Thanks for the extensive discussion. I may
be contacting you off-line. I am writing a text on terrestrial and freshwater
paleoecology (John Wiley & Sons) and need to have some discussion of
sequence stratigraphy, both theoretical and practical.
Judith Harris
From:
paleonet-owner@nhm.ac.uk [mailto:paleonet-owner@nhm.ac.uk] On Behalf Of Michael Simmons
Sent: Wednesday, March 24, 2004
4:11 AM
To: paleonet@nhm.ac.uk
Subject: RE: paleonet Global
sea-level curve
Andrew Rinsberg raises a topic close to my
heart. In the last few years myself and my colleagues have been investigating
the sequence stratigraphy of the Phanerozoic of Arabia and North
Africa (see Sharland et al., 2001; 2004). To quickly answer the
point about global sea-level curves other than Haq et al, for the Mesozoic -
Cenozoic the standard revision is that of Hardenbol et al. (1998) using data
principally from NW Europe. For the Mesozoic there are other sea-level curves
published as well (see, for example, Hallam, 1995; Embry, 1988 and Kauffman
& Cadwell, 1993), these generally being specific to a basin or region and
can be extrapolated globally as you see fit.
For the Palaeozoic there is less consensus
on a single "global" sea-level curve. The Ross and Ross (1988) curves
are being superseded by curves for specific periods (see, for example (amongst
many), Johnson et al., 1985; Loydell, 1998 and papers within Walliser, 1995).
From these a composite Palaeozoic global curve can be drawn depending on how
much confidence you have on extrapolating data from a specific basin or basins
globally.
In our work on North Africa and Arabia, we have now recognised 100+ regionally
correlatable and biostratigraphically calibrated Maximum Flooding
Surfaces. We have made absolutely no attempt to fit these to a Haq
et al. or any other global sea-level curve. We have, however, drawn our own (as
yet unpublished) curve for this Gondwanan/Tethyan margin and compared it to the
Haq et al curve and a composite Palaeozoic curve. This is particularly
useful because (i) from what I know data from Arabia was not used in the
construction of the Haq et al curve, so forming an independent test; (ii)
Arabia was relatively tectonically stable for large periods of geological
time - the onlap margin of the Arabia Shield forming a useful indicator of
eustatically driven sea-level rise and fall. The comparison shows that some Haq
et al events are detectable in Arabia and North Africa (for example many of the
Middle - Late Jurassic surfaces), others are not, either implying that they are
open to revision or that there are subtle tectonic events in Arabia
that mask them. In the Palaeozoic there is often clear correspondence with
"global" events that are well known (e.g. transgression in the
bifidus Zone of the lower Llanvirn). Where there are clear tectonic events in Arabia (e.g. establishment of the Zagros foldbelt and
foreland in the late Cenozoic) there is usually marked departure from the any
global pattern. It would be legitimate to ask how accurate is our
biostratigraphic resolution for both construction of our own curve and
comparison with the Haq et al curve and others. In answer I would say it is as
accurate as it can be - issues such as the calibration of platform zones to
basin zones are issues familiar to most subscribers to this listserver and need
not be debated further here. To improve calibration of events we have utilized
techniques such as Sr isotopes were available.
I would strongly advocate construction of
a sea-level curve for any given basin rather than trying to fit a global curve
to the local rock record. Nonetheless it is then interesting to compare the
results with global curves as this is helpful in unraveling tectonic controls
on sedimentation from eustatic ones.
I should note that Felix Gradstein and his
colleagues are publishing a new Geological Timescale this year. Significant
changes in the duration of not only Palaeozoic stages (already well known to
most interested parties), but also Mesozoic stages are possible. If accepted,
this may well impact on our understanding of the duration of sea-level cycles
and the mechanisms controlling them.
I would be happy to receive comments on
the above both online or offline from interested parties.
Embry, A.F. 1988. Triassic sea-level
changes: evidence from the Canadian Arctic archipelago. In: SEPM Special
Publication, 42, 249-260.
Gradstein, F. et al. 2004 (in press). A
Geological Timescale 2004. Cambridge
University Press, 384pp.
Hallam, A. 1995. Major bio-events in the
Triassic and Jurassic. In: Walliser, O.H. (ed.) 1995. Global Events and Event
Stratigraphy, Springer-Verlag, 333pp
Hardenbol, J. et al. 1998. Sequence
Stratigraphy of the European Basins, SEPM Special Punlication 60.
Loydell, D.K. 1998. Early Silurian
sea-level changes. Geological Magazine, 135, 447-471.
Johnson, J.G. et al. 1985. Devonian
eustatic fluctuations in Euramerica, Geol. Soc. Am. Bull., 96, 567-687
Kauffman, E.G. and Caldwell, W.G.E. 1993. The Western Interior
Basin in space and time.
In: Geological Association of Canada,
Special Publication 39, 1-30.
Sharland, P.R. et al. 2001. Arabian Plate
Sequence Stratigraphy, GeoArabia Special Publication 2, 371pp.
Sharland, P.R. et al., 2004. Arabian Plate
Sequence Stratigraphy - Revisions to SP2, GeoArabia, 9, 199-214.
Walliser, O.H. (ed.) 1995. Global Events
and Event Stratigraphy, Springer-Verlag, 333pp.
Director of Geoscience
Neftex Petroleum Consultants Ltd
71 Milton
Park
Abingdon
Oxfordshire
OX14 4RX
United
Kingdom
mike.simmons@neftex.com
www.neftex.com
-----Original Message-----
From: Martin Farley
[mailto:mbfarley@hal-pc.org]
Sent: 24 March 2004 01:06
To: paleonet@nhm.ac.uk
Subject: Re: paleonet Global
sea-level curve
The following comments on sea level curves derive from my participation
in Exxon's effort to build a new sequence stratigraphic standard (see SEPM
Special Publication 60, Sequence Stratigraphy of European Basins):
1) the Haq, et al. curve is a model and some parts of it were better than
others.
2) the Haq timescale is definitely out-of-date, so that the curve itself would
have to be squeezed or stretched like an accordion to a new timescale, even
assuming the sequence strat model underlying it is unchanged. (see below)
3) a sea level curve derived from a sequence model is several levels of
abstraction removed from real data. First, you take a sequence strat model (at
a minimum, the position of sequence boundaries and maximum flooding surfaces).
You then create a coastal onlap model to capture relative rises and
falls. This coastal onlap model is averaged over several sites. Then you
can fit an estimate of sea level to the coastal onlap model assuming some
external controls on the limits to the sea level fluctuations (for example,
maximum level of the bathtub ring in the Cretaceous highstand). This
shows the uncertainty inherent in sea level curves. It also implies the
enormous work required to build curves with a reasonable precision.
4) Algeo and Seslavinsky (1995) is an interesting (and different) approach, but
doesn't begin to have the resolution of the Haq, et al. curve. For the
upper Paleozoic at least, Charlie and June Ross have been following a sequence
approach of course.
The outcome of SEPM 60 is a sequence stratigraphic standard (i.e., sequence
boundaries and maximum flooding surfaces in places where ages are highly
constrained, so the Haq sequence model is out-of-date too. SEPM 60's
sequences are a standard, but not a globally documented sequence model.
Obviously, Haq, et al. and SEPM 60 were financed in large part by industry. To
make a revision of the sea level curve would require a lot of careful age
control (i.e., biostratigraphy), among other things. Industry interest in
supporting this has disappeared. So no matter how interesting and useful
a eustatic curve would be, progress seems stymied.
PaleoFolks,
Which
global sea-level curve is now considered to be standard? A web search yields a
welter of information that seems to cluster around brief and local
modifications to the sea-level curve of Haq (1987), but it's hard to believe
that anything could last so long without being superseded. After all, in 1987,
I lived in another city working at another job before I was superseded. But
maybe the Haq curve is more lasting.
Andrew
K. Rindsberg
Geological
Survey of Alabama
Univ.
of North Carolina at
Pembroke