Conglomerate Test Research Articles

Paleomagnetism of devonian units from Miguasha, Gaspe, P.Q., Canada.

The area investigated is located at longitude 66°30′W and latitude 48°N, on the northern shore of Chaleur Bay. Some 124 specimens (46 samples) were collected from 9 sites in the following geological formations: La Garde (10 samples, late Early to early Middle Devonian), Pirate Cove (11 samples, early Middle Devonian), Fleurant (15 samples, early Late Devonian) and Escuminac (10 samples, early Late Devonian). These formations are composed of conglomerates, silty and calcareous sandstones deposited in a continental environment. The Upper Devonian geological units are amongst a few found in Canada to the east of Alberta. The main goal of this study is a better definition of the Apparent Polar Wander Path of the Appalachians of North America. This study allowed the identification of geological formations favourable to paleomagnetic sampling. Remanent magnetizations and magnetic susceptibilities were measured; both alternating field and thermal demagnetizations were carried out. After demagnetization, 4 components (A, B, C and D) were isolated and several components were found in each formation. Fold and conglomerate tests were performed; the first is significantly negative except for component B and the second is positive in the Fleurant Formation. The mean direction of the components is: 156°, +31° for A, 52°, -46° for B, 167°, -21° for C and 223°, +25° for D. The corresponding paleopoles are 22°N 141°E, 6°S 77°E, 51°N 137°E and 17°N 83°E. Based on the fold test, degree of alteration and unblocking temperatures, it was found that components A, C and D are secondary. Component B is either primary or secondary. The relative age of acquisition of magnetization is the following: B>D>A>C. The paleolatitudes vary from 17°S to 10°N going from A to B (tilt corrected), to D and to C. The results are discussed in relation to the Devonian Apparent Polar Wander Path of North America.

Late Paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east Tennessee: Evidence for a post‐ore chemical event

Thermal demagnetization studies on samples from mineralized and unmineralized rocks in the Lower Ordovician Knox Group of east Tennessee reveal a mean direction of D/I = 157°/+18° (α95 = 7.7°, k = 52.9) after 60% unfolding, combining stable endpoint and remagnetization circle data. Although not significant at the 95% confidence level, the fold test is suggestive of a synfolding (Alleghenian) age of the magnetization. The magnetization cannot be postfolding, i.e., Permian or younger, because the in situ directions do not resemble those for post‐Carboniferous times. The magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the Early Permian, but unlike poles for pre‐Late Carboniferous times. Characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test. The best estimate for the paleopole resulting from this study falls at 40°N, 126°E, within the Late Carboniferous segment of the North American Paleozoic apparent polar wander path. Thus the magnetization is interpreted as synfolding and secondary due to widespread Kiaman remagnetization. Blocking temperatures of up to 520°C and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization. Scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres. Secondary dolomites have the same magnetic characteristics as the primary limestones, and it is inferred that similar magnetite spheres are the carriers of their magnetization. Because this magnetization, and therefore its carrier, occurs in sphalerite and secondary dolomite, it must be coeval with or younger than the ore, which itself is thought to be of middle to late Paleozoic age. It is argued that the magnetization was acquired during growth of magnetite, associated with a late Paleozoic chemical event.

On the origin and stability of remanence and the magnetic fabric of the Torridonian Red Beds, NW Scotland

Synopsis Primary (compactional) magnetic fabrics and multicomponent remanences are recognized in the Stoer and Torridon Groups. Low temperature (LT) blocking remanences are randomized around 400–600°C and relate to a post-Torridonian magnetic overprint, possibly of early Mesozoic age. In the Stoer Group (and some Stoer boulders in the basal Torridon Group) LT remanences are partly or fully carried by magnetite (titanomagnetite). High temperature (HT) remanences are characterized by discrete unblocking above 600°C, having a specular haematite remanence carrier. Results of a conglomerate test of some Stoer boulders provides a positive stability test for HT remanences in the Torridon Group, and a convergence of evidence suggests that remanence acquisition of both the Stoer and Torridon Groups was facilitated by both detrital and early diagenetic processes. A fold test of the Stoer boulders (inclination data) indicates that the boulders have preserved a primary HT remanence. Secondary fabric is readily observed in the boulders, hence, HT remanences most likely reside in different grain- or mineral fractions than the bulk susceptibility. The overall palaeomagnetic data (HT remanences), indicates that the angular unconformity, separating the Stoer and Torridon Groups, represents a considerable period of time. Palaeomagnetic results from the underlying Lewisian basement have traditionally been interpreted in terms of post-orogenic uplift associated with the Laxfordian event ( c. 1800–1400 Ma). Due to an apparent polar fit between Lewisian and Stoer Group data, we address the question as to the significance of post-Laxfordian magnetic overprinting in the Lewisian Complex.

Eocambrian paleomagnetism of the Boston Basin: Evidence for displaced terrane

A paleomagnetic study of the Late Precambrian Roxbury formation, consisting of extrusives and clastic sediments, has yielded a steeply downward directed and pre‐folding characteristic magnetization (D/I = 219°/71°, α95 = 5°, k = 311, paleopole at 13°N, 267°E, N = 4 sites), which indicates a significantly higher paleolatitude (55°) than would be expected if the Boston basin were part of the equatorial North American craton in the latest Precambrian and earliest Paleozoic. This characteristic magnetization reveals dual polarities and is further supported by a positive conglomerate test. A ubiquitous post‐folding late Paleozoic overprint is present in nine sites (D/I = 183°/14°, α95 = 8°, k = 42), with a paleopole at 41°S, 285°E. The pre‐folding magnetization resides in hematite, which is inferred to have formed during early oxidation of the rocks; the high stability of this hematite may have prevented its magnetization from being reset during the late Paleozoic chemical event responsible for the magnetic overprint. The Boston basin has a marked geological similarity to the Avalon basement terranes in Nova Scotia and Newfoundland, as well as the Armorican Massif in France, and the high paleolatitudes observed for all these terranes suggest a common paleogeographical affinity; a likely paleolocation is near the northwestern margin of Gondwana which was located at the southpole in the latest Precambrian and Early Cambrian.

Palaeomagnetism of the Ordovician Moulin de Chateaupanne formation, Vendee, western France

Summary. A palaeomagnetic study of the Moulin de Chateaupanne formation reveals two characteristic components of magnetization. The first one, isolated by thermal treatment at intermediate temperatures, is strongly resistant to chemical leaching. Its direction (D= 217, I= +25, ag5 = 5.5) and the corresponding pole position (Lat = 21S, Long = 320E) are typical of the Hercynian overprints in the Armorican Massif. The second component, carried by fine-grained haematite, presents a steep downward direction (D = 228, I = + 81, ag5 = 6 ; pole position Lat = 34N, Long = 343E) already recognized in other Ordovician rock units from the same area. It is resistant to high-temperature demagnetization treatment and is believed to be of chemical origin, as assessed by a conglomerate test. A mutual consistency test with the nearby Thouars Massif suggests that the tilt of the formation has a sedimentary origin, and that the Thouars Massif has not suffered significant tilt during the Hercynian orogenic events.

A Lower Ordovician paleomagnetic pole from the Oneota dolomite, Upper Mississippi River Valley

Of 149 oriented specimens of the lower Ordovician Oneota dolomite subjected to stepwise thermal or alternating‐field demagnetization, 50 contained a characteristic shallow east‐southeasterly or west‐northwesterly component of magnetization identifiable on orthogonal vector projections. For 48 other specimens the demagnetization trajectories defined greats‐circle paths which intersect close to the characteristic direction. The site‐mean directions for the characteristic component were determined by using both the characteristic directions and great‐circle data, and these yield a formation‐mean direction obtained from 10 sites with a declination of 105°, an inclination of +2°, and a precision parameter k of 17.6 (α95 = 11.9°). The mean of the site virtual poles falls at 10.4° N, 166.4° E (K = 48.0, A95 = 7.0°), which is close to previously published late Cambrian paleopoles. Both polarity groups have predominantly positive inclinations, suggesting the presence of an uncleaned downward component whose unblocking‐temperature and coercivity spectra overlap very strongly with those of the characteristic magnetization. We interpret this downward component as a drilling‐induced remanent magnetization, and we hope to have removed its contribution by averaging the dual‐polarity results. Multicomponent magnetizations were also present in samples collected from blocks of Oneota dolomite incorporated into the overlying New Richmond sandstone (lower Ordovician), but only six out of eight samples yielded well‐defined greats‐circle paths. The best fit intersection of these defines a northerly and steep recent component, while the directions of the higher‐stability components are different for each block. Due to the small number of samples and imperfect resolution of magnetic directions, the conglomerate test is not conclusive; nevertheless it suggests that the stable remanence in the Oneota dolomite was acquired prior to the deposition of the New Richmond sandstone.

Paleomagnetism of Jurassic rocks in the Western Sierra Nevada Metamoprhic Belt and its bearing on the structural evolution of the Sierra Nevada Block

The western metamorphic belt of the Sierra Nevada consists of two eugeosynclinal terranes separated by the Melones and Sonora faults. Subvertical, bedded Mesozoic volcanic rocks metamorphosed to low greenschist facies predominate to the west, whereas Paleozoic metamorphic rocks of higher grade and greater structural complexity predominate to the east. In order to study the structural development of the faults, 121 samples of basalt and diabase were collected for paleomagnetic analysis from three Jurassic formations, the Logtown Ridge and Penon Blanco formations west of the Melones fault and the Sonora dike swarm to the east of the Sonora fault. A northwesterly, downward directed magnetization occurs in each unit. Three fold tests and a conglomerate test on the two formations west of the faults show that the magnetization is secondary, postdating Nevadan (Late Jurassic) folding and is probably coeval with peak metamorphism. An average of five paleomagnetic poles from the Sierra Nevada, three derived from the secondary magnetizations given herein and two previously published, all of probable Kimmeridgian age, yields λ′ = 67.2°N, ϕ′ = 161.2°E, and α95 = 6.5°. Southeasterly magnetizations also occur in the Logtown Ridge Formation and Sonora dike swarm. Directions from the Sonora dikes are approximately antipodal to the secondary directions and are reversed; magnetizations from the Logtown Ridge Formation yield similar results only if corrected for the tilt of bedding. The Logtown Ridge magnetizations (tilt‐corrected) yield a pole position near to that expected for North America. The data from the Sonora dikes require a tilt correction of 25°–30° toward the south‐southwest about a horizontal axis parallel to the regional structure in order to yield a North American pole position. We conclude that the eastern wall rocks of the Melones and Sonora faults have been rotated 25°–30° in response to Nevadan deformation in contrast to the western wall rocks, which have been rotated about 90°.

Chemical remanent magnetization due to deep-burial diagenesis in oolitic hematite-bearing ironstones of Alabama

Oolitic hematite-bearing ironstones of the Silurian Red Mountain Formation of Alabama are shown to carry a single-component remanence stable enough to have survived major folding (of probable Permian age). Nevertheless, the remanence direction (ten sites yielding a paleopole at 38.0°N, 132.4°E with dm = 3.6°, dp = 1.9°), its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the Pennsylvanian—some 130 Ma after deposition. This remanence is likely a chemical remanent magnetization (CRM) acquired during diagenesis induced by heating due to deep burial under a Pennsylvanian clastic wedge. Two possible mechanisms for acquisition of CRM during deep-burial diagenesis are considered. In hypothesis I, the oolitic hematite transformed from original geothite when heated to about 80°C, acquiring CRM. In hypothesis II, the oolitic hematite originated from ferrihydrite and was too fine-grained to acquire stable CRM until heat raised the solubility of hematite allowing grain growth. Hypothesis I explains the timing of remanence acquisition better, but there is some evidence that oolitic goethites may be stable to considerably more than 80°C. Hypothesis II has some difficulty explaining preliminary paleomagnetic results from oolitic hematite-bearing ironstones of the Silurian Clinton Group, New York State. We prefer hypothesis I but both hypotheses remain plausible. Both hypotheses warn that continental red beds may also acquire CRM during diagenesis induced by deep-burial heating, long after deposition but before folding.

Thermal Windows of Yukon-Koyukuk Basin, Alaska: ABSTRACT

Vitrinite reflectance values from two mid-Late Cretaceous sedimentary sections in the Yukon-Koyukuk basin are within the oil window when plotted on a time-temperature-reflectance nomogram. However, preliminary paleomagnetic studies in other areas indicate a higher temperature overprint. To establish a more consistent record of paleothermometry in the basin, over 500 paleomagnetic samples from 35 localities and 25 K-Ar dating samples were analyzed. The samples include cobbles from conglomerates to test for remagnetization and argon loss. Our analysis indicates that (1) some sample localities fail the conglomerate test (cobbles show consistent magnetic directions) and have experienced temperatures of 300°-500°C, (2) other localities, along the lower Yukon River, display primary paleomagnetic signals that pass both conglomerate and regional fold tests and have reversals, and (3) K-Ar ages of detrital components (volcanic pebbles and muscovite) from the lower Yukon River localities are older than their age of deposition, suggesting that temperatures have not exceeded the K-Ar blocking temperatures of these components (~300°C for muscovite and lower for whole rocks). The complex thermal history of the Yukon-Koyukuk basin is a result of at least three magmatic episodes. The overprinted areas are closest to igneous occurrences; other areas were thermally unaffected by these widespread magmatic pulses. End_of_Article - Last_Page 666------------

Secondary magnetizations from the Clinton-type iron ores of the Silurian Red Mountain Formation, Alabama

From 27 (out of a total of 29) sites a characteristic pre-folding magnetization has been obtained with D = 150°, I = +20°, α 95 = 3.5° , and paleopole at 38°N, 132°E. However, we conclude from a bedding-error test and a conglomerate test, as well as from descriptions of the hematite as a replacement mineral, that the magnetization is a (late) post-depositional chemical remanent magnetization. The age constraints on the magnetization, between Middle Silurian and Early Permian, can be refined by a comparison with the apparent polar wander path for cratonic North America; this comparison suggests a Late Carboniferous age for the magnetization and the hematite, which constitutes the principal component of the iron ores. Similar Late Paleozoic remagnetizations have been noted in other Appalachian and mid-continent formations and suggest a widespread, but as yet ill-defined mechanism for the remagnetization. It is tempting to correlate this event with the early phases of the Alleghenian orogeny in Carboniferous times and with possible fluid migrations resulting from the tectonism.

A Pennsylvanian paleomagnetic pole from the mineralized Late Cambrian Bonneterre Formation, southeast Missouri

One hundred eighty‐eight oriented samples of the Late Cambrian Bonneterre carbonates of southeastern Missouri were taken from Viburnum Trend mines and local surface outcrops. Initial natural remanent magnetization directions have south‐southeasterly declinations and downward inclinations. Progressive demagnetizations using thermal and AF techniques give two directions: one is a viscous present‐day field direction, and the other is of reversed polarity, with south–southeasterly declinations and very shallow inclinations. The resulting paleopole, at 43°N, 126°E, is located near the Pennsylvanian to Early Permian segment of the North American apparent polar wander path. A negative conglomerate test supports this assignment for the magnetization age. Magnetic extracts from the mineralized as well as unmineralized zones of the Bonneterre contain spherical crystal aggregates of pure magnetite of late diagenetic origin. The secondary magnetization resides in the magnetite spheroids, and we infer that the spheroids formed prior to sulfide mineralization.

A paleomagnetic conglomerate test using the Abee E4 meteorite

The meteorite Abee is a type 4 enstatite chondrite with many centimeter-size clasts. The paleomagnetic conglomerate test was applied to these clasts, to study the thermal and magnetic history of the meteorite. The directions of magnetization in mutually oriented clasts are significantly different, suggesting the meteorite was not reheated to temperatures much above 100°C during or after accretion. Paleointensity estimates were made using Thellier's method. Interior samples which were probably not reheated during entry into the earth's atmosphere show paleointensities of several oersteds. The fusion crust is also strongly magnetized, showing paleointensities up to 60 Oe.

Paleomagnetism of Late Archean metavolcanics and metasediments, Abitibi orogen, Canada: volcanics of the Blake River Group and sediments and volcanics of the Timiskaming Group

A paleomagnetic study (159 samples, 30 sites) was done on calc-alkaline volcanics of the Blake River Group, dated at 2703 ± 2 Ma and occupying the central portion of a major synclinorium in the Abitibi Greenstone Belt. Magnetically stable single- and multi-vectorial components of magnetization were resolved in demagnetization; however, there was poor consistency of directions linearly decaying to the origin in vector analysis on the site, sample, and subsample levels (e.g., D = 190.0°, I = −29.2°, k = 1.5, α95 = 25.0°, n = 21 specimens from N = 21 samples). Preliminary results from a conglomerate test performed on the Chaput–Hughes Member of the overlying Timiskaming Group suggest that a pre-depositional remanence was retained and that there was not a widespread regional metamorphic event after the sediments were deposited. Cobbles derived from alkali volcanics of the Timiskaming Group and Kinojevis (tholeiites) and Blake River volcanics display dispersed directions of magnetization between cobbles. Preliminary magnetization data from the matrix of the conglomerate are well grouped, and correction for regional structure results in a direction similar to the characteristic one of Matachewan dikes. Paleomagnetic data from a downfaulted block of Timiskaming volcanics cut by Matachewan diabase dike may be interpreted to suggest that the fault block has been rotated clockwise (80°+) and subsequently tilted to the west (80°).

Evidence for metamorphic remagnetisation of upper Precambrian tillite in the Dalradian Supergroup of Scotland

ABSTRACTA conglomerate test using 36 clast cores (76 specimens) from the upper Precambrian Port Askaid Tillite of the Dalradian Supergroup in the Garvellach–Islay region of Scotland shows that the tillite was remagnetised during orogenic activity in Lower Ordovician time. After 550°C thermal demagnetisation, the clast remanence directions give a mean of 155°, 14° (α95—12°). This is similar to the mean from four tillite matrix sites of 162°, 4° (α95—9°) and the mean ‘primary component direction’ of 166°, −12° (α95—6°) previously determined from the interbedded siltstones. This indicates that the matrix and siltstones were also remagnetised. These mean directions give virtual geomagnetic poles after tilt correction that fall within the cluster of British Ordovician poles from rocks in the Caledonian orogen, and thereby indicate a prefolding Ordovician age for the remagnetisation. Thermal demagnetisation studies and observations from polished and thin sections of tillite clast and matrix specimens show that secondary hematite derived from partially to completely altered magnetite is the dominant carrier of the metamorphic remagnetisation. These results show that remanence data from the Port Askaid Tillite cannot be used to support the hypothesis that a worldwide glaciation occurred during the Late Precambrian.

Remagnetization in Keweenawan rocks. Part I: conglomerates

Conglomerate tests are used to show that Late Precambrian Keweenawan conglomerates on the Keweenaw Peninsula, Michigan and at Mamainse Point, Ontario have suffered a partial chemical remagnetization that predated tectonic tilting. At Mamainse Point the overprint was acquired during the final normal polarity stage of Keweenawan volcanism, whereas on the Keweenaw Peninsula it occurred after cessation of volcanism and has a maximum Late Freda Sandstone age. The extent of overprinting on the Keweenaw Peninsula increases westwards along a 40 km strike length in the Copper Harbor Conglomerate, and may at least in part be related to secondary mineral zonation (including native copper) in the underlying Portage Lake volcanics. It is shown that neither of the two overprinting episodes can be responsible for the reversal asymmetry observed in Keweenawan igneous rocks.

Magnetic components contributing to the NRM of Middle Siwalik red beds

The components of the NRM in Middle Siwalik red beds are carried by two phases of hematite, a red pigment phase and a specular hematite phase. We present evidence in the form of a conglomerate test that the specularite phase carries a remanence acquired during or shortly after deposition. The red pigment, however, post-dates deposition, in many cases by at least one reversal boundary. This secondary component has a higher coercivity but a lower blocking temperature in these rocks and can therefore be selectively removed by thermal demagnetization to reveal a primary magnetization useful for magnetostratigraphic studies.

Early acquisition of characteristic magnetization in red beds of the Moenkopi Formation (Triassic), Gray Mountain, Arizona

Four discrete magnetizations associated with events occurring during deposition, early diagenesis, recent weathering, and cloud to ground lightning have been identified in rocks of the Moenkopi Formation at Gray Mountain, Arizona. A paleomagnetic study of more than 800 samples tied closely to the physical stratigraphy of the Moenkopi indicates that most of the magnetization remaining after partial alternating field and thermal demagnetization was acquired during and shortly after deposition. Directions of this remaining (characteristic) magnetization, in both normally and reversely magnetized rock, are closely similar to directions reported for the Moenkopi elsewhere on the Colorado Plateau. Sampling across the Moenkopi Formation at Gray Mountain has revealed two reversed magnetozones separating three normal magnetozones. Generally sharp boundaries of the magnetozones, their close concordance with physical stratigraphic units, and the rarity of anomalous directions of magnetization within the magnetozones indicate that most of the secondary components of the characteristic magnetization were acquired within 104–105 years after deposition. A conglomerate test based on siltstone clasts in sandstone shows highly scattered but not completely random directions of magnetization following partial thermal demagnetization. A secondary Triassic normal magnetization appears to be superimposed on some of the clasts, as suggested by the approximate coincidence of the resultant magnetic vector from the clasts with a Triassic normal direction. A fold test based on samples from highly deformed silty clay stone and sandstone in an intraformational landslide shows that these rocks acquired most of their stable magnetization before folding. A component of Triassic normal magnetization acquired after deformation also occurs in some of these rocks.

Palaeomagnetic data on the andesitic cover of the Sesia-Lanzo Zone (Western Alps)

The remanent magnetization of the andesitic cover of the Sesia-Lanzo Zone has been studied from 16 sites along three transversal sections. The remanent magnetization of the rock is stable and it appears to be primary after a conglomerate test. The mean direction of magnetization is consistent for all sites, but for one exception, with satisfactory values of statistical parameters. Therefore no deformation structures postdating the acquisition of remanent magnetization can be inferred from palaeomagnetic data. The mean direction of magnetization (11 sites, 152 specimens) is: D=135.9, I=−2.9, with α95=8.8. No tectonic correction can yet be made.

Palaeomagnetic Studies in the British Caledonides?IV Lower Devonian Lavas of the Strathmore Region, Scotland

Summary Mean palaeomagnetic directions after dip-correction at 6 1 sites in lavas, ignimbrites and sediments in the Lower Devonian sequence of the Strathmore region, show a principal clustering around an axis D = 42/222 E, Z = -49/+49 which are regarded as normal and reversed (N and R) polarity groups respectively. The frequency distribution of site mean directions is not simply bipolar, however, having subsidiary maxima around D = 76, Z = +29 (' Group A ') and D = 162, Z = -35 (' Group B '). It is shown that the population can be subdivided into N and R Fisherian distributions (34 sites altogether) plus Groups A and B (1 8 sites together) and a random background of nine ' unclassified ' sites. This analysis leads to a best estimate of the mean N and R directions corresponding to a palaeomagnetic pole 5 N 140 E (dJI = 5, dx = 9). The primary (Lower Devonian) origin for the stable remanent direction, at least in the N, R, A and B groups is demonstrated by fold tests, a conglomerate test, igneous contact tests and by the recognition of a magnetic polarity stratigraphy consisting of at least eight N and R zones and at least five zones of ' A ' or ' B ' polarity. These latter groups are believed to record preferred intermediate directions of the geomagnetic field during polarity transitions. Remanent intensity is similar in the lavas in all polarity groups; it is believed to be the product of oxidation during cooling of the lavas. The more usual situation of lower intensity in rocks formed during polarity transitions is perhaps observed in intermediate polarity samples of the Highland Border Ignimbrite and may indicate an original TRM in these rocks. A composite calculation of the mean N and R direction after AF cleaning based on this and previous studies in the eastern part of the Midland Valley of Scotland (50 sites altogether) gives D = 41, Z = -40 (ag5 = 6); the corresponding palaeomagnetic pole is 4 N, 140 E (dJI = 4, dx = 7).

Palaeomagnetic Results from Palaeozoic Rocks of South-east Australia

Summary Palaeomagnetic results have been obtained from igneous rocks whose ages range from Middle Silurian (Wenlockian) to Lower Carboniferous (VisCan). A collection of 210 samples from four areas of south-east Australia was studied. The use of alternating field demagnetization procedures enabled the directions of presumed original magnetization to be isolated for the various formations. A conglomerate test provided additional stability evidence for the result from one formation. Calculated palaeomagnetic pole positions are as follows: Isismurra Formation and Lower Kuttung Series (VisCan)-34E, 73N with A,, = 21; Lochiel Formation (Upper Devonian)-14OoE, 58N with A,, = 7; Ainslie Volcanics (Lower Devonian)-l73E, 71N with A,, = 10; Bowning Group (Gedinnian Stage, Lower Devonian)-225E, 64N with A,, = 9; Laidlaw Formation, Douro Volcanics and Hawkins ' Series ' (Wenlockian-Ludlovian, Middle-Upper Silurian)91E, 54N with A,, = 7.