 and Plate 2.4 (A)). Thickening upward sequences of up to 10 m are common and

represent progradations into middle fan channels dominated by Ci turbidites with a high

sand/shale ratio (Fig 3.21 and Plates 2.4 (B) and 3.8). The three 100 m thick,

hemipelagic members within this formation represent significant periods of deposition

in an outer fan or possibly basin plain environment. They are composed of Facies G,

Fig 3.20: Juxtaposition of outer fan and slope channel deposition in a

Fig 3.20: continued / 1 A

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the Port Logan Formation at Needles Fig 3.21: Composite sequencing

Saddle Member contains bedded cherts in addition (Fig 2.11). The transition between

middle fan lobe and outer fan deposition is sometimes gradational with thinning or

thickening-upward sequences of up to 20 m while in some places it is abrupt over 1 or

2 metres. A few much thinner (less than 10 m) sequences of outer fan deposits are

intercalated in the succession and in places contain oxidised red mudstones and dark,

carbonaceous, fissile shales indicating periods of anaerobic bottom conditions possibly

related to minor marine transgressions. There is no significant lateral facies variation

over the 3.5 km the beds are exposed along-strike. The marked uniformity and

cyclicity of deposition in the Port Logan Formation is characteristic of middle fan and

outer fan deposition in a 'classical' submarine fan of Mutti and Ricci-Lucchi (1972,

1975). Active deposition switches between different lobes in relation to the sudden

plugging of channels in the middle fan, with intervening deposition in an outer fan

environment.

Clanyard Bay Block - pelagic and hemipelagic Moffat Shales were deposited in a basin

plain environment with a progradational increase in clastic input during the Llandovery.

The dominant anaerobic bottom conditions were temporarily replaced in the Ashgill by

aerobic condtions caused by a major marine regression resulting from glacial advance

(Leggett 1978). During this period oxidised red mudstones were deposited and

spherulitic siderite nodules formed diagenetically causing reduction of the iron oxide

and a green colouration in the surrounding red mu stone (Plate 3.13). As before,

deposition of the Moffat Shale Group was followed by rapid progradation into the

middle fan with the lower 250 m of the Clanyard Bay Formation characterised by

interchanging outer fan, middle fan lobe and rare middle fan channel deposition, in an

overall progradational megasequence. Thin, pelagic, black shale units intercalated in

this succession are thought to relate to minor fluctuations in oxygen content related to

differences in current density and/or an increase in the density of carbonaceous material

of a major retrogradational megasequence, with initial deposition in a channel mouth bar

characterised by thick to massive B2 and Ci deposits with little shale (see Plate 2.7),

and subsequent deposition of Ci and C2 turbidites in a middle fan lobe with the shale

content increasing inversely with age. Towards the top of this megasequence

intercalated bands of interbedded shales (Facies G) and thin turbidites (Facies E) are

suggestive of channel levee or out of channel deposition in a proximal environment.

Their association with middle fan lobe deposits in anomalous.

Cardrain and Mull of Galloway Blocks - the Mull of Galloway Formation is, like all of

the Hawick Group, remarkably uniform lithologically. The vast bulk of the sediment,

consisting of Ci, C2 and E turbidites is characteristic of middle fan lobe deposition (Fig

3.22(A)), though a few thin sequences of Di, D3, G and rare D2 deposits are indicative

of an outer fan environment. The Leucarron Member contains in addition middle fan

channel sequences up to 12 m thick that may display composite cycling but overall thin

upwards (see Fig 3.22 (B)). These sequences have a B2, Ci, C2 and E association at

their base and a G, D and D3 association at their top indicating retrograde movement

into outer fan deposition as the channel fills, before a return to middle fan lobe

deposition. These channelised sequences are typically amalgamated, lenticular bedded

and contain prolapse structures in contrast to the laterally extensive regular bedding of

the middle fan lobe. Sand volcanoes are common throughout the formation indicating

rapid deposition, in addition the marked decrease in greywacke grain size and paucity of

tool formed sole markings, relative to the formations to the N, are noteworthy features.

The high detrital carbonate content of the formation suggests deposition above the

carbonate compensation depth which in turn implies a shallower depth of sedimentation

than in the formation to the N. There is no evidence of a major marine regression at this

time to account for this. The origin of the thin, oxidised red mudstone layers developed

Fig 3.22 (A): 5 m section from Carrickamickie Bay (NX12133151) deposition.

the Mull of Galloway Formation at showing characteristic middle fan

MS FS S M Medium sand pine sand Silt Mud 0-15 - 0-5mm 0-0dl5~ O'15mm 0 0059-006 25mm < 0-0059mm

Discussion - although the Mutti and Ricci-Lucchi (1972, 1975) and Mutti and Walker

(1973) facies analysis schemes have proved of great value in interpreting the sediments

and their distribution and associations in the Rhinns, it is clear that the depositional

environment of the Rhinns sediments is more complex than the simple submarine fan

systems on which the facies schemes are based. Three main points emerge from the

analysis above concerning the sedimentation history of the Rhinns:-

(1) Two fundamentally different environments of deposition are represented in the

sediments of the Rhinns. The older of these is dominated by pelagic, black shales

deposited in a basin plain environment. Deposition continued uninterrupted for up to

35 million years from Llandeilo to mid Llandovery without any substantial clastic input

Such extended remoteness from a clastic source is only possible in an oceanic basin.

Throughout the period of time represented by clastic deposition in the Rhinns, some 12­

14 million years from Ashgill to late Llandovery, a clastic apron, representing the

second major depositional event, prograded across the oceanic basin.

Attempts to laterally relate facies associations in different tectonic blocks of the

same age have proved too subjective and of little value apart from showing the

progradational diachronous onset of turbidite deposition above the Moffat Shale Group

and its related effects.

(2) The progradational transition from pelagic, oceanic basin deposition to proximal

clastic deposition is extremely rapid occurring in just a few metres, rather than a gradual

change with extensive deposition of basin plain and outer fan deposits as would be

expected. The catastrophic nature of the transition is best displayed in the Money Head

Block where pelagic, black shales interbed with Ci turbidites at the top of the Moffat

Shale Group (Fig 2.6). It is less pronounced in the Grennan Point Block (Fig 2.7) and

even less so in the Clanyard Bay Block where hemipelagic mudstones intercalate with

the pelagic, black shales for a few metres prior to the deposition of proximal Facies C

occur continually over such a long period of time special conditions needed to exist and

argues against deposition in a submarine fan system as described by Mutti and Ricci-

Lucchi (1972), This is further supported by the absence of extensive tracts of clastic

outer fan and basin plain deposits which normally characterise submarine fans. Instead

clastic deposition is dominated by proximal high density flows with ’distal style'

sedimentation generally confined to thin sediment starved sequences deposited in what

was essentially a proximal environment

(3) The distribution and vertical and horizontal sequencing of facies associations in the

clastic deposits is much more irregular and complex than those described by Mutti and

Ricci-Lucchi (1972, 1975) and Mutti and Walker (1973). Often seemingly incongrous

environments are juxtaposed together in the succession, eg. the outer fan and inner fan

channel in the 'Stinking Bight beds' and middle fan lobe and inner fan channel in the

Mull of Logan Formation. No clear sequence trends or patterns emerge to the

sedimentation beyond an initial progradational jump at the base of the blocks.

Transitions between different environments are typically abrupt and rapid rather than

gradational and cyclicity is poorly developed in all but a couple of the formations.

These features suggest clastic deposition took place in a tectonically unstable, confined,

high energy basin. In addition facies associations have been identified that are not

described by Mutti and Ricci-Lucchi. The most widespread of these is the association

of Facies E turbidites, that are lag deposits from large bypassing sediment gravity flows

and typically occur as overbank deposits in the inner fan, with Facies D and G deposits,

that only occur together in the outer fan or basin plain. Such an association has been

identified by Underwood and Bachman (1982) as common in subduction zone trenches

In document The geology of the lower paleozoic rocks in the southern Rhinns of Galloway, S W Scotland: studies in an imbricate thrust terrane (Page 173-192)