A sedimentologic-stratigraphic investigation of Marsdenian (Namurian R2a-b) sediments in the Central Pennines

Abstract

The Marsdenian (R2a_b) sediments constitute the third major clastic influx into the Central Pennine Basin, following the first and second influxes during the Skipton Moor Grit (Elc) and Kinderscoutian (Ri) intervals respectively. The Marsdenian influx involves three distinct delta advances during the Scotland Flags (R2a), Pule Hill (R2b) and Hazel Greave (R2) intervals. Each of these Marsdenian fills represents a roughly similar type of deltaic advance into the basin, which subsequently
undergoes abandonment and widespread transgression probably associated with eustatic sea level rise characterised by its distinct goniatite band.
In this sedimentological and stratigraphical investigation of these Marsdenian sediments in the Central Pennine Basin between Wharfedale and Longdendale, 20 lithofacies are recognized, described and interpreted in hydrodynamic terms on the basis of their internal evidence. Five Facies Associations are recognized in which lithofacies succeed one another with variable predictability.
The Deep Water Association, the lowest association recognised in only one locality in the southwest parts of the study area is some 122m thick, and involves interbeds of marine mudstone and turbidite-like sandstones of various thickness scales. The mudstones are considered to represent sourceward ponding of sediments, perhaps during sea level rise.
The Delta Slope Association, the lowest association in most other localities is a coarsening upward sequence with mudstones and siltstones dominating the lower parts, while medium-grained to occasional coarsegrained
sandstones dominate the upper parts, which show great lateral variation. Non turbiditic density current deposits associated with slumps, occur locally. Minor subaqueous extensions of the distributary channels are recognized within the Slope Association, particularly in
the upper parts. The Association is interpreted as a prograding slope (Prodelta-Mouth bar) of a fluvially dominated deltaic coastline. The turbidites, termed Prodelta Turbidites, are products of dense sedimentladen
discharge issuing from the distributary mouth bar areas during flood period and so entered the basin as hyperpycnal flows.

The Distributary Channel Association involves 2 distinct channel types, namely the Major Channels and the Sheet Channels. The Major Channels are lenticular in cross-section when they are fully exposed, while the Sheet
Channels are characterised by extensive planar erosional bases which can be traced in outcrop for hundreds of metres. The Major Channels occur in Complexes of up to 4, with a common thickness range of 3-3Oni, and some of the thick ones may be made up of smaller sized channels termed scour based sand bodies. Several types of sandstone lithofacies whose vertical order is variable occur within the Major Channels. However there is a
preferred upwards lithofacies sequence of Erosion surface - Massive Sandstone -. Large Scale Cross Bedding - Trough Cross- or Tabular Cross- Bedded Sandstone - other smaller scale cross bedding. Most of the massive beds are thought to be trough deposits of large bedforms or products of
scour troughs incised at the basal parts of a slip face of a large bedform.
The Large Scale Cross Beds are regarded as the products of alternate bars developed in rivers of low sinuosity. Dunes and sandwaves occurring in shallower parts of the channel produced the cosets of tabular cross-bedding.
Smaller bedforms superimposed on the larger bedforms during falling stage produced the erosional surfaces within the individual sets of the larger bedform. Side and/or vertical filling mechanisms operated during the infilling of the Major Channels. Most Sheet Channel fill thicknesses lie between 1 and 1Cm (commonly 2-4m) and these channels are interpreted as of low sinuosity.

The following genetic groupings of lithofacies are distinguishable in the Interdistributary Complex Association: Alternating Tabular Bedded Sandstone and Mudstone Units, interpreted as levee progradation units.
Alternating lenticular sandstone bed and mudstone units, considered as crevasse splay sediments; channel unit cutting into sharp based tabular sandstone, regarded as crevasse channels truncating levee deposits;
gradationally based sandstone units, thought to represent minor mouth bar sediments into an interdistributary bay.

The widespread coal on seatearth that characterises the abandonment lithofacies association is taken to indicate the overall colonization of the delta top by vegetation.

The lithofacies analysis is used as the basis for establishing and defining the three Marsdenian deltas mentioned above, based on the construction of the following new chrono- and lithostratigraphical subdivision of the Marsdenian sediments: Scotland Flags Time Stratigraphical Interval involving all the lithofacies occurring between the bases of the bands of
R. gracile and R. bi1ingue typical. All the main sandstone dominated units occurring at the upper parts of the interval and which had hitherto been variously given multifarious rock-stratigraphical names are termed
Scotland Flags. The mudstone dominated unit which usually occurs below the Scotland Flags are called Scotland Flags Mudstone. The same nomenclatural procedure applies to their overlying Pule Hill- and Hazel Greave Time Stratigraphical Intervals.

Plots of bands of goniatites in accordance with their stratigraphical levels suggests that band of R. gracile, R. bilingue typical, R. bilingue late and R. superbilingue are reliable for stratigraphical correlation based on their widespread distribution, whereas bands of R. bilingue early,and R. rnetabilingue are not due to their restricted distribution.

Fourteen ichnogenera are recognized, described and used in palaeoenvironmental reconstruction. Their vertical distributions do not appear to be bathymetrically controlled, excepting perhaps ichnogenus Olivellites.
Rather the mode of deposition of their host sediment exerted a more signi ficant control.

Palaeocurrent indicators suggest currents dominantly towards the southwest but also southeast. The long axes of Pelecypodichnus and plants have a preferred orientation parallel to the current.

Among the many factors that controlled the sedimentation are the following: River processes, limited wave influence, relative basin level changes, subsidence and compaction, climate, salinity and predepositional
topography.

On the whole, the Marsdenian sediments are interpreted as deposits of Fluvial-Dominated High Constructive Lobate Deltas.