Daniel P. Le Heron

Calculating ice volumes and ice flux to constrain the dimensions of a 440 Ma North African ice sheet

Abstract: Geological evidence indicates that the Sahara was glaciated by a large ice sheet c. 440 Ma ago. Two end-member theories have been proposed for the large-scale configuration of this ice mass. In the large ice-sheet hypothesis, the Sahara was covered by a continuous ice sheet that straddled North Africa–Arabia, South Africa and South America. In the small ice-sheet hypothesis, it has been argued that separate ice sheets developed in each of these areas. Taking these end-members and an ice sheet of intermediate dimensions, we investigate this problem, calculating likely ice volumes and the flux of ice to the ice margin. We find that the small ice sheet adequately accounts for the estimated magnitude of post-glacial transgression (c. 45–80 m) associated with ice-mass decay. Additionally, the notion of a North African–Arabian ice sheet within the small ice-sheet hypothesis is supported by an analysis of ice-stream fluxes across the Sahara, accounting for widespread heterogeneous flow within the ice sheet. This paper provides an independent check on the likely size and extent of Late Ordovician ice masses, with potential application to other Palaeozoic ice-sheet reconstructions.

Field-based investigations of an ‘Infracambrian’ clastic succession in SE Libya and its bearing on the evolution of the Al Kufrah Basin

Abstract Field-based investigation of ‘Infracambrian’ rocks cropping out on the eastern flank of Al Kufrah Basin (area 500 000 km2) reveals a an approximately 500 m-thick clastic succession of massive and cross-bedded sandstones, separated by 60 m-thick mudrock intervals. New zircon age data indicate a maximum age of deposition of approximately 950 Ma; furthermore, the absence of zircons of Pan-African age suggests a minimum depositional age older than the Pan-African Orogeny. Previously unreported folding and spaced cleavage affects these deposits to produce a pronounced NE–SW-striking tectonic grain that is interpreted to result from NW–SE-directed orthogonal compression during the Pan-African Orogeny. These Infracambrian rocks are therefore unlikely to be suitable analogues for weakly deformed strata shown to exist beneath the Cambro-Ordovician strata of the Al Kufrah Basin. Earlier work mapped a series of Infracambrian marble outcrops along strike of the clastic deposits; thin section petrography reveals that some of these are basic igneous rocks metamorphosed to greenschist facies. Interpretation of gravity data over the Al Kufrah Basin shows NE–SW-striking faults, parallel to outcrop structures, and secondary NW–SE faults. The data do not support earlier interpretations of a rhomboidal geometry in the deep subsurface of the basin, which has previously been attributed to strike-slip (pull-apart) processes. This research impacts on earlier suggestions that the Al Kufrah Basin opened as one of a series of en echelon pull-apart basins situated along a 6000 km-long shear zone known as the Transafrican Lineament, stretching from the Nile to the Niger Delta.

Neoproterozoic ice sheets and olistoliths: multiple glacial cycles in the Kingston Peak Formation, California

The Kingston Peak Formation is a diamictite-bearing succession that crops out in the Death Valley region, California, USA. An exceptionally thick (>1.5 km) outcrop belt in its type area (the Kingston Range) provides clear insights into the dynamics of mid-Cryogenian (‘Sturtian’) ice sheets in Laurentia. Seven detailed logs allow the lateral and vertical distribution of facies associations to be assessed. We recognize (1) diamictite facies association (ice-proximal glacigenic debris flows), (2) lonestone-bearing facies association (ice-marginal hemipelagic deposits and low-density gravity flows with iceberg rafting), (3) pebble to boulder conglomerate facies association (ice-proximal cogenetic glacigenic debris flows and high-density turbidites), (4) megaclast facies association (olistostrome and hemipelagic sediments subject to ice-rafting), and (5) interbedded heterolithics facies association (low-density turbidites and hemipelagic deposits). The stratigraphic motif allows three glacial cycles to be inferred across the range. Ice-minimum conditions interrupting the Kingston Peak succession are associated with the development of an olistostrome complex, succeeded by a thick accumulation of boulder conglomerates deposited during ice readvance. The data testify to a strong glacial influence on sedimentation within this ancient subaqueous succession, and to highly dynamic ice sheet behaviour with clear glacial cycles during the Sturtian glaciation.

Depositional architecture and sequence stratigraphic correlation of Upper Ordovician glaciogenic deposits, Illizi Basin, Algeria

Abstract Upper Ordovician glaciogenic deposits are profoundly important as hydrocarbon reservoirs across North Africa, such as within the Illizi Basin of SE Algeria. In this study we present a new sedimentological and sequence stratigraphic model for Upper Ordovician glaciogenic deposits based on the analysis of core descriptions and wireline logs from 25 wells in the Tiguentourine Field. Within the glaciogenic succession, two ice advance–retreat cycles can be defined, consisting of glaciomarine ice-contact fan deposits and tillites. Deposits of the marine ice-contact fan systems generally show a retrogradational stacking pattern from ice-proximal to ice-distal deposits. This pattern is attributed to the deposition in front of a retreating ice sheet. The proximal marine ice-contact fan deposits consist of massive or low-angle cross-bedded pebbly sandstone. They are interpreted as the deposits of turbulent, high-energy plane-wall jets, emerging from subglacial meltwater conduits. These jet-efflux deposits are up to 60 m thick and interbedded with deposits of cohesive and non-cohesive debris flows. The jet-efflux deposits are overlain by fine-grained, thick-bedded massive sandstone. These mid-fan deposits build up the bulk of the glaciomarine fans and are interpreted as deposits of underflows, generated at the point of flow-detachment, where marine meltwater jets become buoyant and large volumes of sediment fall-out from suspension. In the upper part of the fan succession massive sandstones pass upwards into mud-prone massive sandstones, interpreted as deposits of cohesive sandy debris flows. The most ice-distal deposits are muddy sandstones and mudstones deposited by waning low-density turbulent flows and suspension fall-out. The best reservoir properties within the glaciogenic succession are attributed to the proximal and medial deposits of the ice-contact fans such as coarse-grained jet-efflux deposits and sustained high-density turbulent flow deposits. However, the mud content within the massive sandstones is highly variable and influences the reservoir quality. Both glacial depositional sequences infill 60–175 m deep, elongated depressions, which are interpreted as subglacial tunnel valleys. These tunnel valleys acted as depocentres for the glaciomarine fan deposits. After final deglaciation and post-glacial transgression, organic-rich shale was preferentially deposited in underfilled tunnel valleys.

Did lingering ice sheets moderate anoxia in the Early Palaeozoic of Libya

The Hirnantian glaciation of West Gondwana produced a glacially sculpted topography, which is draped by organic-rich latest Ordovician and early Silurian ‘hot shales’. Although these are the most important Early Palaeozoic source rock in North Africa, organic enrichment is distributed unevenly. For example, in Al Kufrah Basin, Libya, ‘hot shales’ are elusive, but outcrop analysis at the western basin demonstrates why this is the case. The topmost Mamuniyat Formation, of Hirnantian age, comprises glaciogenic sandstones, passing upward into mixed facies of the Tanezzuft Formation, which has a latest Ordovician–early Silurian age. The basal Tanezzuft Formation contains a shelly carbonate (cool-water deposits accumulated under oxygenating conditions) and bioturbated sandstone succession. Above, hummocky cross-bedded and graded sandstone intervals are intercalated with shale and siltstone (storm influx onto a muddy shelf). These are interrupted by several lonestone-bearing intervals (ice-rafted debris), a striated pavement (of subglacial origin), and manganese oxide crusts and concretions. The concretions and bioturbation imply oxygenation of the sea floor during transgression. These putative glacial deposits were deposited following the main phase of the Hirnantian glaciation, at the same stratigraphic level as ‘hot shales’ elsewhere in northern Gondwana. Lingering ice caps may have produced well-oxygenated marine waters precluding ‘hot shale’ deposition.

Normalograptus kufraensis, a new species of graptolite from the western margin of the Kufra Basin, Libya

Normalograptus kufraensis sp. nov. occurs as monospecific assemblages in the Tanezzuft Formation at the western margin of the Kufra Basin (Jabal Eghei), southern Libya. These graptolites have parallel-sided rhabdosomes with long, straight virgellae, climacograptid thecae and a full straight median septum. N. kufraensis is intermediate between Ordovician graptolites from the N. angustus (Perner) lineage and the younger sister species N. ajjeri (Legrand) and N. arrikini Legrand. N. kufraensis differs from these taxa as follows: it is broader than N. angustus; it has greater thecal spacing than N. ajjeri or N. arrikini. A table comparing measurements of N. kufraensis with 44 other Normalograptus taxa differentiates it from other members of this morphologically conservative group. Even though N. angustus and N. ajjeri are very long-ranging graptolites, a stratophenetic approach suggests that the specimens from Jabal Eghei may be of late Hirnantian or younger age. The faunal composition and preservation suggests these graptolites occupied the ‘cratonic invader’ biotope. The stratigraphic succession records deglacial flooding and fluctuating of redox in the Tanezzuft Formation, with the graptolites indicating a short-lived interval of anoxia.

Neoproterozoic deglacial sediments and their hydrocarbon source rock potential

Abstract Many Neoproterozoic successions contain viable hydrocarbon source rocks, even though they were deposited before most extant life forms evolved. Eukaryotic microalgae, bacteria, chlorophyte micoalgae, marine pelagophyte algae and dinoflagellates may have contributed organic matter. Major global-scale glaciations, which are commonly attributed to a ‘snowball’ or ‘slushball’ Earth scenario, or deposited under a ‘zipper rift’ scenario, are believed to have played an important role in the deposition of hydrocarbon source rocks during the mid- Neoproterozoic (Cryogenian). Phases of Cryogenian deglaciation may have culminated in the deposition of high total organic carbon shales and ‘cap carbonates’ in restricted anoxic basins, which may have been carved by ice sheets themselves or, alternatively, formed as restricted extensional half graben as Rodinia began to fragment. One example of these organically enriched deglacial sediments comprises shales and dolostones deposited following the Sturtian glaciation in the Centralian Superbasin of Australia, an amalgam of basins that extends almost continent-wide across Australia. Data from the Marmot MMDD-1 drill core on the Stuart Shelf in the southern part of the Centralian Superbasin, together with previously published data on organic enrichment in the Amadeus Basin in the central part of the Centralian Superbasin, suggest that the deposition of organically enriched shales was widespread during the Sturtian deglaciation.

Microbial carbonates in space and time: introduction

DAN BOSENCE1*, KATHRYN GIBBONS2, DANIEL P. LE HERON1, WILLIAM A. MORGAN3, TIM PRITCHARD4 & BERNARD A. VINING1,5 Department Earth Sciences, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK Nexen Petroleum UK Ltd., Prospect House, 97 Oxford Road, Uxbridge, Middlesex, UB8 1LU, UK Morgan Geoscience Consulting LLC, 132 W. Ellendale Estates Drive, Houma, LA 70360, USA bg Group, 100 Thames Valley Park Drive, Reading, Berkshire, RG6 1PT, UK Baker Hughes, Bentley Hall, Alton, Hampshire, GU34 4PU, UK

Interpretation of Late Ordovician glaciogenic reservoirs from 3-D seismic data: an example from the Murzuq Basin, Libya

Understanding the recessional behaviour of ancient pre-Cenozoic ice sheets based on seismic reflection studies is generally difficult through scarcity of data. In North Africa, however, hydrocarbon exploration has produced high quality seismic reflection datasets that permit an analysis of the morphology and internal sedimentary architecture of incisions of Late Ordovician age related to the Hirnantian glaciation. Analysis of a high-resolution 3-D seismic dataset covering a small area in western Libya (the N Murzuq Basin) reveals a sharply defined, WNW–ESE-oriented palaeo-escarpment, with a higher (cliff-forming) western margin and a lower (basin-forming) eastern margin. The palaeo-escarpment defines the western flank of a sub-basin extending up to 60 km in width, known as the Awbari Trough. The escarpment and the trough are interpreted as the morphological expression of a major unconformity dividing pre-glacial sediments below from Late Ordovician (?Hirnantian) glacially related sediments above. Two hypotheses are considered for the origins of both the escarpment and the Awbari Trough: (1) as a tectonic feature such as a half graben that was active during sedimentation and (2) a glacially related palaeotopography, with the latter interpretation preferred, owing to the lack of evidence for syn-sedimentary fault activity. The width of the Awbari Trough compares to the large-scale cross-shelf troughs in modern high latitude settings, such as the Barents Shelf, produced by ice streams. The Awbari Trough was progressively filled in by gravity flow deposits throughout the course of the glaciation, until the initial incision became filled in with sediments during an overall glacial retreat phase and ceased to influence sedimentation patterns. Glacial re-advance across the basin produced a second unconformity observed in seismic data. Above this unconformity, meltwater processes incised a shallow (~ 20 m) and wide (~ 5 km) subglacial tunnel valley. Stabilization of the ice front prior to its ultimate retreat resulted in the deposition of a delta complex prior to the Early Silurian transgression.

An exhumed Paleozoic glacial landscape in Chad

In northern Chad, an outcrop belt of Paleozoic rocks occurs in the Ennedi-Bourkou range. There, satellite image interpretation reveals a series of clearly expressed paleo–ice stream pathways, which are encased in sandstone plateaux. At least five paleo–ice stream pathways are recognized, measuring 5–12 km wide. Each contains well-expressed belts of mega-scale glacial lineations (MSGLs) with occasional drumlins. The paleo–ice stream tracks are confined to present-day low-lying areas, representing ancient valley networks, and have sinuous geometries. The features occur on multiple plateau and/or stratigraphic levels. Their dissection by late Neogene rivers discounts a modern-day origin as eolian features, and offset suites of MSGLs by east-west–striking faults confirms their geologic antiquity. The paleo–ice stream pathways appear to have drained a newly discovered late Paleozoic paleo–ice sheet of probable Visean age that flowed northward toward presentday Libya, with an estimated <250-m-thick tidewater ice margin. This discovery has wide-ranging implications, increasing the known extent of late Paleozoic ice sheets, and potentially their effects on sea-level changes.