Figs 3, 4, 5 Global Reconstructions for the Devonian and Carboniferous by Christopher Scotese

Figs 3, 4, 5 Global Reconstructions for the Devonian and

Carboniferous by Christopher Scotese

Figure 3

Figure 4

Figure 5

Fig. 6. European Variscides Tectonic Zonation by Leveridge and Hartley. Black arrows are summary tectonic transport directions and are broadly symmetric between the north and south. This pattern is consistent with the NNW-SSE convergence and collision of Laurasia in the north with Gondwana in the south.

Figure 7. Tectonic evolution of the southern part of SW England between 410 and 360ma by Leveridge & Hartley 2006.

Figure 7 shows an instructive and sophisticated model by Leveridge and Hartley of the tectonic evolution of the southern part of SW England as the Rheic Ocean finally closed between 410ma (Early Devonian) and 360ma (Early Carboniferous). The continental crust to the south belongs to the large Gondwana-derived fragment of Armorica/Britanny. To the north there is the continental crust of Laurasia showing large steep normal faults that have extended that crust because this is a passive margin on the edge of an ocean. The Rheic Ocean with its oceanic crust lies in between.

By 395ma, compression associated with the convergence of the two plates initiates subduction of the oceanic crust beneath Gondwana and a large piece of oceanic crust is sheared off and becomes attached to the overlying Armorican fragment (395-390ma). This critical piece of evidence of ocean closure is preserved as the ophiolite of the Lizard Complex in southern Cornwall (see map, figure 1). During and following this, sediments shed off the advancing Gondwanian continental edge and starts to fill the subduction trench above the downgoing oceanic plate. Such “Trench Basins” contain impure and immature siliclastic rocks such as mica bearing and feldspathic sandstones as well as deep water mudstones and are otherwise known as Flysch Basins. As the trench is filled it becomes deformed by the overiding Gondwanan plate and large thrusts develop and propagate northwards through this until by 360ma (early Carboniferous) they overide the Laurasian margin. The trench sediments also become gradually overlain by Continental-derived sediments shed from the high ground of the Gondwana margin and the trench converts from a deep oceanic subduction Flysch basin into a shallower successor/foreland basin of non marine(terrestrial) sediments of continental type.

STRATIGRAPHY

As the Gondwanan/Armorican plate was thrust gradually northwards over the English Laurasian plate the weight of the southern plate pushed the northern plate down and the former small extensional basins there deepened, widened, subsided and received sediment much of which was derived from the erosion of the leading edge of the southern plate, but also included material from the northerly Laurasian plate. These basins therefore opened in sequence from the south to the north (Fig.7):- the Gramscatho flysch Basin (the oldest-Devonian), the Looe Basin (413-394ma Lr Devonian), the South Devon Basin (394-362ma, Mid. to Upper Devonian), the Tavy Basin (377- 362ma, Upper Devonian), the Culm Basin (362-265ma Tournasian-Namurian, the youngest of the basins), the North Devon Basin (Devonian). We will be spending most of our field time (stratigraphically-speaking) in the Culm Basin and specifically in the Upper Carboniferous Crackington Formation and the overlying Bude Formation. We will also spend time in one locality (Boscastle), in the older (Dinantian) Tintagel group rocks that are found on the immediate southern boundary of the Culm basin in our investigation of the nature of the important Rusey Fault.

Figure 8. The six main depositional basins of the Devonian and Carboniferous sediments in SW England. These developed with time from south to north as the

southern plate (Gondwana) overrode the southern edge of Laurasia.

The Culm Basin: The Crackington Formation and Bude Formation

Turbidites are clastic sediments (muds, impure silts and impure sands) that are carried by, and deposited from, turbidity currents. Such currents, being a mixture of water and sediment, have high densities compared to ordinary sea or fresh water. They can form dense sub-aqueous flows that move, often rapidly, down the continental slope and along the seabed, which they have the power to erode. When they reach the flat of the abyssal plain, they lose energy and deposit their material. Please look in the appendix for a more detailed account of turbidity currents and turbidite rocks.

The Crackington Formation, which is of Namurian age (see stratigraphy columns Figure 12, page 13) is generally accepted by workers to be a marine turdidite that formed much in the manner of the foregoing paragraph. On the basis of the predominance of mudstone and the thin-bedded nature of the sandstones in the formation, It is interpreted to be distal (distant from its source).

The Bude Formation (of Westphalian age) lies conformably above the Crackington Formation has, by contrast, proved much more controversial. The Geological Survey and many academics see it also as a plain and simple turbidite which, on the basis of the abundance of its thick-bedded sandstones (which are interpreted as turbidite flow channels), is a more proximal turbidite unit compared to the Crackington Formation. Dr. Higgs and others (Oxford) take issue however. They have shown firstly that on the tops of the beds of many Bude Formation sandstones there are symmetric wave ripples (see appendix 4 for descriptions of sedimentary structures) and also preserved crab trails. This implies a shallow water setting and the authors infer that these sediments belonged to a delta top environment with the deltas built by rivers flowing out from a terrestrial shore line in the north towards deeper water in the south. Additionally, sulphur and organic carbon analyses indicate the water that these rocks were deposited in was fresh/brackish and not marine and that this water was in a lake (which they called Lake Bude), and not a marine setting, although it lay very close to one (see page ). Turbidites do however have a place in their model. They believe that seismic shaking has affected the delta sediments (producing chaotic beds known as seismites) and from time to time the frontal part of the deltas collapsed and created subsea turbidite flows with fan deposits at deeper levels in the lake.

2.TECTONICS AND STRUCTURE

Basin Inversion

Figure 9

As we have seen the overriding of Gondwana on to the southern Laurasian margin, bent that margin downwards, created horizontal extension and initiated a series of extensional basins there which developed in sequence northwards. Other phrases for extensional basins are referred to, in the above diagram (Fig. 9), as “grabens” or “full grabens”, where there are two large, bounding faults inclined towards each other. These downthrow towards each other, and allow the central area to move down and accumulate younger sediment. A half graben, on the other hand, has only one steep normal fault and this often dips more gently at deeper levels creating a curved or “listric” shape. With full grabens the space in between is more or less symmetric, whereas in half grabens it is asymmetric and triangular in section.

The structures in Figure 9 are produced by horizontal extension of the crust/lithosphere. If one were now to apply horizontal compression to this diagram, new low angle thrust faults could, in theory, form and cut across the pre-existing faults. Alternatively, and as discovered about 20 years ago, the movement direction on the old normal faults could simply be reversed. The latter is called “inversion tectonics” and it is caused by ”fault reactivation”. Such processes are now understood to be a very

8 common feature of global tectonics. Turning back to Fig. 7 you will remember that as the Laurasian passive margin was overridden by Gondwana, that particular margin was bent down, extended and half grabens were formed and filled by sediment from the Gondwana plate and from within the Laurasian plate. In many other cases however the movement direction on the steeply inclined half graben normal faults were simply reversed and the faults became high angle reverse faults (steep thrusts with dips of about 600). Figure 10, which is a theoretical model of the reactivation process, shows the reversal of tectonics with the process of the swapping of the direction of movement of the faults moving progressively to the left.

Figure 10. Fault reactivation and ductile folding of sediment fill in basins.

What is also suggested in this diagram is that whereas the rather hard and strong basement rocks simply move by brittle faulting, the soft, weak and ductile basin fills are asymmetrically folded as they are firstly pushed up over the steep faults and then are pushed out over the long limbs of adjacent folded basin fills. Because the extreme left-hand fault dips in the opposite direction to all the others, this results in two large sediment folds facing towards each other. We shall return to this later in the story.

A model of the reactivation process for the Culm basin is shown in simplified cartoon format in Fig. 11. The half arrows on the faults show that the hanging walls are moving up relative to the footwalls (for terminology-see appendices) and are high angle reverse faults (steep thrusts). The full arrows show sediment supply.

Figure 11

Firstly we can see that the Crackington and Bude formations lie in a single symmetric full graben in which the bounding faults RF (Rusey Fault) and GF (Greencliff Fault) are normal/extensional faults that have been reactivated under horizontal tectonic compression orientated north-south. On the extreme north the BCBF (Bristol Channel Bray Fault) has been reactivated and its dip direction has made it an upthrust to the north. The wedge shaped sediment fill of the half graben here is still evident and its low ductility has allowed it to form a substantial north facing fold. The BF (Brushford fault) appears not to have been reactivated and the GF (Greencliff Fault) only slightly so. On the far south is a large north dipping fault where the half graben fill has thrust up strongly southwards and has generated a south facing asymmetric anticline. On the back of this the Rusey fault has similarly generated a smaller fill fold of the same symmetry. This fold has created a sea/basin floor “sill”, that in Higgs’s Bude Formation model separates Lake Bude to the north from a small marine flysch basin to the south. Thus at times of low water level, the lake and the basin are kept separate whereas at times of high water in the marine basin there is overspill and the marine waters contribute to the brackish conditions of Lake Bude.

The Rusey Fault

The current model for this structure is that it formed as the southern synsedimentary extensional/normal fault to the Culm Basin and this was then reactivated during the later Variscan northward thrusting episode. Part of the reactivation deformation is as a southward directed high angle thrust and the other part is as a southward verging (see appendices for structural terms) asymmetric fold in the basin fill rocks themselves. The fault is shown on most maps as a fault zone of variable thickness. It is extremely well exposed (in general terms) on the west coast between Boscastle and Tintagel. Within the zone and south of it the stratigraphy changes with the occurrence of Lower Carboniferous (Dinantian) units not commonly seen elsewhere in North Cornwall eg.Cherts, alkaline basalts, other volcanics. There are also several different fine-grained units (referred to as “slates”) that may possibly be the result of extreme deformation. Some of these rock units occur as tectonic slices bounded by low angle faults. Steep extensional faults are also ubiquitous and the coastal section is regarded as very complex. No modern structural work has been done here and so we will pay it a visit.

Figure 12. Showin Figure 12 The Rusey Fault on the southern margin of the Culm Basin

Figure 13 STRATIGRAPHIC COLUMNS

BUDE AND OVERALL CRACKINGTON Fm STRATIGRAPHY STRATIGRAPHY

Figure 14. Leveridge and Hartley’s (2006) N-S structural cross section across SW England.

The Leveridge& Hartley section is the most detailed and sopisticated section that has ever been drawn of the SW England Variscides. Bear in mind it is also a model (basically a big idea). Scientific models reflect a) the state of knowledge at that time and b) other prominent ideas (in this case popular structural ideas of the early 2000s about the mechanics and geometries of fold and thrust belts. Beginning on the left (north),there is a big upright synclinal fold with the fan-like arrangement of the axial planes of the secondary folds (such a structure used to be called a “synclinorium”). This fan is also shown by the arrangement of the symmetric basin extensional faults which have all been reactivated by N-S shortening as high angle reverse faults. In the thrust terminology of the times this would have been referred to as a “pop-up” structure and it would have been connected downwards into a flat lying/gently inclined basal thrust. The latter is not represented in any guise.

Moving now onto the extreme right (south) we have a set of about 8 major thrusts dipping at moderate angles to the south and carrying the Gondwana/Armorican plate over the Gramscatho Flysch basin (the original oceanic trench), the Lizard ophiolite and the edge of the passive margin of Laurasia with its Devonian-Carboniferous basins. This type of thrust arrangement is called an “Imbricate Stack”. The folds within it and above it verge and face north. Normally these imbricate thrusts should connect down into a flat-lying/gently inclined basal thrust. Again, no hint of this appears. There are some more steeply-dipping parts of these thrusts and, further north the thrusts have a fairly uniform high angle, nearly as steep as the reactivated basin margin faults on the extreme left of the section. This, it seems to me, reveals that the authors have made an implicit assumption that all or most of the shortening (in terms of faults in the section) is by reactivation of steeply inclined (around 600 dip) pre-existing extensional basin–forming faults.

In the middle part of the section between the imbricate stack and the Culm synclinorium the thrusts confront each other. Such a structure is known in the terminology of fold and thrust belts as a “Triangle Zone”. Again, the two closest of these faults with opposite dip directions should connect down into a basal thrust.

What, you may ask, is so important about basal thrusts and steeper faults connecting into them? The simple answer is that the the presence of a basal low angle thrust allows the tectonic force/stress to be distrbuted more evenly around the lithosphere. It provides physical connections (“connectivity”) between the different fault and fold elements and permits the lithosphere to deform homogenously- thus thickening the lithosphere and shortening it in response to the convergent movement of plates. So Leveridge and Hartley’s section is not a very realistic one. Basically there should be geological evidence in this entire area of large low angle thrusts

THE ITINERARY

SATURDAY 19TH MAY

1.HARTLAND QUAY. a) large upright folds in the main cliffs. Some are rounded folds most are chevron folds. Why the difference? Understanding fold terminology. b) a smaller antiform, synform fold pair behind the quay wall. c) understanding more fold terminology.

The Hartland Quay Upright Folds. Why does a big rounded

fold become a chevron fold?

2.BUDE FORESHORE. a) the “Whaleback” Fold.(an antiformal pericline). b) sedimentary structures c) the sedimentary environment of the Bude Formation.

3. SALTHOUSE-NORTH END OF WIDEMOUTH SAND. a) a medium scale, tight, south facing fold in the Bude Formation with an inverted limb. b) north dipping southerly directed thrusts and similarly orientated extension faults. Reactivation? c) sedimentary structures.

Published sections from Phillips Strand to Salthouse. South is to the right.

4. MILLOOK HAVEN. Classic chevron fold locality. a) fold axial planes are inclined northwards b) what is the fold vergence. c) what shape of very large fold have we crossed since the last locality? d) more on Chevron fold formation.

SPOT THE FAULTS!!

5.CRACKINGTON HAVEN. a) steeply north inclined extensional faults with wide deformation zone between the fault blocks. Downthrow to the north. b) a gently dipping extensional and possibly listric fault with a broad fault zone and top to the west movement.

6. BOSCASTLE. a) North side of the harbour. Spectacular cylindrical/non cylindrical folds in slatey rocks (see front cover). b) these deform an earlier set of structures associated with the slates. So D1 and D2? c) South side of the harbour and a walk along the cliffs to see unusual stratigraphy and low dip angle faults and ductile shear zones. Are there any mylonites? We might leave this walk until Sunday morning.

Survey map and section of the Boscastle area. Stratigraphic log on next page

Field guide map from GA guide (Dearman et al 1970) for the area west of Boscastle.

Kilve, Bristol Channel Basin

Sunday 20th May

The c.18km coastal section between Hinkley Point power station and Blue Anchor Bay contains exceptional exposures of a wide range of brittle tectonic structures. The area is a world class natural laboratory for studying the geometry and development of normal and compressionally-reactivated normal faults, and extensional relay ramps were first discovered here in the 1990’s.

The Bristol Channel Basin (mainly offshore) accumulated a thick succession (up to 8km thick) with Triassic evaporates of the Mercia Mudstone Fm overlain by Late Triassic Rhaetic mudstone and limestone up to the alternating grey shales and limestones of the Liassic Blue Lias Fm. (Lower Jurassic). Basin inversion occurred in the Tertiary.