I know you said that "I am going to have to refrain from responding any further
to this..." but you raised some important issues which should be addressed. As
you said you are still lurking, hopefully you will read this and find something
interesting to reflect on. My apologies for a tardy reply, however your
comments deserve discussion in some detail and I wanted to check some things
To begin with some introductory comments. I have had a long interest in
catastrophic geological processes, especially impacts. I am also very
interested the role that tsunami's may play in coastal history, especially as
the east coast of Australia appears to have been significantly effected by such
processes in the past. So I am not adverse to the possibility of tsunami
deposition in these sediments, whether caused by major impacts or by tectonic
movements. The question is, given the fact that a lot is understood about the
wide range of carbonate depositional environments and something is known about
both the Devonian at Wee Jasper and the characteristics of tsunami deposits,
whether such processes have played a role in the deposition of the Wee Jasper
THE DEVONIAN AT WEE JASPER
The main source for the stratigraphy of this area is Peddar et al. (1970). The
lowest unit of Devonian age at Wee Jasper consists of the Sugarloaf Creek
Formation. This consists of fragmental and coherent felsic volcanic lithologies
and is conformably overlain by the carbonate dominated Murrumbidgee Group. The
Devonian volcanics rest on lithologically similar Silurian volcanics.
The lowest formation in the Murrumbidgee Group is the Cavan Formation, which at
Wee Jasper is 157 m thick. The base Cavan Formation consists of calcareous
shales interbedded with thin limestones. Carbonate content increases upwards
and the bulk of the Cavan consists of limestone. The limestones are cyclic
consisting of centimeter to decimeter scale beds of either laminated lime
mudstones or fossiliferous wackestones, packstones, floatstones and rudstones.
The Cavan is overlain by the Majurgong Formation, which at Wee Jasper is 180-200
m thick (rather thicker than I had remembered). The formation consists of red
siltstones and shales, interbedded with cross-bedded sandstones and thin
limestones. Bedding is again centimeter to decimeter scale.
The uppermost unit of the Murrumbidgee Group is the Taemas Limestone. The
formation is at least 890 m thick. Further west the Formation thins somewhat
and various members become distinct within it (Browne 1959). At Wee Jasper,
however, the entire formation consists of decimetre to metre scale beds of
bioclastic wackestones and packstones. Within this formation are scattered
bioherms, the largest are probably 100 of so m long and 20-30 m thick. They are
(plano-convex with the convex surface uppermost) and often standing as modern
topographic highs. The bioherms consist of rudstones and floatstones with local
stromatactis cavities in the muddier units.
Given the centimetre to metre scale bedding of the Murrumbidgee Group the total
thickness of 1250 m represents of the order of 10,000 depositional events. For
the biostratigraphers the succession is regarded as Early Devonian throughout
(Praguian to Emsian).
So this is the succession. Do any of these sediments show signs of being
deposited by impact induced tsunamis? To evaluate this we need to consider the
criteria for evaluating whether or not a particular depositional unit is due to
CHARACTERISTICS OF TSUNAMI DEPOSITS
Tsunamis are giant waves and when they are in water less than half their
wavelength they begin to rework the bottom. Because wave motion is ellipsoidal
there is initially little lateral transport. As they break on the shore there
can be a considerable run up onto land, carrying with it sediment entrained from
the shore face. The water then flows back into the sea, carrying with it most
suspended marine sediment and material eroded from the land. This sediment is
then deposited in the shore and out onto the shelf or, in places where the shelf
is narrow or non-existent, can be carried into deep water. Because tsunamis
almost always consist of trains of waves this pattern may be repeated with
Shoreline tsunami deposits are characterised by erosional bases, often overlain
by rip-up clasts and are normally graded (Gelfenbaum and Jaffe 1999). The
substrate is non marine, whether sedimentary or of basement. Depending on the
source material the grain size ranges from sand to boulders. Boulders
(including large shells and corals) can be chaotic in a sand matrix or
imbricate. Mud is a minor component, as it remains in suspension and is carried
out to sea. Upper units in sandy deposits can be laminated (Bryant and Price
2001). Thicknesses are typically 10's of cm. The biota consists of a mix of
open marine shallow marine, shoreline and terrestrial organisms (Jacobs 2001).
Sometimes they can be recongised by reversals of age in the sediment (Young et.
Shelf tsunami deposits are very similar to shoreline deposits except that they
do not overlie terrestrial deposits or rocks. They can also be thicker, up to
several metres in thickness. Tsunami deposits on shelves can contain a mix of
organisms from diverse environments (Schnyder and Baudin 2001). Boulder rich
units are probably unlikely to be imbricate. If it is a low energy shelf, then
mud suspended by the tsunami may settle out.
Deep water tsunami deposits, like those of shorelines and shelves typically
consist of erosional bases and are graded. Depending on the sediment source the
sediment size varies from sand to boulders. Boulder rich units are typically
poorly sorted and chaotic. Sandy units are superfically featureless, but can
contain subtle oscillation in grain size (Goto et al. 2001). Breccia clasts can
be very large, especially in proximal situations (Barnett 2002). They can also
be very thick, the Peñalver Formation of Cuba is interpreted as a deep water
tsunami deposit from the K-T boundary impact on the Yucatan Peninsula. It is at
least 180 m thick and the thickest K-T boundary deposit known (Goto et al.
2001), outside the fall-back deposits in the actual crater. These authors note
that while the lower part of the Peñalver Formation is related to the tsunami,
the upper sandy part is more likely due to the settling out of airborne sandy
material.There may be a settling out of mud in deep water deposits as well.
Proximal tsunami deposits are associated with liquefaction features and other
indicators of seismic disturbance. These are found in deposits linked to both
tectonic events and impacts. Tsunami deposits formed by tectonic events and
impacts are very similar in character, and can only be differentiated if a
particular horizon can be tied to an impact stratigraphically,
geochronologically, or by the presence of petrological or geochemical features
suggesting and impact origin. This is the case for the Peñalver Formation (Goto
et al. 2001) and the Acraman impact layer
(http://www.newscientist.com/hottopics/disaster/secretstri.jsp) in South
DEPOSITIONAL ENVIRONMENTS OF THE MURRUMBIDGEE GROUP
There has been no published bed by bed analysis of the depositional environments
throughout he succession, and I am certainly not that intimately familiar with
it. But I have seen enough to begin to glimpse some of the characteristics of
There are many features of the succession that are not compatible with
catastrophic processes. Some will be mentioned later, so I will focus on some
features of the Cavan Formation. These base of the formation is conformable and
gradational with the underlying Sugarloaf Creek, marked by the appearance of
carbonate and the cessation of volcanic input. How is this consistent with
As already mentioned the limestones are cyclic consisting of centimeter to
decimeter scale beds of laminated lime mudstones and bioclastic units. The
mudstones are finely laminated, these laminae in thin section show evidence of
microbial precipitation. The laminae are usually flat, but occasionally form
small domal stromatolites. More commonly they are broken up by mud cracks,
whose upturned rims indicate desiccation, and teepee structures. How can this
be explained by catastrophism? The patterns are much more consistent with
deposition on an intertidal flat. There is evidence of local catastrophism.
Occasional thin beds of bioclastic debris may represent storm events.
The bioclastic beds consist of three types: very poorly sorted bioclastic
wackestones to floatstones, poorly sorted rudstones, and sorted rudstones.
These beds appear to have sharp contacts with underlying and overlying laminated
lime mudstones. The basal contacts are not erosion or, if they are are
erosional on a scale of only mm, the upper surface is concordant. the beds are
laterally continuous form 100's of m or km. This indicates an abrupt change of
environment but before we jump on any band wagon we need to consider what sort
of change to what sort of environment, dominated by what sort of process. The
concordant nature of the contacts is not particularly suggestive of catastrophic
processes. The easiest way to deposit mud and large bioclasts is for the
fossils to have grown in place. The fossils in the wackestones and floatstone
include many decimetre sized corals and stromatoporoids, some of which are
digitate and in growth positions. There are also partly articulated crinoid
stems. These cannot have undergone transport as they fragile forms would have
been fragmented. How is all this consistent with a catastrophic event?
The rudstones are in contrast dominated by large, robust corals, often
fragmented. This is consistent with higher energy deposition. The events were
energetic enough to break up the fossils and winnow out the mud not not to
significantly erode the underlying sediments. So there is evidence of at least
moderate energy, (open to swell waves or storms) but not necessarily
catastrophic. Or each bed could have been deposited in a single catastrophic
event, such as a tsunami or large storm. These fossils in such cases are still
most likely to have been locally derived, the high energy event reworks and
sorts existing sediment rather than transporting it from elsewhere. One would
have to examine each bed in detail to try and find more evidence to select
between these hypotheses. But however these beds were deposited, we must
remember that they are separated by units of laminated mudstone that and
wackestone to floatstone that do not indicate catastrophic deposition or even
Allen Roy wrote:
> It should seem obvious that the Bioherms, wackestones and packstones were
> high energy deposits, which would not be much of a problem from a
> catstrophic viewpoint. Most gradualists would consider the siltstone to be
> very slow deposition. The Catstrophist would look to a turbidite or tsunami
> for it's deposition source. Tsunami depositions often have biological
> remnants scattered through out the deposit. Turbidites flow below water
> because of their great density. It is that high density that allows for
> rapid deposition of whatever size sediment the flow is carrying.
Allen, it is important to do geology, not simply to wave your arms. You say
that "Bioherms, wackestones and packstones were high energy deposits" - why?
Wackestones and packstones by their very nature are moderate to low energy
deposits. They contain mud. How do you deposit bioclasts centimetres to
decimetres across together with mud? As I said earlier, the easiest way is for
the bioclasts to have grown in situ. The well preserved fossils indicates
minimal transport. The only way you can transport mud and large bioclasts
together is with a debris flow. I have studied carbonate debris flows and these
look nothing like them, furthermore the well preserved delicate organisms argues
against transport even in a debris flow.
As for bioherms, these can form in high energy environments. Indeed, one of the
definitions of a reef includes a wave resistant structure. The critical feature
of a bioherm is not whether it is high or low energy, but that it is
biologically constructed in place. The bioherms are composed of large
(decimetre to metre) sized corals and stromatoporoids. The include tabular,
digitate, and domal growth forms These organisms are often in growth position
and are inter grown with each other. Together with the preservation of delicate
branching taxa and the geometry of these units this strongly suggests they have
grown in place. Feel free to disagree, but please explain how a catastrophic
process can produce lenticular masses of organically bound bioclasts of often
delicate structure. One way is to form them elsewhere and then transport them
here. However do do this you would have to have a bioherm forming environment
elsewhere (for which there is no evidence) and a transport process, again for
which there is no evidence. I have seen large displaced bioherm blocks, and
these look nothing like them.
As for siltstones, you are making a artificial distinction between "gradualists"
and catastrophists". It should not be a question of blindly following a
paradigm, but going where the evidence points. I cover both gradual and
catastrophic processes in lectures. There are features of the Majurgong
Formation which are not consistent with deposition by tsunamis or turbidity
currents. The beds are not graded, do not have erosional bases, or mixed
biota. The biotic assemblage is characteristic and very different from that
found in the carbonates, there is no mixing. The contacts between the Majurgong
and the Taemas and Cavan are gradational, there is interbedded. This is not
consistent with the entire unit being a deposited by a single catastrophic
event. There are many sandstone beds that show ripple and trough cross
stratification, each separated by shale or siltstone from each other, clear
evidence for different depositional events.
> They are natural documents that can be read only if you have an interpretive
> paradigm. The paradigm determines what will be understood.
Of course paradigms are important. But they are not strait jackets - or should
not be. Paradigms in science are capable of revision. If this were not the
case then there would be no point presenting data and discussing anything, would
there? The fact that you have been discussing these matters shows that you do
not believe that paradigms are unchangeable.
> > They also should get a sense of the vastness of geological time
> Not hardly. Only if they have been brainwashed with gradualism.
Allen, this is an unworthy comment, it insults the students, saying that they
have been brainwashed, me, that I have been brainwashing, and the course leader,
allowing them to be so treated. The course included sizable blocks of lectures
from a YEC, and OEC, and a CC. They get exposure to a wide range of views in a
course that is designed to encourage people to critique world views, both
secular and Christian
In my original post I said:
"After some explanation about the significance of biota, sorting, grain size,
and sedimentary structures for interpreting sedimentary environments they see
Do you accept that grain size and sorting is a function of energy and rapidity
in depositing sediments? Do you accept that sedimentary structures indicate
process? Do you accept that different organisms live in different
environments? If you agree, how can you call is this brainwashing? If you do
not, then you have nothing meaningful to say about the science of sedimentary
geology, because what you say does not engage the issues.
Back to the students, I tell them what basic sedimentary features mean, how I
interpret them, how others might interpret them and they can draw their own
conclusions. Is this brainwashing or education?
> > Two years ago there was one lady in tears because of the
> > way in which her world view had been challenged by the discovery of the
> > reality deep. Her faith survived, whether by denying geology or by
> > changing her world picture, I do not know.
> I cannot see what she would be in tears about, but for possibily not being
> told that deposition of siltstone and shale need not take a long time as
> shown by turbiditeds and tsunami.
I understand she wept because her faith had been tied to belief in a young earth
and that a global flood was responsible for fossils. She wept because her
informed reason told her otherwise when her eyes saw evidence to the contrary.
She had been educated in the standard YEC approach and it failed. Her faith
survived, I am glad to say, but whether by rejecting geological evidence or YEC
I do not know.
Allen, if you want to seriously engaged "gradualists" and move the science of
sedimentary geology along, you need to draw up criteria that allows people to
identify the processes that form sediments. Sediments are deposited by both
gradual and catastrophic events, we need to distinguish them in the record. You
must critically engage the literature on depositional processes, both
catastrophic and non catastrophic, and not try and shoe horn everything into a
turbidite or tsunamite model. You will also need, unless you want to say that
all sediments everywhere are the result of the flood, to develop criteria to
differentiate pre, syn, and post flood deposits that are accepted by post flood
geologists. Until then you will be talking past other geologists and even your
fellow flood geologists.
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Devonian Duffin Bed, New Albany Shale, South-Central Kentucky North-Central
Section and Southeastern Section, GSA Joint Annual Meeting (April 3–5, 2002)
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stratigraphy in the Wee Jasper region of New South Wales. Journal of
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