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Facies Relationships in Sandstone Intrusions: Examples From the Numidian Flysch, Northern Tunisia
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DOI：10.1111/bre.70116.png)
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Abstract
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The Tabarka Injection Complex (TIC) consists of sandstone intrusions sourced from slope sandstone channels and emplaced into deep-marine mudstone and siltstone (Oligo-Miocene) of the Numidian Flysch, Northern Tunisia. The internal and external architecture of these sandstones is extremely complex due to the range of sand remobilisation and intrusion processes that modify depositional units and generate sandstone intrusions. Depositional facies record deposition from gravity flows in sand-rich slope channel complexes. Remobilised facies demonstrate post-depositional dewatering and fluidisation in the parent sandstones with abrupt lateral and vertical transitions into intrusive sandstones emplaced into hydrofractures in host mudstones. Surface marks developed along the walls of intrusions and other sedimentary structures in sandstone intrusions are interpreted to document high velocity erosive sand-laden turbulent flows and the transition to higher concentration laminar flow. Post-injection, sandstone intrusions were modified by dewatering, compaction and local bioturbation. The TIC is 50 m thick and is divided into five stratigraphically constrained units: (i) units 1 to 3 are 4 to 18 m thick and comprise turbiditic channel sandstone, which transition upward and toward the channel margins into extensively de-watered and remobilised sandstone, which in turn feed sandstone dykes and sills; (ii) unit 4 and 5 are 12 and 15 m thick and contain mostly sandstone intrusions that form an intricate complex of sills and dykes (up to 1.2 m thick). Previous outcrop studies on the origin of sandstone intrusions in the Numidian Flysch proposed syn-depositional downward injection or early post-depositional liquefaction and intrusion into adjacent host mudstone. Our data, instead, indicate that the sandstone intrusions formed by post-depositional upward and lateral injection (10's of meters) of fluidised sand from shallowly buried slope channels into the adjacent host mudstone, most likely in two or three injection events. The presence of sandstone sill-dominated units confirms that supra-lithostatic pore-fluid pressure (Pf) was attained in the parent sandstone during early burial and dyke strike orientation was controlled by the regional stress field related to the Magrhebian compression.
Keywords:
flow regime
fluidisation structures
hydrofracturing
overpressure
post-depositional deformation
sand fluidisation
sandstone intrusions
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