Use of trace fossil in stratigraphy and Sedimentology

        

Use of trace fossil in stratigraphy and Sedimentology

Trace fossils are characterized by long time range but narrow facies range. These characters together with the fact that trace fossils show no secondary displacement
and often occur abundantly in rocks, which have only a few body fossils, make them
invaluable for stratigraphic and sedimentological investigation.
When preserved as surfacial features trace fossils help determining the depositional
top of a sequence. Under suitable conditions trace fossils also provide clues to the
physical and chemical conditions prevailing at the time of sedimentation. Following
are some of the examples.
Depositional Facies and Palaeo bathymetry
The animals responsible for' generation of particular types of trace fossils are sensitive
to environmental and bathymetric conditions. Following this clue Seilacher proposed
that assemblages of trace fossils might be used for interpretation of depositional facies
(,ichnofacies') and palaeobathymetry (Seilacher 1964, 1967). Following is a summary
of Seilacher's major fIndings (Fig. 6.9).
1. In marine beach and intertidal zones, which are areas of rough water erosion
and sedimentation, organisms like Skolithos and Arenicolites build deeply
penetrating vertical tubes for self-protection. content Both dwelling and feeding burrows
dominate this zone.
2. Shallow shelves have Cruziana facies.
3. Relatively deep, tranquil water is characterized by Zoophycos facies. Zoophycos
in general forms 3·D spiral structures. They branch out to build horizontal
burrows. Such structures help them to collect food particles, which settle down
from shallower levels and become part of the substrate.
4. In still deeper parts and within pelagic mud layers between turbidities, organisms
like Helminthoida and Nereites splay out to collect the food trickling down
from the water column above.
As an objection to this apparently idealized interpretation of palaeobathymetry, it
was pointed out that ichno-assemblages are controlled, not only by water depth, but
also by other physical parameters like the quality of light, salinity and oxygen content of water. Direction of water current may also play an important role in controlling
orientations of resting and grazing organisms (Ekdale 1988).
Ichnofossil as Indicator of Energy Leuel
Some organisms churn-up the sediments, thereby disturbing the layers. The
phenomenon is called bioturbation. Excessive bioturbation suggests vigorous organic
activity together with a slow rate of sedimentation. Conversely, total absence of
bioturbation indicates lack of organic activity possibly because the prevailing condition
was anoxic. An environment devoid of nutrients will also have the same effect. Vigorous mechanical agitation, as in a beach foreshore, might also retard organic growth.
Ichnofossil as Index of Toxicity Burrowing activity is promoted in well-oxygenated environment. Increase of toxicity
due to lack of oxygen or emission of H2S may cause suppression of organic activity.
Burrowing activity itself aerates freshly deposited sediments. Of all the animal species
the O1)e producing the trace Planolite is least tolerant of low oxygen level. Thalassinoides
is somewhat more tolerant, followed by Zoophycos and then Chondrites (Savrda and
Bottjer 1986).
lchnofossil as Indicator of Strength of Substrate
Mechanical strength and stability of the substrate can be inferred from the depth and
condition of footprints of vertebrates, provided the weight of the maker (say, a bird or
a dinosaur) of the print can be estimated. Corrugated vertical burrows and flattened
horizontal tubes indicate that the sediments underwent post-depositional compaction.
Ichnofossil as Indicator of Rate of Sedimentation
Seilacher (1962) interpreted absence of burrows within a sequence of flysch as indicativeof very quick, essentially instantaneous, sedimentation. In a sequence of laminated,undisturbed sand and bioturbated shale, the sand is interpreted to have deposited quickly,and the mud very slowly, allowing it to be exposed to burrowing activity over a long
period of time (Pettijohn, 1975, p. 131). It should be remembered however, that
bioturbation of mud over a sufficiently long time may also leave a homogeneous residual
layer.Trace Fossil Occurrences and Reports
Mu.:h of the pioneering studies on trace fossils and similar biogenic sedimentary
structures are due to the German scholars like R. Richter, W. Hantzchel, and A.
Seilacher. These studies were followed ih the United States by R.W. Frey, S.G.Pemberton and others.In India Arya and Rao (1979) reported bioturbation structures from the Middle
Proterozoic Narji Formation, Kurnool Group in Andhra Pradesh. (Fig. 6.10). These
structures, occurring in the form of worm burrows, were tentatively classified as
belonging to the Skolithos and Glossifungites assemblages.Mauhk and Chaudhun (1983) reported three types of trace fossils from the Tnassicred beds of the Gondwana sequence of the Pranhita Godavan Valley These mclude (a)
honzonta! burrows (b) lllchned passageways between succeSSIve gallenes of honzontal
burrows (c) vertIcal to slIghtly lllc1med burrows The morphologIes of these burrows
are enVIronment senSItIve The traces are vertIcal In the Shlfilllg condItIOns of the channelfacIes but they are honzonta! m the stable flood pi am areasBurrows and bIOturbated sedIments were reported from the slhClc1astlc tIdal flatdepoSIts of the of the Bhander Group of the Precambnan Vllldhyan Supergroup near MaIhar, Central IndIa (Chakrabartl, 1990) The burrows vary from large-dIameter, near vertIcal stubby forms to mIcroSCOPIC thread-lIke features cuttmg across the sedImentary
structures Intense bIOturbatIOn of the sedIments housmg the burrows suggest that thet race makers were 'depOSIt feeders'
Das and Rao (1992) reported numerous bIoturbatIon structures from the MIddle
ProteroZOIC Chamuna LImestone beionglllg to the Chattisgarh Group, IndIa These
burrows of mIcroSCOPIC dimenSIOns were obtamed from fine gramed, pyntiferous
lImestone, depOSIted m dysaerobIC lagoonal carbonate mud enVIronment Based on
their shape, Internal architecture, and theIr relatIOnshIp to the host rock, these burrows
are belIeved to belong to a number of Ichnogenera SkolIthos, Monocratenonl
Rluzocoral1lUm, and Polybessurus
Sarkar and Chaudhun ( 1992) reported several morphological vanetles of trace fossus
from the MIddle and Late Tnassic flUVIal redbeds of the Pranhtta-Godavan Valley
The dIstnbutlOn of burrows here was shown to be faCIes controlled SkolIthos, a type
of vertIcal dwellmg burrow, occurs both In channel and floodplam deposl ts Taellldl urn,
the horoizontal strucutures representing deposit-feeding organisms, are restricted to
catastrophically emplaced sand sheets in channels and proximal floodplains. Vertical
escape burrows are confined to continually accreting parallel laminated sands of channel
bars. Horizontal dwelling burrows are confined to smaller sandsheets of floodplain
drainage systems.
Burrows of wormlike animals were reported from the Mesoproterozoic Chorhat
Sandstone within the Vindhyan Supergroup of Central India. These burrows, unlike
other trace fossils of similar age, always follow a bedding plane, which is a few
millimetres below the original surface. Hence these were interpreted as the work of
microscopic, worm like "undermat miners" that excavated tunnels underneath microbial
mats. Their presence within the Vindhyan rocks suggests that triploblastic animals
existed more 'that a billion years ago.