Geology terms: Basal Lag?

[JJT3]

Hi Everyone,

 

Looking for some help with understanding what a basal lag layer is?

I have come across this term a lot when trying to identify different formation layers I see while out on the hunt. 
While I know a layer exists because fossils I find come from it, I don’t believe I have ever actually seen this basal lag layer.

 

Thanks,

John

[Fossildude19]

Basal (Geology) - "Located at the bottom of a geologic unit".

Lag Deposit - "the deposition of material winnowed by physical action. aeolian (relating to or arising from the action of the wind.) processes, fluvial (flood) processes, and tidal processes can remove the finer portion of a sedimentary deposit leaving the coarser material behind.  Lag deposits are found in processes such as central island formation in streams and rivers. "

 

So, a Basal "lag" layer is a layer at the base of a geological formation, that was formed by wave, wind, current, or tidal forces.

These forces have winnowed similar sized/weighted material (matrix and/or fossils) together by the action of water, wind and sand movement. 

Often times, you will see similar sized fossils collected together in lag deposits.

[Nanosaurus]

This usually refers to a layer or series of layers that has a more coarse/conglomeratic base and then fines upwards into fine sands, silts, and muds. These lags are sometimes good sources of fossils that concentrated along that bottom layer before being buried. This is a common way vertebrate fossils are found/preserved. I hope this helps.

[doushantuo]

Edited October 12, 2021 by doushantuo

[doushantuo]

of course not all lag deposits are fluvial in origin....

 

Edited October 12, 2021 by doushantuo

[Oxytropidoceras]

Basal lags can also be associated with ravinement / basal marine flooding surfaces within shallow marine deposits. Go see:

 

Rogers, R.R. and Kidwell, S.M., 2000. Associations of vertebrate skeletal concentrations and discontinuity surfaces in terrestrial and shallow marine records: a test in the Cretaceous of Montana. The Journal of Geology, 108(2), pp.131-154.

 

More PDFs of similar papers

 

Boessenecker, R.W., 2011. Comparative taphonomy and taphofacies analysis of marine vertebrates of the Neogene Purisima Formation, Central California (Masters thesis, Montana State University, Bozeman, College of Letters & Science).

Boessenecker, R.W., Perry, F.A. and Schmitt, J.G., 2014. Comparative taphonomy, taphofacies, and bonebeds of the Mio-Pliocene Purisima Formation, Central California: strong physical control on marine vertebrate preservation in shallow marine settings. PLoS One, 9(3), p.e91419. (open access)

 

Yours,

 

Paul H.

[FranzBernhard]

6 hours ago, Wayan Man said:

This usually refers to a layer or series of layers that has a more coarse/conglomeratic base and then fines upwards into fine sands, silts, and muds.

Is this definition sufficient for a "basal lag deposit"? This definition may include basal lag deposits, but, generally, I am imaging just a fining-upwards sequence, without winnowing involved.

Franz Bernhard

[JJT3]

Thank you all for the replies.

 

On the east coast of the US most streams containing fossils also contain large pebble gravel ranging from 1/8” to 1.5” pebbles. Can anyone add if these quartz pebbles come from fossil bearing layers or if they come from a more recent surface layer?

 

Im just trying to put together what my eyes are seeing verses what is really going on. I don’t dig in ton stream walls and maybe that is why I have a lower understanding of the layers in the areas I hunt (NJ). I honestly don’t think I have seen any of the fossil bearing layers. 

[FranzBernhard]

4 hours ago, JJT3 said:

Can anyone add if these quartz pebbles come from fossil bearing layers or if they come from a more recent surface layer?

I will try to give a general answer:

Some fossils you will find in recent gravel bars etc. in a river may come from reworked lag deposits, some fossils may come from reworked, fine-grained fossiliferous layers, which are usually more extensive than lag fossil deposits.

A few of the pebbles may come from reworked lag deposits, but most of them will come from "normal" conglomeratic layers, which are usually (nearly) free of fossils and more extensive than fossiliferous layers. The conglomeratic layers can be stratigraphically below or above the fossiliferous layer(s). Or both, of course.

And the recent gravel bar in the river may represents a newly formed lag deposit on its own!

Hope, other members will comment too, thanks!

Franz Bernhard

[doushantuo]

"basal lag" is semantically,epistemiologically,sedimentologically and stratigraphically ambiguous.

you might try measuring clast orientation,bTW

as an alternate term  ,but with perhaps less petrographical connotations,one could use "ravinement diastem"

edit: not entirely satisfied with the previous sentence ,but I'll let it stand'

Edited October 13, 2021 by doushantuo

[MarcoSr]

Lag layers in marine deposits are caused by transgressions and regressions.  Per Wikipedia: “A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling with water or decreasing in capacity. Transgressions and regressions may be caused by tectonic events such as orogenies, severe climate change such as ice ages or isostatic adjustments following removal of ice or sediment load.”  As the sea level rises it reworks the sediments deposited in the previous transgression/regression cycle.   As the sea level falls, it reworks what was deposited in the previous transgression.  This reworking can cause a lag layer and concentrate fossils.  Basal lag layers are deposited during the beginning of a transgression/regression cycle.  Dr. Robert Weems, who studied and described marine formations in the Atlantic coastal plain for the USGS, commented to me that his research on the different layers in the Atlantic coastal plain suggested recurring transgression/regression cycles every 400,000 years or so.

 

Marco Sr.

[Plax]

Marco says it very well. A basal lag is best described in Sequence Stratigraphy. The elements in the basal lag are time averaged and can be reworked many times like the accompanying quartz pebbles or they were current inhabitants of the lag environment. That is why preservation is so variable. It's also important to realize that any resistant vertebrate clasts such as shark teeth from missing time between the formations can be found in the lag. An exaggerated example of the missing time vertebrate clast would be the meg teeth found at the base of the Goose Creek and Waccamaw/James city formations here in NC. No Miocene rocks are in the section but the teeth are resistant clasts found in the lag between two formations. One also gets Cretaceous teeth in these basal lags with the meg teeth where the neogene formations lie on the Peedee or earlier formations. Meg teeth are similar to big quartz pebbles as far as resistance to erosion and dissolution.