T3S2 01 Wright Cherns

8/11/2019 T3S2 01 Wright Cherns

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The Cambro-Ordovician evolution

of the TAZ (Taphonomically Active

Zone) and its impact on carbonate

facies

Paul Wright (1) & Lesley Cherns (2)

Natural Sciences, National Museum of Wales, Cardiff,UK ([email protected])

Earth & Ocean Sciences, Cardiff University, Cardiff, UK([email protected])
mailto:[email protected]:[email protected]


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Linking three major features of

Ordovician marine carbonates

1. Increase in burrowing depth and degree ofbioturbation in marine sediments through theOrdovician

2. Demise of subtidal flat pebble conglomerates

3. Abundance of hardgrounds

1 & 2 have been linked and interpreted as reflecting thephysical disruption of thin cemented beds by burrowers

But should focus be on geochemical aspects of changes inbioturbation linked to the thickening of the TAZ(Taphonomically Active Zone)?


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Take Away PointsIncreased depth of bioturbation through the Ordovician was

critical to early carbonate diagenesis, thickening the TAZ and sodeepening the zone of syn-depositional cementation that

reworking by waves, etc, became less likely

Evidence from three features of shallow ramp sea floors:

Pre-Floian shallow depth of cementation, short residence

times and frequent reworking to produce subtidal flat

pebble conglomerates

Floian increase in depth of bioturbation, lowered depth of

cementation, less prone to reworking, thicker cementationzones associated with hiatus nodules and hardgrounds

Post-Floian further increase in burrowing depths, zone of

cementation below wave effects, marked decrease in

hardground frequency


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TAZ and associated early cementation related to

aragonite dissolution and calcite precipitation


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Droser & Bottjer 1988

Increase in degree of disturbance

by bioturbation seen through the

Cambrian of the Great Basin, USA

In

creasing

Increase in bioturbation through the Cambrian


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Lower Cambrian Upper Ord.

Cambrian Ordovician

Droser & Bottjer 1989

6=complete

homogenization

Increase in bioturbation through the

Cambro-Ordovician


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Degree and depth of bioturbation

Contemporaneous with the increase in thedegree of bioturbation was an increase in thedepthof bioturbation

Droser & Bottjer 1989: Depth of bioturbationis consistently less than 6cm through the

Middle Ordovician measurements for thedepth of bioturbation observed in strata ofLate Ordovician age are as much as 30cm


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Hardground data from Taylor 2008

and Wilson 2008

Flat pebble data exclude

peritidal examples

And the other two features in question


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Can we be more precise about when

these changes were taking place?


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In China theFloian is an

interval of

major change

in sea floorcharacter

From Liu & Zhan,

2009, Acta Geol

Sinica, 83, 513-523


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In China theFloian is an

interval of

major change

in sea floorcharacter

From Liu & Zhan,

2009, Acta Geol

Sinica, 83, 513-523


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In China the Floian is an interval of major

change in sea floor character

From Liu & Zhan,

2009, Acta Geol

Sinica, 83, 513-523

Flat pebble

conglomerates

Bioturbation

Subtidal microbial

carbonates


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Flat pebble conglomerates


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Hiatus nodules


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Bryozoan

on Md Ord

Kanosh

Fm Utah

Bryozoan on

Up Ordhardground,

Kentucky

Ordovician,

Utah

The appearance of encrusters

makes hardgrounds more

recognizable after the early

Ordovician (Brett & Liddell, 1978).This likely reflects availability of

such stable substrates as

compared with the irregular and

less stable, fragmented cemented

layers characteristic earlier in the

Palaeozoic

Hardgrounds


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The Ordovician was agolden age for epizoans

on hard substrates, at

least for those which left

skeletal evidence. Much

of the increased

abundance and diversity

of Ordovician hard

substrate organisms is

due to the increase inhard substrate availability

since the Cambrian.

Taylor & Wilson, 2003,

ESR, 62, 1-103Brett & Liddell 1978,Paleobiology, 4, 329-348

Exhumation of larger and more stable cemented

surfaces provided a new niche for invertebrates


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Ordovician Hardground Abundance

Previous focus on theory that Ordovician seas wereundersaturated with respect to aragonite so sea-floor dissolutionreleased carbonate for local cementation Carbonate hardgrounds reach their peak abundance in shallow marine

environments largely due to the prevailing Calcite Sea conditions whichfacilitated early aragonite dissolution and synsedimentary calcite

cementation Taylor & Wilson, 2003, ESR, 62, 1-103 The widespread development of hardgrounds during the Ordovician was

related to the extensive and pervasive precipitation of low-magnesiumcalcite on shallow-water marine seafloors (Wilson and Palmer, 1992;Palmer and Wilson, 2004) ; dissolution of aragonite may have been thesource of the calcite cement. Harper 2006, Palaeo3, 232, 148-166

However, Kenyon-Roberts 1995 found no direct evidence for seafloor dissolution associated with Ordovician hardgrounds, andmany studies have now established that early aragonitedissolution takes place in the TAZ where undersaturation isgenerated largely by oxidation of H2S


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PREC-C-LATE EARLY ORD

Thin TAZ, shallow sub-sea

floor zone of cementation

(ZOC) , readily and

frequently reworked

MID-LATE ORD

Thicker TAZ, deeper ZOC, less

commonly reworked so more

developed; reworking produced

hiatus nodules and exposed

surfaces as hardgrounds

LATE ORD

Thicker TAZ, even deeper

ZOC; rare reworking and

exhumationLMA only


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With shallow-depth cementation , thicker diagenetic bedding (LMAs)

was restricted to deeper water settings less affected by reworking. As

the TAZ thickened and depth of cementation increased, thicker beds

were now likely to be preserved in shallower settings..LMA facies

migrated into shallower water settings


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Thanks to Carl Brett, Noel James and Brian Pratt for many images

This talk will be available on the Carbonateworld web site in two weeks

T3S2 01 Wright Cherns

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