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Current-aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso-arenacea Formation

Current-aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso-arenacea Formation
Current-aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso-arenacea Formation
Turbidite sandstones of the Miocene Marnoso-arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0.5 to 2.0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium-grained and fine-grained sandstone beds composed of crude to well-defined stratified facies (including corrugated, hummocky-like, convolute, dish-structured and dune stratification) and are aligned sub-parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic-related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineation formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro-topography compared to classic primary current lineation and are not associated with planar-parallel, finely-laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineation, but under relatively high near-bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water-escape following the primary depositional grain fabric. For the Marnoso-arenacea beds, sheet orientation may be genetically linked to the enhanced primary current lineation structures. Current-aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction-dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher-concentration grain or hindered settling dominated regimes.
Dewatering, palaeoflow, sedimentary structures, turbidite, upper stage plane bed
0037-0746
1260-1279
Amy, Lawrence A.
857f581c-1529-424c-a948-91ab2fd476eb
Talling, Peter J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Sumner, Esther J.
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Malgesini, Giuseppe
a3728cb8-1570-471b-b6d1-306d3b683b8a
Amy, Lawrence A.
857f581c-1529-424c-a948-91ab2fd476eb
Talling, Peter J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Sumner, Esther J.
dbba4b92-89cc-45d9-888e-d0e87e5c10ac
Malgesini, Giuseppe
a3728cb8-1570-471b-b6d1-306d3b683b8a

Amy, Lawrence A., Talling, Peter J., Sumner, Esther J. and Malgesini, Giuseppe (2016) Current-aligned dewatering sheets and ‘enhanced’ primary current lineation in turbidite sandstones of the Marnoso-arenacea Formation. Sedimentology, 63 (5), 1260-1279. (doi:10.1111/sed.12260).

Record type: Article

Abstract

Turbidite sandstones of the Miocene Marnoso-arenacea Formation (northern Apennines, Italy) display centimetre to decimetre long, straight to gently curved, 0.5 to 2.0 cm regularly spaced lineations on depositional (stratification) planes. Sometimes these lineations are the planform expression of sheet structures seen as millimetre to centimetre long vertical ‘pillars’ in profile. Both occur in the middle and upper parts of medium-grained and fine-grained sandstone beds composed of crude to well-defined stratified facies (including corrugated, hummocky-like, convolute, dish-structured and dune stratification) and are aligned sub-parallel to palaeoflow direction as determined from sole marks often in the same beds. Outcrops lack a tectonic-related fabric and therefore these structures may be confidently interpreted to be sedimentary in origin. Lineations resemble primary current lineation formed by the action of turbulence during bedload transport under upper stage plane bed conditions. However, they typically display a larger spacing and micro-topography compared to classic primary current lineation and are not associated with planar-parallel, finely-laminated sandstones. This type of ‘enhanced lineation’ is interpreted to develop by the same process as primary current lineation, but under relatively high near-bed sediment concentrations and suspended load fallout rates, as supported by laboratory experiments and host facies characteristics. Sheets are interpreted to be dewatering structures and their alignment to palaeoflow (only noted in several other outcrops previously) inferred to be a function of vertical water-escape following the primary depositional grain fabric. For the Marnoso-arenacea beds, sheet orientation may be genetically linked to the enhanced primary current lineation structures. Current-aligned lineation and sheet structures can be used as palaeoflow indicators, although the directional significance of sheets needs to be independently confirmed. These indicators also aid the interpretation of dewatered sandstones, suggesting sedimentation under a traction-dominated depositional flow – with a discrete interface between the aggrading deposit and the flow – as opposed to under higher-concentration grain or hindered settling dominated regimes.

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Accepted/In Press date: 16 December 2015
e-pub ahead of print date: 28 April 2016
Published date: August 2016
Keywords: Dewatering, palaeoflow, sedimentary structures, turbidite, upper stage plane bed
Organisations: Geology & Geophysics, Marine Geoscience

Identifiers

Local EPrints ID: 387368
URI: http://eprints.soton.ac.uk/id/eprint/387368
ISSN: 0037-0746
PURE UUID: 12ca54dc-11be-4738-bfe8-fb60ba56ee9b

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Date deposited: 12 Feb 2016 16:14
Last modified: 18 Nov 2019 20:08

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