Friday, July 15, 2011

Different types of grounds

So I started taking a closer look at how a transgressive lag would be identified. A transgressive lag (Fig. 1) is a deposit that you find at the base of a sequence of rocks that occur from the rise of the sea level or as the shoreline moves landwards.

Transgressive lags are associated with ravinement surfaces. A ravinement surface (Fig. 1, 4) is the surface that gets scoured off as the "front" of the sea makes it's way landwards.
Fig. 1 shows a general idea of how a transgressive lag with a ravinement surface is formed.

Fig. 1
Regression is the opposite of transgression, when the sea level drops or the shoreline moves towards the sea.
You can find this picture in the SEPM website

At this ravinement surface, certain types of ichnofacies (eg. Glossifungites in Fig. 1, 4) or rocks that contain traces left behind by organisms (eg. as they burrow) can be found. The type of ichnofacies (which in turn depends on the type of organism creating the trace fossil or ichnofossil) depends on the different types of grounds.

So let's look at 4 types of grounds:
1) Softground
2) Firmground
3) Hardground
4) Woodground

All these grounds are exactly as the word says.

1) A softground is loose material that is wet and has not been compacted and consolidated into rock yet. For eg. the top layer of a beach (Fig. 2, 3). Many different ichnofacies can occur here. Refer to the picture here to get an idea. A softground is probably not going to show up in a ravinement surface or any other geological record because it is easily eroded away.

Fig. 2
Softground. Beach sand forming dunes (that will turn into cross bedding if it gets compacted and preserved) with organic material (dark layers in the top right) from the lagoon behind.
Northeast Trincomalee, Sri Lanka 2009.

2) A firmground is loose material that has been compacted and most of the water been removed by the compaction from the sediment at the top. Fig. 3, 4.

Fig. 3
Softground is found at the top and firmground a little below the surface. The black layers are organic material. Of course since this picture is showing the layers being eroded away by the water, you don't see the firmground being very firm!
Note the possible cross bedding being formed at the bottom of the pic where the water is. This is a little behind in location from Fig. 1.
Northeast Trincomalee, Sri Lanka 2009.

Fig. 4
Glossifungites ichnofacies on firmground from the Pleistocene (2.6my to 11,000y). These trace fossils have been made by mud shrimp (Upogebia pugettensis).The softground has been eroded away. This surface would be a ravinement surface.
Goose Point at Willapa Bay, WA. Photo by M. K. Gingras from Catuneanu, 2006, p. 38.

3) A hardground is material (mostly those that trickle down onto the ocean floor) that has been compacted, de-watered, and cemented into rock and can usually be found at the bottom of a column of water like the ocean floor where the water has been above the sediment for a while. You can find more on hardgrounds here.

Fig. 5
Hardground formed during the Middle to Late Ordovician (472-444my) in an open marine environment. Bivalve fossils present.
This picture does not however show a ravinement surface.
Viola Limestone in the Arbuckle Mountains, OK, 2008.

4) A woodground is separate from the soft-firm-hard-ground trio. It is material that is formed from the compaction of xylic (wood) material. These can usually be found where wooden material such as logs break down and compact or where compacted peat occur. These woodgrounds can contain ichnofossils called Teredolites thought to be created by certain bivalves. I don't have any clear pictures of this and none i can find on the internet either. Maybe if someone has one, they can contribute it? Here's the reference to woodgrounds. Unfortunately i don't have access to that paper, so if you do, do check it out and tell me about it. :D

Update I recently came across on Woodgrounds & Teredolites can be found here on a recent post by Eric from The Dynamic Earth.

There is a 5th type of ground called Soupground where the material is just slurry and constantly wet. But I would probably just put that in with softground as it is probably not going to show up in the geologic record! But it can most definitely be used as a descriptor for present times. So it should not be totally overlooked.

You'll find some of the ichnofacies terms i have mentioned here. And here's a blog on a trace fossil on a hardground (during Middle Jurassic - 176 to 160my) in Israel by the Wooster geologists. They are HUGE!

If anyone has other pics to contribute of these different types of grounds, please do so! :D

1) Posamentier, Henry W., and George P. Allen, 1999, Siliciclastic Sequence Stratigraphy - Concepts and Applications, Society of Economic Petrologists and Paleontologists, p. 216.

2) Catuneanu, O, 2006, Methods of Sequence Stratigraphy Analysis. In Principals of Sequence Stratigraphy, Elsevier, p. 375.

No comments:

Post a Comment