Chemostratigraphical Characterisation of the Lower Silurian Formigoso Formation: A Case Study from Aralla (Cantabrian Mountains, Province Leon, NW Spain).Tim Ferriday and Michael Montenari

Basin Dynamics Research Group, Earth Sciences and Geography, Keele University, Newcastle-under-Lyme, Staffordshire, ST5 5BG, United Kingdom.

1. Introduction

2. The Formigoso Formation

4. Lower Silurian ‘hot’ shales of Aralla 5. Geochemical cyclicity - orbital forcing and the 413ka problem

6. Interpretations and Conclusions

3. Geochemical signals - Aralla case study

References

0 1 2 3 4 5 6 7

K [Wt%]

Th

[p

pm

]

0

5

10

15

20

25

30

Heavy

Thorium

-Bearing M

inera

ls

Kaolinite

Mon

tmor

illonite

Mixed Layer Clays

Ilite

Micas

GlauconiteFeldspar

Potassium Evaporates

Th/K = 0.5Th/K = 0.6

Th/K = 2

Th/K = 3.5

Th/

K =

12

Th/K

= 2

8

1

2

3

4

1

2

3

4

~ 70% Ilite Line

~ 40% Micas Line

~ 30% Glauconite Line

~ 30% Feldspar Line

Aralla shale composition Th plotted against K

The Cantabrian Mountain belt is located in northern Spain in a region known as Castilla y Leon. The mountains form a part of the geologically distinct region known as The Cantabrian Zone (or Cantabrian Arc) as seen in Figure 2 . The zone is an area of thin-skinned tectonics (the tectonics were a result of the combination between Variscan and Alpine Orogenys). The thrusts and folds throughout the region are affecting rocks ranging in age from Precambrian to Carboniferous (Aller et al. 2005).The Cantabrian Zone can be sub-divided into a number of smaller thrust fault units. Julivert (1971) and Pérez-Estaún et al. (1988) separated the Cantabrian zone into five units, these being the: Folds and Nappes, Central Asturian coal-field, Ponga Nappe, Picos de Europa and the Pisuerga-Carrión domains, as seen in Figure 2 (Abalos et al. 2002). The Folds and Nappes unit has been further subdivided into the; Somiedo-Correcilla, La Sobia-Bodon, Aramo, Elsa nappe and Valsurvio (Abalos et al. 2002). Each of these sub-divided units consists of a distinctive stratigraphic succession. The Somiedo-Correcillas unit (Figure 2 - Highlighted in Blue) which is located towards the centre of the Cantabrian Zone, is the area of interest (The Location of the Aralla Section has been marked) as it contains the Palaeozoic sequence spanning from the Precambrian to the Carboniferous including the Silurian black shales of the Formigoso Fm. and the underlying Barrios Fm. (refer to Figure 3 for stratigraphic column). The aim of this study is to highlight the geochemical cyclicity seen within the Silurian Formigoso Formation (Figure 1) and to determine the factors controlling the cyclicity.The globe in the centre of the poster (Figure 7) shows a palaeogeographical reconstruction of the Lower/Middle Silurian. The project area within Iberia has been marked including where the 'hot' shales of the Lower Silurian were deposited along the northern most rim of Gondwana.

Thickness: 50-800m+ (massive lateral variation in thickness. May be controlled by the underlying glacial relief of the Barrios Fm.)Age: The Formigoso Fm. yields an abundance of graptolites and palynomorphs as well as scarce brachiopods, bivalves, cephalopods and trilobites. It extends from the Aeronian-Telychian boundary (Upper Aeronian to be confirmed) into the lower Sheinwoodian as determined by graptolite assemblages (Truyols et al. 1974). These dates are confirmed by organic walled microfossils (chitinozoans) by (Cramer-Diez and Diez 1978). Verniers et al. (1995) confined the lower chitinozoan assemblage to the Conochitina alargada Biozone corresponding to the middle Aeronian. The Graptolite data requires re-evaluation (Robardet and Gutierrez-marco 2002).Lithology: Black and grey siltstones and shales with sand intercalations towards the top of the Fm. that show ripple laminations, hummocky cross-stratifications and trace fossils. Kegal (1929) proposed two members: the lower member, Pizarras del Bernesga (meaning black shales with an abundance of graptolites) and an upper member Capas de Villasimpliz (meaning shales and siltstones intercalated with sandstone beds). Environment: Shallow marine shelf deposits or shelf margin deposits (Robardet and Gutierrez-Marco, 2002) determined by the sand intercalations (Tempestites or Turbidites. The environment of deposition is related to a rapid marine transgression followed by a shallowing upwards sequence (represented by the sandstone beds) into the gradational boundary of the overlying San Pedro Fm. The lateral variation in the thickness of this Formation is very dramatic.

560

535

510

485

460

435

410

385

360

335

(Ma)

Neopr.

Cam

brian

Ord

ovi

cian

Silu

rian

Devo

nia

nC

arb

onifero

us

Early

Mid

dle

Late

Tre

m.

Are

nig

Lla

nvirn

Cara

d.A

sh.

Lla

ndover.

Wen.

Ludl.Pridoli

Loch.

Prag.

Em

sia

n

Eifel.

Giv

.F

rasn.F

am

enn.

Tourn

ai.

Vis

ean

Serp.

Bash.

RSL - 200M + 200M

Regression Transgression

Narcea Gp (Comte 1937 and Narcea Schists of Lotze 1956, 1961c)

Herreria Gp (Comte 1937,1959)

Lancara Fm (Comte 1937, Lotze 1961c)

Oville Fm (Comte 1937)

Barrios Fm (Comte 1937)

Formigoso Fm

Furada (San Pedro) Fm

La Vid or Raneces Gp

Upper Ordovician units do not existin most areas of Cantabria. Post-Arenigian Ordovician rocks are known along the Narcea antiform in the western part of the ‘folds and nappes’ region.

Santa Lucia or Moniello Fm

Huergas or Naranco Fm

Portilla or Candas Fm

Nocedo or Pineres Fm

Fueyo Fm

Ermita Fm

Vegamian Fm(West)

Baleas Fm(East)

Alba Fm (also called Genicera Fm orCarboniferous griotte limestones)

Barcaliente and Olleros Fms

Valdeteja Fm

San Emiliano Fm

Hirnantian Glaciation

Luna Quartzite + Diamictites

Getino Fm (Or Getino Bed)

Figure 3: Chronostratigraphy and palaeobathymetry of Cantabria, northern Spain

4+The 'hot' shale is termed so because of the high gamma radiation content, that of the insoluble U . Gamma values may exceed that of 700+ API (American Petroleum Institute) units in the 'hot' shale

6+horizons (Figure 6). In oxygenated conditions U is dissolved in seawater. Under the anoxic black 6+ 4+'hot' shale conditions the U is reduced to insoluble U which is adsorbed to organic content.

4+Therefore the enrichments in authegenic U in Silurian 'hot' shales can be used as a proxy for the organic content and anoxic phases. The Silurian 'hot' shales were deposited along the majority of the northern rim of Gondwana. The palaeo-reconstruction (below - Figure 7) shows the locations of the 'hot' shale and infers where else they may have been deposited.Luning (2000), states that the threshold value for the 'hot' shales of northern Africa is >200API as seen in Figure 6.In order to create the gamma log for Aralla (Log section Figure 1) geochem values (U & Th ppm + K Wt%) had to be converted to API Values. The following factors were utilised to convert to API: 1ppm Th = 2.54 API, 1ppm U = 6.69 API and 1% K = 10.64 API (factors from Technical University of Denmark – Accessed online). The sum of the API values for U, Th and K gave the Gamma Log. The total organic carbon (TOC) curve for the Aralla log (Figure 1) was generated by using the U ppm values as a proxy. Fertl & Chilingarian (1988), proposed a correlation between U ppm values and the Wt% of organic carbon. Fertl & Chilingarian’s correlation curve was applied to the U ppm values of the Aralla log to approximate the TOC values.When comparing the Aralla section to the 'hot' shales of Africa it is clear that the gamma values at the base exceed Lunings 200 API threshold, indicating the occurrence of 'hot' shales at Aralla.

From looking at the Aralla log (Figure 1) the contrast at the base of the log section between the underlying lithology (Barrios Fm) and that of the black shales of the Formigoso Fm. is drastic. The log section (Figure 1) represents U, Th, K, Gamma and TOC values for the entireity of the basal Formigoso Fm. It is evident that there are eight prominent cycles within the black shales of the Formigoso Fm. The eight cycles are apparent in the majority of the trace element geochemistry (the eight cycles are indicated through the log section Figure 1).The basal black shales (before the sandstone intercalations) of the Formigoso Fm. represent a duration of approx. 4 Myrs, spanning from the Aeronian/Telychian boundary (436Ma) to the mid-Telychian Monoclimacis Graptolite Zone (~ 432Ma). Therefore, each single cycle seem to represent approximately <500 kyr. This consistent cyclic signal is tentatively interpreted to represent the dynamic sedimentological response to the Earth's 413ka eccentricity, sub-cycles within may also represent the earths 100ka eccentricity (as seen in Figures 8 + 10).Sea-level fluctuates in response to precessional (Axis wobble - Figure 10) cycles where the rise and fall of sea level is proportional to the degree of eccentricity of the Earth's orbit (shape of Earth's elliptical orbit oscillates from more circular to less circular (called eccentricity - Figures 8 + 10). Variations in eccentricity affect the seasonality. When eccentricity is high then there are large changes in the summer insolation (effecting sea-level massively if large ice masses are apparent) between the perihelion and aphelion summer positions (Figure 9). As the Earth’s orbit approaches round (no eccentricity) the amount of change in insolation through a precessional cycle would approach zero leading to stable sea levels. The ellipticity of the Earth's orbit varies in a cyclic pattern from near round to as much as 4% with a period of about 100 ka . These 100 ka eccentricity cycles occur with enhanced peaks at 413 ka intervals (Figures 8 + 10). Periods of high sea-level are represented by high U - (and corresponding TOC-) values as seen within the Aralla log (Figure 1). The cyclic behaviour of the Formigoso Fm. is therefore related to sea-level changes. As sea-level drops and indeed rises, the stratified water column will be pertubated – leading to oxygenation of the anoxic bottom waters; this is represented by lower U and TOC values (U is no longer reduced and organic content is oxidised).

Unlike the gamma ray log, which measures total radioactivity (Aralla Gamma Log - Figure 1), the spectral log reads the relative concentrations of radioactive potassium, thorium, and uranium (Figure 1). The thorium-uranium ratio measured by this log has been found to be a valuable indicator of depositional environment (Fertl 1979).A thorium-uranium ratio greater than 7 is thought to indicate a continental, oxidizing environment and a ratio of less than 7 to imply marine deposits, most likely gray and green shales. For thorium-uranium ratios less than 2, the presence of black, probably organic, shales deposited in anoxic marine environments is suggested. The Th/U ratio plot (Figure 12) clearly indicates that the Aralla section contains black, organic rich anoxic marine shales. Figure 13 represents V/(V+Ni) ratios for Aralla, the ratios allow the discrimination between dysoxic-oxic, anoxic and euxinic conditions. The Formigoso Fm. falls predominantly into the anoxic phase (as suggested by Figure 13) with extreme euxinic phases at the base and dysoxic-oxic phase at the top where deltaic sandstone intercalations become prominent. Figure 13 displays the basin becoming more oxic towards the top of the black shale formation as the terrestrial influx begins.The gamma ray spectral log may also be used for lithological identification, particularly for clay-typing. The crossplot chart in Figure 11 (Thorium/potassium cross-plot for mineral identification) maps a number of radioactive minerals according to their thorium and potassium concentrations. From the mineral identification chart the Formigoso Fm. consists of mostly 'Ilite' and 'Mixed Layer Clays'.Environmental conditions suggested by the Aralla log and geochemical data are interpreted as follows: as stated above Th/U ratios indicate 'black, organic rich anoxic marine shales'. The black organically rich anoxic shales form as a result of a stratified water column (anoxic bottom water). The stratified water column is generated by restricted oceanic circulation; circulation may have been hindered by freshwater influxes capping the ocean (from the Hirnantian meltwater) combined with the complex relief of the basin. The sandstone intercalations towards the top of the formation indicate terrestrial influx (and the beginnings of a shallowing-upwards-sequence), this terrestrial influx reduces U values as the water column is perturbated (oxygen is introduced to the anoxic bottom waters). Thickness changes of the Silurian Formigoso Fm. throughout the Cantabrian Arc are thought to be controlled by the palaeo-topography of the underlying formations. This relief is regarded to have been generated by the erosion caused by the relative sea-level drop during the Hirnantian glaciation event, in combination with the extensional tectonics related to the widening of the Rheic ocean (as seen in Figure 14).

Contact

J

413, 000years

100,000years

Eccentricity (e)

Myr

ag

o

e = 0 e = 0.06

0.00 0.03 0.060

0.5

1.0

1.5

Figure 8: Eccentricity cycles; 100K yr periodicity, 413K yr periodicity. If orbit was circular then e=0. Today e=0.017 Modified after Berger & Loutre 1991.

Sun at one focus

EquinoxSeptember 22

Aphelion158 million km

Perihelion153 million km

Emptyfocus

July 4

SolsticeJune 21

EquinoxMarch 20

January 3

SolsticeDecember 21

Figure 9: Showing the difference between Aphelion and Perihelion orbital positions. Modified after Pidwirny (2006)

Eccen

tric

ity

Low

High

Precessional Signal

0 400 ky

THE MILANKOVITCH HIERARCHY

100K Cycles

Sea L

evel

Rise

Fall

413 ky CycleFigure 10: Showing 100ka and 413ka eccentricity cycles and the effects on sea-level. Modified after Berger 1988.

Figure 11: Crossplot between K (Wt%) and Th (ppm) discriminating the Radioactive minerals present according to their thorium and potassium concentrations. Modified after Schlumberger Well Services 2009

1

2

N S

Ice masses progressing

North

Late Ordovician - Hirnantian (palaeo-relief generated by Hirnantian and extensional tectonics)

Barrios Fm.

Sea-level is at a minimumduring the Hirnantian glaciation.

Relief of the Barrios Fm created by the progressing Hirnantian ice masses.

Early Silurian - Transgression as a result of the melting of the Hirnantian ice masses

Ice masses haveretreated

Sea-level rise

Initial deposition of the Cantabrian Early SilurianBlack Shales

3 Early/Middle Silurian - Maximum sea-level, and continuing deposition of the black shales.

Barrios Fm.

Sea-level maximum

The Silurian Black shales are deposited above palaeo-relief. The resulting thicknesschanges are drastic within the Formation.

Extensional regime Rheic Oceans southern passive margin

Barrios Fm.

Extensional regime Rheic Oceans southern passive margin

relief generated by extensional tectonics.

Relief as a resultof Hirnantian

Glaciation

Black shale depositsmuch thicker in the North

Continuation of black shale depositionmaybe not as organically rich as the basal deposits.

HirnantianGlaciation

Figure 14: Modelling how the thickness changes have been controlled in the Formigoso Fm. The parameters controlling the thickness are relief generated by the Hirnantian event and extensional tectonics related to the opening of the Rheic ocean.

SCO-1 (Govindaraju 1989)

Average Shale (Turekian & Wedephol 1961)

Average Shale (Turekian & Wedephol 1971, 1991)

North American Shale Comp - NASC (Gromet 1984)

Post-Arch. Aust. Shale - PAAS (Taylor & McLennan 1985)

Upper Cont Crust - UCC (Taylor & McLennan 1985, 1995)

Figure 15: Spider-plots showing the locality geochem averages. The locality averages are normalised to the various world standards. Each graph represent the enrichments and depletions of the various elements in comparison to the normalising standard. Aralla - Grey,

Caldas - Red, La

Majua - Green, Sena - Blue, Villanueva - Light Blue

Tim Ferriday Basin Dynamics Research Group,Earth Sciences and Geography,Keele University,Keele, Staffordshire, ST5 5BGUnited KingdomOffice: +44 (0) 1782 733620t.ferriday@epsam.keele.ac.uk

Michael MontenariGeography, Geology and the Environment,William Smith Building,Keele University, Keele, Staffordshire, ST5 5BGUnited KingdomOffice: +44 (0) 1782 733162m.montenari@esci.keele.ac.uk

ABALOS, B., CARRERAS, J., DRUGUET, E., VIRUETE, J.E., PUGNAIRE, M.T.G., ALVAREZ, S.L., QUESADA, C., FERNANDEZ, L.R.R. and GIL-IBARGUCHI, J.I., 2002. Variscan and Pre-Variscan Tectonics. In: W. GIBBONS and T. MORENO, eds, The Geology of Spain. London: Geological Society, pp. 155-183. ALLER, J., VALIN, M.L., GARCIA-LOPEZ, S., BRIME, C. and BASTIDA, F., 2005. Superposition of tectono-thermal episodes in the southern Cantabrian Zone (foreland thrust and fold belt of the Iberian Variscides, NW Spain). Bulletin de la Societe Geologique de France, 176(6), 487-497. BERGER, A. 1988. Milankovitch Theory and climate. Reviews of Geophysics, Vol, 26, No. 4, pp. 624-657 BERNÁRDEZ, E., GUTIÉRREZ-MARCO, J.C. and HACAR, M., 2006. Sedimentos glaciomarinos del Ordovícico terminal en la Zona Cantábrica (NO de España). Geogaceta, 40, 239-242. CRAMER-DIEZ, F.H. & DIEZ, M.C. 1978. Iberian chitinozoans. 1, Introduction and summary of pre-Devonian data. Palinogia, no. Extraord. 1. 149-201 FERTL, W.H. & CHILINGARIAN, G.V. (1988): Total organic carbon content determined from well logs. SPE 1562, 61st Annu. Tech. Conf., 407-419, Houston. FERTL, W.H (1979). Gamma Ray spectral data assist in complex formation evaluation Proc, 6th. European Symp., March 1979GUTIERREZ-MARCO, J.C., GHIENNE, J., BERNARDEZ, E. and HACAR, M.P., 2010. Did the Late Ordovician African ice sheet reach Europe? Geology, 38(3), 279-282. JULIVERT, M., 1971. Decollement tectonics in the Hercynian Cordillera of Northwest Spain. American Journal of Science, 270(1), 1-29. LÜNING, S., CRAIG, J., LOYDELL, D.K., STORCH, P. and FITCHES, B., 2000. Lower Silurian hot shales' in North Africa and Arabia: regional distribution and depositional model. Earth-Science Reviews, 49(1-4), 121-200. KEGEL, W. 1929. Das Gotlandium in den Kantabrischen Ketten Nordspaniens. Zeitschrift der deutschen geologischen Gesellschaft, 81, 35-62. PÉREZ-ESTAÚN, A., BASTIDA, F., ALONSO, J.L., MARQUÍNEZ, J., ALLER, J., ALVAREZ-MARRÓN, J., MARCOS, A. and PULGAR, J.A., 1988. A thin-skinned tectonics model for an arcuate fold and thrust belt: The Cantabrian Zone (Variscan Ibero-Armorican Arc). Tectonics, 7(3), 517-537. ROBARDET, M. and GUTIERREZ-MARCO, J., 2002. Silurian. In: W. GIBBONS and T. MORENO, eds, The Geology of Spain. London: Geological Society, pp. 51-66. SCOTESE, C.R., BOUCOT, A.J. and MCKERROW, W.S., 1999. Gondwanan palaeogeography and pal˦ oclimatology. Journal of African Earth Sciences, 28(1), 99-114. TRUYOLS, J., PHILIPPOT, A. and JULIVERT, M., 1974. Les formations siluriennes de la zone cantabrique et leurs faunes. Bulletin de la Societe Geologique de France, 16(1), 23-35. VERNIERS, J., NESTER, V, PARIS, E DUFKA, R, SUTHERLAND, S. & VAN GROOTEL, G. 1995. A global Chitinozoa biozonation for the Silurian. Geological Magazine. 132, 651-666.Technical University of Denmark: (accessed online 26/03/12) http://server4.oersted.dtu.dk/research/RI/SNG/SNG-logs.html PIDWIRNY, M. (2006). "Earth-Sun Geometry". Fundamentals of Physical Geography, 2nd Edition. 26/03/12. http://www.physicalgeography.net/fundamentals/6a.html Schlumberger Well Services: Log Interpretation charts 2009 (ed)-(accessed online 26/03/12) http://www.slb.com/

0 200 400 API

200 ft

100 ft

0

50m

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hot shale

Mem

ou-

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Tan

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Awayn atWanin Fm.

BRD-4

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gammaray

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TOC (WT%)m 0 1 2 3 4 5 6 7

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465050 100 150 200 250 300 350 400

Figure 2: Overview of the geology within the Cantabrian region; [a] showing the Iberian peninsula and grey shaded representing the palaeozoic outcrops the blue box indicates [b] showing the major thrust units and stratigraphy of the Cantabrian region. The location of the Aralla section is marked. Modified after Julivert (1971) and Perez-Estaun et al. (1988)

MESO/CEN COVEROZOIC

PISUERGA CARRION PROVINCE/Unit

PICOS DE EUROPA PROVINCE/Unit

NAPPE (PONGA NAPPE) PROVINCE/Unit

CENTRAL ASTURIAN COALFIELD PROVINCE

Aramo Unit

La Sobia-Bodon Unit

Esla-Valsurvio Unit

Somiedo-Corrcillas Unit

GRANITOIDS

UNCONFORMABLE STEPHANIAN

NARCEA ANTIFORM (PRECAMBRIAN)

FOLD AND NAPPE PROVINCE UNDIFFERENTIATED PALAEOZOICOF THE CANTABRIAN ZONE

50km

BAY OF BISCAYNProject area overview

Aralla

[b] [a]

Oviedo

Gijon

29m gap

Cycle

2C

yc

le 5

Cy

cle

6C

yc

le 3

Cy

cle

4C

ycle

1C

yc

le 7

Cy

cle

8

Aralla Log: , , + TOC U Th, K γ - Ray

Ho

t S

hale

200 API

0 50 100 150 200 250

γ - Ray APITh ppm0 5 10 15 20 25

U ppm

0 5 10 15 20

K ppm

0 1000

2000

3000

4000

5000

6000

7000

1 m

0 1 2 3 4 5 6

TOC Wt%

Figure 1: Aralla log section showing the concentrations and signals of U, Th, K, γ - Ray and TOC. The cylicity is highlighted

3 Wt%

0 m base

89.25 m top

Figure 7: Palaeoreconstruction of the Lower/Middle Silurian showing the position of the project area and the location of the ‘hot’ shales

‘Hot’ Shales

Siberia

Kaza

kstania

Rheic Ocean

Iape

tus

Oce

an

SP

Avalonia

Project Area

Iberia

Figure 13: Showing the V/(V+Ni) ratios for the Aralla section. The base of the curve represents the base of the log section. Modified after Hatch + Leventhal 1992.

0 0.2 0.4 0.6 0.8 1 1.2

Shale

sS

ST

Ratio > 0.84 signifying Euxinic

conditions

Ratio 0.54-0.82

indicating Anoxic waters

Ratio 0.46-0.60For Dysoxicconditions

C D EB

JF G H I

A

Shales SST intercalations Shales SST intercalationsShales SST intercalations

Shales SST intercalationsShales SST intercalations

Shales SST intercalations

Shales SST intercalationsShales SST intercalations

Shales SST intercalationsShales SST intercalations

Figure 5: Geochemical profile of the Aralla section - Graphs A - J represent the elemental concentrations (in ppm) against stratigraphy (base of the shales to the left of the graphs, the sandstone intercalations apparent at the top of the section can be seen to the right of the graphs). Graph A represents the vanadium ppm values the cylicity is clearly evident (redox sensitive) and corresponds to the eight cycles seen in Figure 1. B shows the titanium values again the eight cycles are clearly evident. The peaks in Iron (graph C) do not correlate to the sulphur peaks (graph E) suggesting that the signal is representing mineralisation events (hydrothermal activity). The contrast between the shale sedimentation and that of the sandstone intercalations seen in graph H is drastic - showing the impact of the continental detrital influx.

Figure 4: Overview photo of the Aralla section clearly showing the boundaries of the underlying Barrios Fm. and the overlying San-Pedro Fm. The yellow line on overview photo represents the Aralla log section. Top right displays the nature of the contact between the underlying Barrios Fm. and the Formigoso Fm. - this boundary marks the begining of the Aralla log section seen in Figure 1. The photo in the bottom right of the figure displays the mono-graptids found within the basal Formigoso Fm. (compass clino scale (4 inches).

Boundary photo looking east

Formigoso Formation

San Pedro Formation

Barrios Formation

Formigoso Formation

Barrios Formation Getino Bed

N S N S

Over view photo looking east

1 2 3CM

4 5 6 7 8 9

0 1 2 3 4 5 6 7

K [Wt%]

Th

[p

pm

]

0

5

10

15

20

25

30

Heavy

Thorium

-Bearing M

inera

ls

Kaolinite

Mon

tmor

illonite

Mixed Layer Clays

Ilite

Micas

GlauconiteFeldspar

Potassium Evaporates

Th/K = 0.5

Th/K = 0.6

Th/K = 2

Th/K = 3.5

Th/

K =

12

Th/K

= 2

8

1

2

3

4

1

2

3

4

~ 70% Ilite Line

~ 40% Micas Line

~ 30% Glauconite Line

~ 30% Feldspar Line

Aralla shale composition Th plotted against K

Figure 6: ‘hot’ shales showing gamma ray and TOC, modified after

Luning (2000)

0.2 0.4 0.6 0.8 1

Sh

ale

sS

ST

Ratio > 0.84 signifying Euxinic

conditions

Ratio 0.54-0.82

indicating Anoxic waters

Ratio 0.46-0.60For Dysoxic -

oxicconditions

0 0.5 1 1.5 2 2.5 3

Th/U

Ratio =

2

SS

TS

ha

les

Th/U Ratio

Figure 12: Th/U Ratios for Aralla, any value <2 implies anoxic, organically rich black shale genesis modified after Fertl 1979.

1

2

3

4 56 7

8

12 3

4 5 6 7 8

Cycle

2C

yc

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cle

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ycle

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Cy

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8Aralla Log: , ,

+ TOC U Th, K γ - Ray

Ho

t S

hale

200 API

0 50 100 150 200 250

γ - Ray APITh ppm0 5 10 15 20 25

U ppm

0 5 10 15 20

K ppm

0 1000

2000

3000

4000

5000

6000

7000

1 m

0 1 2 3 4 5 6

TOC Wt%

Figure 1: Aralla log section showing the concentrations and signals of U, Th, K, γ - Ray and TOC. The cylicity is highlighted

3 Wt%

0 m base

69.25 m top

Ara

lla lo

g s

ect

ion g

eoch

em

istr

y as

belo

w c

om

pre

sse

d to

cle

arly

rep

rese

nt th

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yclic

ity

Geochemical cyclicity of the Formigoso Fm. at Aralla

1

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3

4

5

6

7

8