Sedimentology of mud - Canadian Society of Petroleum Geologists

Are shales really that dull? Shining 

light into dark places and the 

effects of opening Pandora’s Box. 

Joe Macquaker 

Memorial University of Newfoundland

Aims 

a) Discuss our perceptions about the origins of “black shales” 

b) Provide you with microtextural and compositional data 

(optical and electron optical techniques) from thin sections of 

some “classic black shales”. Supplement these data by 

geochemical analyses. I will be “Shining light into dark 

places!” 

c) Review from where the sediment components in siliciclastic 

mudstones are derived. 

d) Review mechanisms likely responsible for sediment transport 

and dispersal in these organic carbon-rich successions. 

e) Discuss what happens to organic-carbon rich mudstones 

shortly after they have been deposited. 

f) Discuss the implications of these data for our existing “black 

shale” depositional models - i.e. the effects of opening 

Pandora’s Box.

Before I start I would like to thank. 

• AAPG and Memorial University for funding this trip 

and allowing me to be away from my teaching duties 

• My co-conspirators: including Kevin Taylor (MMU), 

Kevin Bohacs (ExxonMobil), Sam Bentley (LSU), 

Margaret Keller (USGS), Guy Plint (UWO), Per-Kent 

Pedersen (U of C), Quinn Passey (ExxonMobil), Bruce 

Hart (ConocoPhillips), Juergen Schieber (Indiana) and 

Andy Aplin (Newcastle) 

• My recent students: Megan Laracy, Samer Ghadeer, 

Said Al-Balushi, Matt Johnson. 

• This research has been funded variously by NSERC, 

ExxonMobil, ConocoPhillips, Nalcor, and Statoil.

Real world observations - the problem! 

i) Toarcian aged Mulgrave Shale Member including the Jet 

Rock (32 m thick) exposed on the Yorkshire Coast (UK), 

and deposited over approximately 1 million years. 

ii) Very large volumes of mud deposited in these settings. 

iii) Base of this succession contains the Toarcian OAE.

Real World Observations: sub-log scales 

(i) Fine-grained! 

(ii) No obvious evidence of any 

physical sedimentary 

structures e.g. ripples, 

erosion surfaces, rip-up 

clasts. 

(iii) “Varved deposit” 

composed of thin 

intercalations of clay 

mineral and organic 

carbon-rich material and 

silt-rich material. 

(iv) Fine grained populations of 

pyrite framboids. 

(v) Many anoxic proxies (trace 

elements, stable isotopes 

and biomarkers). 

Jet Rock, deposited during 

Toarcian OAE, up to 14% TOC

Real world observations - the problem! 

i) Kimmeridge Clay Formation (Upper Jurassic, 500 m thick in 

Wessex Basin deposited over 8 million years). 

ii) Main source rock in North Sea (much of it apparently anoxic). 

iii) Very large volumes of mud deposited in these settings. Mud 

was deposited both at high and low sea-level stand.

Real World Observations: sub-log scales 

Fine-grained succession! That exhibits high gamma, low densities and high 

neutron porosities, no obvious sedimentary structures other than preserved 

lamination, although rare shell pavements, concretionary horizons, 

populations of small pyrite framboids, anoxic photic zone biomarkers are 

all present 

Kimmeridge Clay Formation up to 52% TOC

Perceptions and models 

These attributes mean that geologists using proxy data commonly interpret: 

• Organic carbon-rich mudstones deposited on continental shelves away 

from obvious slopes, as having been deposited by suspension settling, in 

low energy environments as a continuous rain of detritus, in settings 

where the bottom waters were either persistently anoxic or dysoxic. 

• Deposition occurring from buoyant plumes derived from either rivers or 

zones of high primary production in the photic zone. Very little 

evidence of advective transport at the sediment water interface. 

• During deposition fine-grained clastic sediment in these environments 

bypassed the proximal portions of the continental shelf resulting in there 

being little lateral lithofacies variability and lithofacies exhibiting 

widespread draping geometries. 

• Climate driven processes linked to clastic dilution versus primary 

production and variable bottom water anoxia were responsible for 

temporal facies variability. 

• Variability in these successions using Milankovitch forcing mechanisms.

Current paradigm for sediment transport 

continental shelves is: 

• Marginal marine sands dispersed basinward by either storms (ripcurrents, 

relaxation-curents), tides or gravity-driven density flows 

(turbidites). 

• No mechanism to transfer large volumes of fine-grained sediment 

offshore, particularly at high sea-level stands, because sediment 

transport is dominated by geostrophic flows forming a near shore 

mud-belt. Insufficient slope on continental shelves (

Shining light into dark places

Kimmeridge Clay Formation 

All KCF samples collected from this succession

Kimmeridge Clay Formation - intraclasts 

Blackstone up to 52% TOC


>TOC in intraclasts host sediment quartz rich 

Blackstone up to 52.3% TOC 

Top of Blackstone 36.1% TOC


In-situ. Agglutinated, benthic foraminifer 

Blackstone up to 52.3% TOC 

Top of Blackstone 36.1% TOC 

Middle of Blackstone 44% TOC


Normally graded, starved ripple laminae. Note curved 

lower contacts. 

0.45 m above Blackstone, 4.5% TOC 

Macquaker and Bohacs, 2007


Burrow mottled silt-bearing, clay-rich siliciclastic mst 


1.05 m above Blackstone, 4.6% TOC


Stacked succession of thin bedded, sharp based 

carbonaceous mudstones with downlapping laminae. 


0.45 m above Blackstone, 13% TOC 

1.05 m above Blackstone, 4.6% TOC


Discontinuous wavy laminae composed of algal 

bodies, organic carbon and calcareous pellets. 


Horizontal thin section of discrete organo-minerallic 

aggregates in silt-rich matrix 


BSEI of coccolith rich fecal pellet and enclosing 

organic matter. 

Shining more light into dark 

places

Mulgrave Shale 

Member (Jet 

Rock), Lower 

Jurassic, 

Toarcian 

Most of the following 

Mulgrave Shale samples 

collected from this 

succession

Mulgrave Shale Member 

Sharp based, beds with wavy laminae and erosional 

truncation of laminae. Base of Millstone. 

Erosion 

Erosion 

Erosion


Sharp based, beds with wavy laminae and erosional 

truncation of lamina. Note shell pavements 

Erosion 

Erosion 

Erosion 

MC0207 6.7% TOC


Normally graded beds with discontinuous wavy laminae that 

exhibit downlapping geometries. 

MC0201, 14% TOC 

Termination Termination 

Termination


Normally graded beds with sharp bases, pellets and organominerallic 

aggregates. 

MC0201, 14% TOC 

MC0203, 5.5% TOC 

Termination Downlap


BSEI image, detail of coccolith-rich pellets and organominerallic 

aggregates. 

MC0201, 14% TOC 

MC0203, 5.5% TOC 

Downlap Downlap MC0203, 5.5% TOC


Thin bedded organic carbon bearing clay rich mudstone. Individual beds 

are sharp based, have discontinuous silt lags at their bases, overlain by 

discontinuous pelleted laminae and capped by homogenized laminae 

Ravenscar 6, 3.5% TOC 

Homogenized laminae 

Pelleted laminae 

Silt lag


Sub-horizontal thin section through partially homogenized organiccarbon 

bearing clay-rich mudstone with organo-minerallic aggregates. 


PM127 middle 

Homogenized laminae Silt lag 


Ghadeer and Macquaker, 2011


Detail of sub-horizontal thin section scan with organo-minerallic 

aggregate in clay-rich matix. Note some framboidal pyrite. 


PM127 middle 

Homogenized laminae Silt lag 


Ghadeer and Macquaker, 

2011 

PM127 middle

Grey Shale Member 

Normally graded beds with sharp bases, discontinuous homogenous laminae that 

downlap onto the underlying bedding planes, overlain by wavy continuous intercalated 

silt and clay rich laminae and clay-rich drapes (triplets). 

Grey Shale 10-14

Cleveland Ironstone Formation 

Normally graded beds with sharp bases, discontinuous homogenous laminae that 

downlap onto the underlying bedding planes, overlain by wavy continuous intercalated 

silt and clay rich laminae and burrow mottled clay-rich drapes (triplets). 

Grey Shale 10-14 

Cleveland Ironstone Formation


Normally graded beds with sharp bases, discontinuous silt- and clay-rich laminae close to 

their bases (some subtle downlap) organo-minerallic rich laminae in their middle portions 

and more homogenized laminae towards their tops (triplets). Note shell pavement. 

Grey Shale 10-14 

Cleveland Ironstone Formation 

Mulgrave Shale Member

Shining even more light into 

dark places!

Second White Speckled Shale Member, Alberta 

(Cenomanian - Turonian) 

Howard Creek



Stacked succession of normally graded thin beds that contain 

downlapping non-parallel laminae 

WS2 30 borehole 

Downlapping laminae terminations

Second White Speckled Shale Member, Alberta (Cenomanian - 

Turonian) 

Clay-bearing, silt-rich lithic siliciclastic mudstone. The laminae contain 

abundant silt-sized partially compacted aggregates composed of clay 

minerals and organic carbon and lithic grains. 

WS2 borehole



Stacked succession of thin bedded mudstones. The sample contains parallel 

laminae and an homogenous interval. Note abundant fine sand-sized lithic 

grains 

WS2 borehole

Greenhorn Formation, Cenomanian - Turonian (Kansas) 

Stacked succession of thin bedded, normally graded calcareous mudstones. The 

sample contains both ripple laminated and homogenized beds. 

RKB#1 1040.4 

RKB#1 1040.4

Greenhorn Formation, Cenomanian - Turonian (Kansas) 

Silt-bearing clay-rich calcareous mudstones. Foraminifer, 

pellets and aggregate grains form the silt-sized fraction 

RKB#1 1006.7 

RKB#1 1006.7

Looking in the dark: key observations 

a) All these mudstone units (classic source rock intervals) are very 

heterogeneous at scales ranging from 10 -2 m to 10 -5 m. 

b) They are organized into stacked succession of very thin beds 

(typically each

What processes were dispersing the sediment? 

a) The preserved microtextural fabrics (ripples, gutter casts, sharp 

based beds, triplets, etc.) indicate that the sediment once deposited, 

was being eroded and then dispersed by advective processes. These 

sediments cannot just be the products of suspension settling through a low energy 

water column. 

b) The settings in which these successions were being deposited were, 

at least episodically, much more dynamic than most geologists have 

modeled. 

c) Normal grading in beds suggests that sediment was also being 

deposited from waning flows. 

d) On the distal parts of continental shelves there are not many 

obvious mechanisms that transport sediment offshore. Storms are 

very effective in the more proximal reaches and the products of 

these processes (e.g tempestites, gutter casts, “lamscram” fabrics) 

are well known, but current veering in response to the Coriolis 

Effect typically produces geostrophic flows with shore-parallel 

sediment dispersal patterns. Away from obvious geomorphic 

features such as deltas, fault scarps, canyon heads, gradients are 

typically insufficient to drive gravity driven density flows.

Wave enhanced gravity flows of fluid mud. 

• Energy for maintaining sediment in suspension is derived from 

orbital motion of surface-gravity waves, instead of slope-induced, 

self-generated turbulence. Different to turbidity currents (Wright 

et al. 2001; Traykovski, 2007). Very common in muddy settings on 

continental shelves, because usually not enough energy to entrain 

grain sizes > fine sand in the wave boundary layer. 

• We know a great deal about the likely physics that underpin this 

process, however, we do not know what the specific products 

of these flows are! 

• So we (Macquaker et al., 2010) sampled mud-dominated locations 

on modern continental shelves where wave enhanced fluid mud 

flows were occurring. We then impregnated the sediment and 

made thin sections to image textures present. 

• We compared these textures with those present in the ancient 

mudstone record.

Wave-enhanced gravity flows of fluid mud 

a) Erosion, followed by combined 

flow traction transport in 

turbulent conditions. 

b) Turbulence in wave boundary 

layer dampened by increasing 

weight of mud. Laminar 

sediment gravity flow 

established. Shear and 

subsequent reworking at base of 

flow leads to deposition of 

alternating clay/silt laminae - 

some material transported as 

grain aggregates. 

c) Wave activity subsides leading to 

gradual lutocline collapse and 

upward-fining. 

d) Lutocline collapses - clay drape 

Modern Eel Shelf sediment 

C 

B 

A

These fabrics are commonly encountered in ancient 

“black shales” 

Very thin examples are present in the Mulgrave Shale 

Mowry Shale 

C 

B 

A

Advective sediment transport of mud? 

• Where the primary fabrics present in muds deposited on 

continental shelves have not been completely 

homogenized by burrowing they commonly contain a 

great deal of evidence that wave enhanced gravity flows 

of fluid mud were dispersing the sediment. These flows 

are not the same as turbidites. 

So does mud move as bedload? 

• Yes! There is also some evidence that deposited mud is 

being reworked forming aggregate grains (hardened before 

reworking) and then transported and sorted in bedload. 

• This is consistent with the facts that (a) following 

deposition it is bound together (EPS and surface charges) 

and (2) with Schieber’s (2010) flume tank experiments.

Ripple laminae composed of intercalated Inoceramus 

prisms and floccule aggregate grains (Colorado 

Group). These are not low energy clay drapes! 

Manitoba

Mud dispersal processes in these successions 

• The muddy component in these successions was being dispersed 

by advective processes that were transporting sediment both as 

fluid mud in suspension and in bedload as aggregate grains. 

• Storm driven flows and WESGFs likely underpin mud dispersal. 

• Schieber (2010) has shown that flow rates at the sediment water 

interface were at least 20cm/s (sufficient to transport fine sand) 

• The common occurrence of these fabrics in the studied source 

rocks indicates that they were neither exclusively deposited in low 

energy nor deep settings - conditions at the sediment water 

interface were dynamic. 

• These laminae are not classic varves 

So does low energy suspension settling occur? 

• Yes! But not quite in the way we expect! 

• Texturally the mud is packaged into organo-minerallic aggregates, 

that are interpreted to be “marine snow” - it is not delivered as 

“dispersed fine-grained continuous rain”.

Micro-textures in the Jet Rock 

MCO209. Discontinuous, wavy lamina composed 

of calcareous pellets, clay and organic matter. 

6.1% TOC.

Micro-textures in the Mulgrave Shale 

MCO209. Discontinuous, wavy lamina composed 

of calcareous pellets, clay and organic matter. 

6.1% TOC. 

MCO201. Calcareous pellets forming part of an 

organo-minerallic matrix n.b. abundant, 

diminutive framboids. 14.1% TOC.

Suspension settling processes. 

• In modern oceans most of the sediment delivered to the sediment 

water interface by suspension settling is packaged into organo - 

minerallic aggregates. Particularly during phytoplankton blooms. 

• These aggregates sink rapidly to the sea floor, minimizing the time 

organic matter is exposed to oxic degradation processes and 

maximising the chances of non-refractory om being preserved. 

• Anoxic micro-environments may develop within these aggregates.

Sediment disruption post-deposition. 

• In these “type examples” of organic carbon-enriched 

mudstones the presence of sharp based normally graded 

thin beds that contain contain organo-minerallic 

aggregates, an in-situ. benthic foraminifer and are 

burrowed by organisms colonizing from bedding planes is 

very significant because: 

– Firstly, it indicates that much of the organic carbon was likely produced 

during relatively short events (blooms) and that there was insufficient time 

between the events for either all the organic matter to be oxidized or all the 

sediment fabrics to be destroyed. 

– There was sufficient oxygen in the bottom waters to allow a diminutive 

infauna to populate the sediment and there was sufficient time for the 

burrowing organisms to impart a secondary biological fabric to the sediment 

that overprinted the primary depositional fabrics.

Microfabric data and the effects of opening 

Pandora’s box 

• Detailed microfabric investigations of these rocks coupled with 

geochemical data reveal that these organic carbon-rich rocks are 

much more heterogeneous than most geologists have previously 

inferred using data derived from proxy data sets alone. 

• This heterogeneity indicates that while these sediments were being 

deposited there were certainly changes in the balance of sediment 

being supplied to the basin, and produced within the basin. 

• They also show that sedimentary systems, even where significant 

volumes of organic carbon was being preserved, were also 

commonly very dynamic and not dominated exclusively by low 

energy processes. Sediment, once produced was dispersed both as 

fluid mud flows and as bedload to sites where accommodation was 

available. 

• Some sediment was delivered by suspension settling processes. 

This material, however,was likely not transported as a continuous 

rain through a low energy water column but rather settled rapidly 

through the water column as marine snow, following algal blooms.

Microfabric data and the effects of 

opening Pandora’s box 

• The fabrics preserved in thin section data show that burrowing, 

commonly overprints earlier fabrics in these organic carbon-rich 

rocks. 

• The importance of persistent bottom water anoxia as a fundamental 

control on facies variability in shale settings where enhanced organic 

carbon preservation is occurring has been overestimated. 

• Because sediment is being dispersed advectively down-dip finegrained 

sediments can be directly connected with up-dip coarser 

grained sediments - so sequence stratigraphy is possible 

• It is not necessary to consider climate forcing as the dominant 

control on facies variability in these settings - the physical 

mechanisms that we know are operating on modern continental 

shelves can be observed in ancient settings too.

Where do we go now, now that Pandora’s 

box has been opened? 

• To enhance our models of how these rocks formed and specifically 

begin to produce geological models that effectively predict where 

source rocks and efficient shale gas reservoirs are likely to be located 

it is vitally important to integrate data obtained from traditional 

proxy methods and high-resolution microtextural techniques. Great 

deal of potential here. 

• We can now reasonably interpret these rocks using sequence 

stratigraphic principles and there is no need for them to be 

considered as “separate, detached entities” in basinal models. 

• We can’t put these microtextural data back in the “box”, however, 

inconvenient they are. We have to consider them in the next 

generation of facies models. 

• We have the skill sets to do this - but it will take time and effort. 

• We live in very interesting times but at least we now have something 

to look at - Shales are perhaps not as boring as we feared!

Mud clinoform: Bell Island

References - to current research 

Al-Balushi S.K. and Macquaker J.H.S, (2011) Sedimentological evidence for bottomwater 

oxygenation during deposition of the Natih-B intrashelf-basin sediments: Upper 

Cretaceous carbonate source rocks, Natih Formation, northern Sultanate of Oman. 

GeoArabia, V. 17, p. 48-84. 

Aplin A.C. and Macquaker J.H.S. (2011) Mudstone diversity: origin and implications for 

source, seal and reservoir properties in petroleum systems. AAPG Bulletin 

Ghadeer S.G. and Macquaker J.H.S. (2011) Sediment transport processes in an ancient 

mud-dominated succession: a comparison of processes operating in marine offshore 

settings and anoxic basinal environments. Journal of the Geological Society of London, V. 168, 

p. 835-846 

Macquaker J.H.S., Bentley S.J. and Bohacs K.M. (2010). Wave enhanced sedimentgravity 

flows and mud dispersal across continental shelves: reappraising sediment 

transport processes operating in ancient mud-dominated successions. Geology. V. 38: p. 

947 – 950. 

Macquaker, J.H.S., Keller M.A., and Davies, S.J., (2010) Algal blooms and “Marine 

snow”: mechanisms that enhance organic carbon preservation in ancient fine-grained 

sediments. Journal of Sedimentary Research. V. 80: p. 934 – 942 

Taylor, K.G and Macquaker J.H.S. (2011). Iron Minerals in Marine Sediments Record Chemical 

Environments, Elements, V. 7: p. 113 – 118.

Sedimentology of mud - Canadian Society of Petroleum Geologists

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?