Circular Structures in SE Libya

8/8/2019 Circular Structures in SE Libya

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CircularCircular StructuresStructures inin SESE LibyaLibya



OriginOrigin of of circularcircular structuresstructures

EndogenicEndogenic mechanismsmechanisms::

MagmatismMagmatism

((JabalJabal UwaynatUwaynat,, JabalJabal ArkenuArkenu))

Salt/ Salt/ GypsumGypsum diapirismdiapirism

KarstKarst dissolutiondissolution

GlacialGlacial erosionerosion

ImpactImpact originorigin

HighHigh--pressure shock waves (50pressure shock waves (50--100100GPaGPa)) generate high temperaturesgenerate high temperatures

(>1500(>1500--20002000°°C)C)



Definite Indicators of ImpactDefinite Indicators of Impact• Shatter cones

• Planar Deformation Features

• High-Pressure Diaplectic Glass

glassy phase (isotropic in polarized light)

that is produced by shock • High-Pressure Minerals

decomposition of zircon (ZrSiO4) to

baddeleyite (ZrO2) (T>1850°C) and the

melting of sphene (CaTiSiO5) (T>1400°C). High-Temperature Glasses

melting of quartz (SiO2) to a glass

(lechatelierite) at temperatures above 1750°C

Meteorite Fragments

Chemical Signatures from the Projectile

The element iridium (Ir) is probably the

best-known impact indicator because of its

importance in recognizing the Cretaceous-

Tertiary impact event, but other elements (e.g.,Os, Au, Cr) have also been use



TypicalTypical samplessamples of of shattershatter conescones

Quartzite from Vredefort impactSouth Africa

Dolomite shatter cones fromKentland quarry, Indiana USA



Arkenu Arkenu 11 -- Arkenu Arkenu 22 structuresstructures



Arkenu Arkenu 11 -- Arkenu Arkenu 22 structuresstructures were identified by airborne geophysical

survey in 1973, and designated as R33 andR34A

Location: 110 km west from Jabal Arkenu,250 km southof Kufrah

22°04’ N , 23° 45’ E

Flat area covered aeolian deposits andQuaternary soils with a NE-SW trendingdunes crossing through

The total relief is less than 20 m

Diameters: Southwestern (Arkenu 2) 10,3 km

Completely flat, hardly visible crater rims(possibly three concentric annular ridges)

Northeastern (Arkenu 1 ) 6,8 kmInner and outer rims are similar to the

Aorounga crater in Chad.

Host rock is coarse grained to conglomeatic

sandstone of Lower Crtaceous age withplant fossils and thin shale interbeds.



Arkenu Arkenu 11 -- Arkenu Arkenu 22 structuresstructures

Landscape as seen from the center of the northeastern crater, facing tothe northeast. Horizon corresponds to the inner ridge of the crater.

The structure in the middle of the image is an outcrop of breccia close tothe crater center. P.Paillou 2003 April



DifferentDifferent opinionsopinions of of originorigin

Philippe Paillou et al.

2003 site investigattion

Proof of impact origin

Shatter cone structures

Impact Breccias

Recrystallized region with

subisotropic area (up)

Planar Deformation in the

quartz grain in the breccia

(down)



Carbonatite plug origin

supported by Dr M.Baegi from the Libyan Remote Sensing Centre Libya

„Hunting, in 1974 was the first to report the presence of a carbonatitic bodyassociated with the radiometric and magnetic high. Such association hasinvoked interest in these structures. Detailed investigation by C.R.G 1980,reported presence of distinctive abundance of apatite, magnetites, rare earth

elements, nobium, uranium ,thorium and zirconium and document theircarbonatitic origin.”

„The presence of numerous gas tubes, zoned complexes of alkaline igneous

rocks, kaolinized syanites, bleaching zones, magnetite minerals which producesthe positive magnetic anomalies at the central parts of the plugs, and presenceof many volcanic plugs as well as small carbonatite plugs in the surroundingarea that are following a tectonic trend, in NE direction confirm the carbonatite

origin.”




NON-IMPACT ORIGIN OF THE ARKENU CRATERS (LIBYA), 2007By M. Di Martino INAF-Osservatorio Astronomico di Torino

C. Cigolini Dipartimento di Scienze Mineralogiche ePetrologiche, Università di Torino

L. Orti Dipartimento di Scienze della Terra,

Università di Firenze


November 2007

Preliminary geologicaland structural survey.

Petrographic andtextural observations

- no PDF in the collected quartz grains

- shatter cones have been likely produced

by the erosion of sandstones which are

oriented by the dominant winds ( “pseudo-

shatter cones”).

-concretions of diagenetic hematite

-(from mm to 10xcm size).

-magnetite deposits (diagenetic hematite

recrystallized as magnetite),

Suggestion: subvolcanic intrusive body

digisted the sandstone



NON-IMPACT ORIGIN OF THE ARKENU CRATERS (LIBYA), 2007

By M. Di Martino INAF-Osservatorio Astronomico di Torino

C. Cigolini Dipartimento di Scienze Mineralogiche e

Petrologiche, Università di TorinoL. Orti Dipartimento di Scienze della Terra,Università di Firenze

„We, therefore, suggest the craters forming episode is the result of intrusion of apaired, nearly cylindrical subvolcanic stocks (coupled with ring dike injectionin the surroundings) accompanied by hydrothermal degassing. This process wasthen followed by local structural adjustments, likely due to thermal contractionof the whole edifices along circular fractures.

This produced moderate folding and subsidence of the “crater sectors” (currentlydelimited by the crater rims) as well as the origin of the outer circularstructures. Erosion did its cycle and finally revealed the architecture we arenow observing.”




JebelJebel HadidHadid structurestructure

20°52’12.30”N; 22°42’27.47”



JebelJebel HadidHadid structurestructure

StructureStructure descriptiondescription::

LocationLocation:: Jef Jef Jef Jef PlateauPlateau onon thethe westwest andand ErgErg

IdrisiIdrisi onon thethe easteast

DiameterDiameter: 4,7 km: 4,7 km

HeightHeight: 625: 625--630 m630 m aboveabove seasea levellevel

FiveFive concentricconcentric annularannular ridgesridgesNo terrain relief No terrain relief indicationindication onon SRTMSRTM

HostedHosted byby postpost HerzynianHerzynian NubiaNubia SandstoneSandstone

((LowerLower CretaceousCretaceous))

CrossedCrossed byby a NEa NE--SW trendingSW trending fluvialfluvial valleyvalleyconnectedconnected toto thethe EosahabiEosahabi paleoriverpaleoriver systemsystem

((latelate MioceneMiocene-- PliocenePliocene))



HydrocarbonHydrocarbon PotentialPotential

TwoTwo main playmain play conceptconcept

1.1. PaleozoicPaleozoic playplay

SourceSource:: SilurianSilurian hothot shalesshales((TanezzuftTanezzuft F.F.))

ReservoirReservoir::

CambroCambro--OrdovicianOrdovician SandstonesSandstones

((HassawnaHassawna F.)F.)UpperUpper OrdovicianOrdovician glaciogeneticglaciogenetic

depositsdeposits ((MemuniatMemuniat F. )F. )

SilurianSilurian--CarboniferousCarboniferous sedimentssediments

((AkakusAkakus F.,F., TadrartTadrart F.,F., BinemBinem F.,F.,

Dalma F.)Dalma F.)

CretaceousCretaceous sandstonessandstones ((NubianNubian F.)F.)



HydrocarbonHydrocarbon PotentialPotential

2.2. InfracambrianInfracambrian playplay

Black Black shalesshales andand organicorganic--richrichlimestoneslimestones

Peak Peak oiloil generationgeneration::

LateLate TriassicTriassic--EarlyEarly CretaceousCretaceous

ThicknessThickness of of thethe sedimentssediments bybyseismicseismic explorationexploration ininSouthern KufrahSouthern Kufrah BasinBasin

NubianNubian toto recentrecent series 2,5 kmseries 2,5 kmUpperUpper palaeozoicpalaeozoic 0,8 km0,8 km

CambroCambro--OrdovicianOrdovician 2,5 km2,5 km



NonameNoname structurestructure

24°32’45.57”N ; 21°07’59.47”E

StructureStructure descriptiondescription

ElongatedElongated circlecircle shapeshape

2,4 km2,4 km largestlargest diameterdiameter

1,7 km1,7 km smallestsmallest diameterdiameterLocatedLocated InIn aa hillsidehillside,, followingfollowingthethe generalgeneral terrainterrain elevationelevation, no, nocentralcentral upliftuplift ((erodederoded))

FieldField investigationinvestigation shallshall evaluateevaluatethethe structurestructure



PaleoriverPaleoriver systemsystem inin KufrahKufrah BasinBasin

DataData werewere obtainedobtained byby RadarsatRadarsat--11 usingusing aaSyntheticSynthetic ApertureAperture Radar (SAR)Radar (SAR)singlesingle frequencyfrequency 5,7 cm5,7 cm wavelengthwavelength

horizontallyhorizontally polarizedpolarized CC--bandband..

PenetrationPenetration of of thethe radarradar waveswaves dependdepend onontwotwo factorsfactors inin desertdesert conditionsconditions::

electricalelectrical propertiesproperties ((dielectricdielectric constantconstant,,determineddetermined byby thethe moisturemoisture contentcontent))

MoistMoist soilssoils reflectreflect thethe radarradar waveswaves,, wetwetonesones absorbabsorb themthem

GrainsizeGrainsize of of thethe reflectingreflecting surfacesurface

PenetrationPenetration isis possiblepossible if if thethe drydry surfacesurface

grainsizegrainsize isis lessless thanthan 1/5 of 1/5 of thethewavelengthwavelength andand physicallyphysicallyhomogeneoushomogeneous

I t t tiInterpretation fof ththe d iradar image



InterpretationInterpretation of of thethe radar imageradar image

DescriptionDescription ofof thethe megasystemmegasystem



DescriptionDescription of of thethe megasystemmegasystem

The western system extends south into Chad and extends for 317 km

Drainage channel exits to the north towardTazerbo.

Eastern channel is over 100 km.

Two large lake systems, one 79.2 km by 79.6 km,6304 km2

the other 37.3 km by 141 km and covering 5260km2 in area.

This lake extends east beyond the scene border inthis

Both show elongation along an ENE trend.

Smaller tributaries are represented as single lines.

Channel widths average 7–8 km and the length of

the Ttopography in this area decreases northward

consistent with the direction of Wadi Kufra.

Topographical variations along the main channel

path are ranging from 650 m down to 300 m

I t t tiI t t ti ff thth SRTM iSRTM i



InterpretationInterpretation of of thethe SRTM imageSRTM image

The SRTM imageThe SRTM image clearlyclearly showsshows thetheterrain of terrain of thethe basinbasin delineatingdelineating thethe

riverriver branchesbranches

TheThe photogeologicalphotogeological mapmap showingshowing

thethe aeolianaeolian depositsdeposits inin lightlight colorcolor

corresponscorrespons withwith thethe SRTMSRTM

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Circular Structures in SE Libya