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The Upper Ordovician Glaciation in Sw Libya – a Subsurface Perspective

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The Upper Ordovician Glaciation in Sw Libya – a Subsurface Perspective J.C. Gutiérrez-Marco, I. Rábano and D. García-Bellido (eds.), Ordovician of the World. Cuadernos del Museo Geominero, 14. Instituto Geológico y Minero de España, Madrid. ISBN 978-84-7840-857-3 © Instituto Geológico y Minero de España 2011 ICE IN THE SAHARA: THE UPPER ORDOVICIAN GLACIATION IN SW LIBYA – A SUBSURFACE PERSPECTIVE N.D. McDougall1 and R. Gruenwald2 1 Repsol Exploración, Paseo de la Castellana 280, 28046 Madrid, Spain. [email protected] 2 REMSA, Dhat El-Imad Complex, Tower 3, Floor 9, Tripoli, Libya. Keywords: Ordovician, Libya, glaciation, Mamuniyat, Melaz Shugran, Hirnantian. INTRODUCTION An Upper Ordovician glacial episode is widely recognized as a significant event in the geological history of the Lower Paleozoic. This is especially so in the case of the Saharan Platform where Upper Ordovician sediments are well developed and represent a major target for hydrocarbon exploration. This paper is a brief summary of the results of fieldwork, in outcrops across SW Libya, together with the analysis of cores, hundreds of well logs (including many high quality image logs) and seismic lines focused on the uppermost Ordovician of the Murzuq Basin. STRATIGRAPHIC FRAMEWORK The uppermost Ordovician section is the youngest of three major sequences recognized widely across the entire Saharan Platform: Sequence CO1: Unconformably overlies the Precambrian or Infracambrian basement. It comprises the possible Upper Cambrian to Lowermost Ordovician Hassaouna Formation. Sequence CO2: Truncates CO1 along a low angle, Type II unconformity. It comprises the laterally extensive and distinctive Lower Ordovician (Tremadocian-Floian?) Achebayat Formation overlain, along a probable transgressive surface of erosion, by interbedded burrowed sandstones, cross-bedded channel-fill sandstones and mudstones of Middle Ordovician age (Dapingian-Sandbian), known as the Hawaz Formation, and interpreted as shallow-marine sediments deposited within a megaestuary or gulf. Sequence CO3: Is the uppermost Ordovician section assigned in Libya to the Melaz Shugran, Mamuniyat and Bir Tlacsin formations. In most cases subsurface biostratigraphy confirms a possible late Katian to Hirnantian age for this sequence. As a whole, the uppermost Ordovician package is bounded at the base by a major unconformity defining a series of paleovalleys; ranging in width from ca. 1km to >20km, and remnant Mid-Ordovician palaeohighs. Given the glacial context, it is generally assumed that erosion was associated both with tunnel valleys and ice streams. 347 N.D. McDougall and R. Gruenwald A B Figure 1. Upper Ordovician palaeovalleys in (A) outcrop for comparison; Iherir, Tassili N Ajjers, SE Algeria and (B) in subsurface (3d seismic data) from the central Murzuq Basin of SW Libya. Both vertical and lateral (along strike and downdip) facies changes are typically rapid resulting in a complex of glacially-influenced fluvial to glaciomarine environments. The whole succession is terminated by a major post-glacial flooding event, in the earliest Silurian (Llandovery-Rhudanian) during which graptolitic shales were deposited across the region forming a sealing horizon and, locally, organic-rich source rocks. In most cases the late Ordovician glaciogenics can be subdivided into several distinctive packages (McDougall and Martin, 2000; Ghienne et al., 2003), each bounded by Type I unconformities and are effectively a higher order depositional sequence recognizable across the entire area and associated with a series of glacial advances and retreats. Figure 2 summarizes this basic stratigraphic subdivision, as derived from both outcrop and subsurface, by reference to five genetic packages; known as Melaz Shugran, Lower Mamuniyat, Middle Mamuniyat, Upper Mamuniyat and Bir Tlacsin. 348 ICE IN THE SAHARA: THE UPPER ORDOVICIAN GLACIATION IN SW LIBYA – A SUBSURFACE PERSPECTIVE SEDIMENTOLOGY A B This paper is based on the study of significant volumes of seismic data, cores, image logs and conventional wireline logs interpreted within the framework of fieldwork from several key outcrops across the area of SW Libya, notably the Qarqaf Arch and Ghat-Tikiumit areas. Melaz Shugran This is a mud-prone, often het- erolithic unit, locally >250m thick; comprising several unconformity- bound subunits, termed MS1, MS2 and MS3 possibly ranging in age from late Katian to early Hirnantian. Out- crop observations, coupled with both core and extensive image log analysis show this package to comprise sever- al key facies types; (1) Massive, deformed sandy to pebbly mudstones, (2) Thick, generally fine-grained, rela- tively clean sandstones with channel Figure 2. (a) Summary stratigraphic column for the Upper Ordovician in SW geometry and (3) thinly interbedded Libya and (b) wireline log signature typical of the Upper Ordovician in the fine-grained sandstones and mud- central Murzuq Basin. stones all associated with pervasive soft sediment deformation. Comparison with modern analogues suggests these facies most probably repre- sent deposition from debris flows, density underflows, turbidity currents and possibly iceberg rain-out, in morainal banks or subaqueous glaciomarine fans. In broader terms it is assumed that Melaz Shugran deposi- tion was associated with both high relative sea levels and sediment fluxes; presumably a response to a major glacial retreat following the initial platform-wide incision event. Lower Mamuniyat This major sand-dominated package, up to 150m thick, sharply overlies the underlying argillaceous sediments of the Melaz Shugran, along a regional surface interpreted from seismic data and high resolution correlations to vary from a major erosional unconformity (it may even completely truncate the Melaz Shugran resting directly on the older pre-glacial sediments of Middle Ordovician age), to a minor unconformity or diastem. Outcrop observations suggest a subdivision into: (a) a lower unit (LM1) comprising fine- to coarse-grained, even locally conglomeratic, sandstones forming channel-bar complexes of probable tidal origin; and (b) Fine grained sandstones forming sheetflood complexes (LM2) which pass laterally into climbing megaripples, associated with pro-glacial outbursts (Ghienne et al., 2010). However, 349 N.D. McDougall and R. Gruenwald this is perhaps less clear in the subsurface where many wells show abundant soft sediment deformation and locally significant downdip changes in lithology associated with deposition from density underflows and sediment flows in sand-rich glaciomarine fans. Middle Mamuniyat This distinctive heterolithic package is separated from the sand-prone Lower Mamuniyat by a major subaerial unconformity, which, in many cases, displays evidence in outcrop for glacial erosion and associated deformation (folds, step-faulting, injection structures) attributed to glaciotectonism. It records a significant episode of glacial re-incision, most probably associated with the generation of tunnel valleys, and generation of accommodation space followed by rapid post-glacial flooding and the subsequent progradation of braid-delta systems fed by the retreating ice. Initially, deposition appears to have occurred in relatively steep slope-type braid-deltas but with reduced accommodation space deposition occurred principally in lower gradient braid-delta systems. Upper Mamuniyat This sand-prone package is areally the least extensive of the Mamuniyat sequences. It tends to occur almost exclusively in the axes of relatively narrow, deep palaeovalleys. As such it is observed to erosively truncate the underlying sequences, locally resting directly on Middle Ordovician sediments, in response to a significant episode of base level fall and ice advance. The fill of the subsequent incised valleys is typically subdivided in outcrop into 3 units, bounded in proximal areas by significant unconformities: UM1: in the most proximal outcrops, comprises coarse to very coarse, pebbly and locally conglomeratic sandstones infilling palaeorelief in the form of megachannel bodies tens to hundreds of metres in width and up to 10m thick. Internally these channel bodies are typically massive with abundant mudchips or intraclasts overlain by large-scale, low angle cross-stratification. Sequence architecture and facies combine to suggest deposition from major glacial outburst events or jokulhaups. UM2: again in the most proximal areas this forms a single coarsening-upwards package, up to 50m thick, comprising poorly sorted, coarse-grained sandstones characterised by a dense deposit-feeding ichnofauna gradually replaced by medium to large-scale trough cross-bedding with marked bipolar palaeocurrent distribution. The whole assemblage records a significant base level fall, erosion of UM1 and the subsequent progradation of tidally-influenced braid-deltas or, in some cases Gilbert deltas. UM3: In outcrop the least extensive of the three subunits, this package forms anastomosing channel bodies, each tens of metres in width and up to 5m in thickness. Channel bodies are notably incised into underlying Upper Ordovician sediments in response a further fall in base level and generation of small- scale tunnel valleys. In the subsurface, correlations supported by detailed core descriptions and image logs confirm in many cases the existence of the 3-fold subdivision observed in the more proximal outcrops of the Ghat area. Grain sizes are generally finer and unit boundaries defined by the presence of mudchip-rich conglomeratic horizons. Integration of these datasets with geobody detection in 3D seismic strongly suggests
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