What Are Your Stratigraphic Markers?

[Missourian]

Some units in the Pennsylvanian Lansing and Kansas City Groups of the Kansas City metro have characteristics that aid in stratigraphic identification:

 

Eudora Shale - Black & platy shale

Captain Creek Limestone - * Enteletes brachiopods common

Farley Limestone - Osagia in upper part

Liberty Memorial Shale - Siderite concretions

Muncie Creek Shale - * Phosphatic concretions; black and platy shale

Cement City Limestone - * Large horn corals (Caninia torquia)

Quivira Shale - Black & platy shale in places

Wea Shale - Siderite concretions in a few spots

Wea Shale - * In lower part, hash with Crurythris brachiopods and ammovertellid forams.

Winterset Limestone - * Dark bluish gray to black chert

Stark Shale - Black & platy shale

Bethany Falls Limestone - * Massive, bus-sized blocks on weathered outcrops

Hushpuckney Shale - Black & platy shale

Middle Creek Limestone - * Meekella brachiopods common

 

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Asterisk (*) for particularly diagnostic features.

[Missourian]

What are some in your hunting grounds?

[Bullsnake]

OK, Missourian. Got to give me a 'T' for trying. And I can't get nearly as detailed as you can, if I even got the formations right to begin with .

Lansing Group, Stanton Limestone.

I believe I've identified the South Bend limestone here, and Rock Lake shale member indicated by Myalina and Aviculopecten (per KGS).

Possibly Vilas shale in lower part.

Very tentatively... Virgilian stage, Douglas group, possible exposure of Stranger formation in loess.

Tonganoxie sandstone, Weston shale.

What I really love about this place is the sand/gravel bed is loaded with tiny fossils.

Shale

Sandstone

Creek bed

And thanks for your inspiration!

[Missourian]

OK, Missourian. Got to give me a 'T' for trying. And I can't get nearly as detailed as you can, if I even got the formations right to begin with .

Lansing Group, Stanton Limestone.

I believe I've identified the South Bend limestone here, and Rock Lake shale member indicated by Myalina and Aviculopecten (per KGS).

Possibly Vilas shale in lower part.

Very tentatively... Virgilian stage, Douglas group, possible exposure of Stranger formation in loess.

Tonganoxie sandstone, Weston shale.

What I really love about this place is the sand/gravel bed is loaded with tiny fossils.

Shale

Sandstone

Creek bed

And thanks for your inspiration!

Those pics look familiar. I'd like to see another 'backyard trip' thread.

Regarding your strata, it's hard for me to say. Good exposures of Rock Lake and South Bend have been elusive for me so far. I've had trouble confirming them unless I could find some black Eudora Shale nearby (with the Stoner Limestone in between). I'm not sure about id'ing units with myalinids and Aviculopecten, though. Those seem to show up together in a lot of places.

(Edit: Myalinids can be useful as markers in some cases. Large shells of Orthomyalina helped me id the Farley Limestone close to home. Aviculopecten, on the other hand, is just too common to be of use, unless they happen to occur in great numbers in a horizon.)

I did had a favorite Rock Lake spot in a quarry just south of KCI, but they absorbed it into their restricted zone after 9/11.

Determining your stratigraphic level can get tricky when you get up into the Douglas Group. The Weston Shale can be pretty thick, and the Tonganoxie Sandstone can cut down through several units into the Lansing. That's when I have to pull out the geologic map.

Edited January 25, 2012 by Missourian

[Wrangellian]

As you know, Missourian, my local Nanaimo Gp up here on Vancouver Island is not so finely graded/divided as your Central NA fms, and I'm not as adept at distinguishing them, mainly I just know by the location/geo map. There are some generalizations, but even within the Haslam Fm for example, it can vary quite a bit from sandy shale on Herd Rd to fine, 'baked' black shale on Somenos Rd, to a mix of unbaked, baked and sandy at Mt Tzuhalem. We have to use a combination of the general rock types and the fossils (if any) found therein to get an idea of the stratigraphy, for example Uintacrinus and Marsupites Crinoids identify the age as Upper Santonian and therefore Haslam (on south-Island) or Lower Trent Riv Fm (north-Island). I can't think of any spots in my area where 2 or more of the Nanaimo Gp fms are visible in the same exposure.

Basic SE Vancouver Island strat chart:

South-Island Fms: -- age -- Equivalent North Island Fms:

Gabriola (sandstone), Mayne (shale) -- Maastrichtian -- Hornby (cong.), Spray (shale)

Northumberland (shale)/ Galiano (conglomerate) -- Camp/Maast. -- Lambert (shale w/fossils), Geoffrey (cong.)

DeCourcy (sandst.), Cedar District (shale w/fossils) -- Campanian -- Oyster Bay/Denman (sandst.)

Extension (cong.), Pender (shale), Protection (sandst.) - Campanian - Upper Trent Riv. (shale)

Haslam (shale) - lots of fossils -- Upper Santonian -- Lower Trent Riv. (shale)

Benson (sandst./cong.) -- Santonian -- Comox (sandst./cong.)

Lower Trent Riv Fm is distinguished from Upper by presence of Polyptychoceras ammonites

Protection Fm has abundant plant fossils.

More detail could be given but this is just what I have for now.

Edited January 31, 2012 by Wrangellian

[Missourian]

As you know, Missourian, my local Nanaimo Gp up here on Vancouver Island is not so finely graded/divided as your Central NA fms, and I'm not as adept at distinguishing them, mainly I just know by the location/geo map. There are some generalizations, but ....

Stratigraphic markers may be even more useful in areas like yours where there is a lack of stratigraphic context. For example, and I'm using a hypothetical here, the presence of Uintacrinus or abundant plants could be indicative of a certain formation or member, or even a narrow zone within.

But then if you are in a mountainous area with folded and faulted strata, then all bets are off.

[Wrangellian]

Right.. It certainly is mountainous here, though the Nanaimo Gp layers are not too deformed or mixed up, just generally tilted toward the NE (Georgia Strait, parallel with the inside coast of the Island) if not level. I don't have a lot of experience with this but I guess when you come upon a rock unit that you can't see the relation to another, known unit, you have to use the different lines of evidence togther to tell which Fm it is, ie the rock type, the fossils (or lack of them), etc. as you say.

I wonder if our local Fms would be divided as finely as yours are if the rocks were easier to differentiate. There is no limestone up here, just alternating, thick units of shale and sandstone (and conglomerate mostly at the bottom and top of the Gp). Some are divided into members and zones, though.

I'm not sure any photos I would be able to take would be useful, otherwise I'd post some. The map is the most useful thing for my area!