Friday, July 29, 2011

Zebraic Chalcedony

Some pictures of zebraic chalcedony from a silicified (turned into chert) stromatolitic sample from Missouri.

Zebraic chalcedony in a silicified stromatolitic sample in plain light. Same view as the cross-polarized view below. Refer to that for scale.
The black spots are remnant dolomite and some hydrocarbon (probably bitumen).

4x close up - the same crystal as below is 0.125mm
This zebraic chalcedony is thought to replace evaporites (McBride et. al, 1977) (e.g gypsum or halite).

10x close up - the brightest (white) crystal in the center is 0.15mm.
Megaquartz is forming in the middle of the zebraic chalcedony splays.

Thus, the chances are that this stromatolite formed in an upper intertidal environment. There is no other evidence as the dolomitization that occurred before the silicification of this sample has destroyed all other fabric (if) present.


Mcbride, E. and Folk, R. 1977, The Caballos Novaculite revisited: Part II: Chert and Shale Members and Synthesis. Journal of Sedimentary Petrology, v. 47, n. 3, p. 1261-1286.

An addition of cooler chalcedony:

Chalcedony radiating fans of fibrous crystals. 10x magnification in plain light. Same stromatolite as the previous pictures. No idea what it is replacing though as dolomitization occurred prior to silicificaiton.

Same as above picture. 10x in cross polars. Megaquartz filling in the rest of the pore.

Fans are so cool! No pun intended there :D

Friday, July 22, 2011

Plane geology and other observations

My flight to San Francisco starting from Wichita led me to Minneapolis which in turn flew to SFO.

On the flight to Minneapolis, being a day flight, I stole the window seat on the plane. I doubt the other passenger noticed, especially since he had an aura of alcohol which i could smell before he even sat down!

Since I did not have my camera on me, I kept a note of all the different streams that I came across as the plane flew below the clouds. A 105° day with no clouds was a great day to fly! Cloud cover mostly started to appear closer to Minneapolis and then on the flight to SFO.

The most noticeable geology features on the flight to Minneapolis were the different types of streams. So there are 4 different types of streams in general. Straight, alluvial fan type, braided, and meandering.

Meandering streams are mostly common on flatter lands. Most of the meandering streams on this route had bars within them. These bars as you will see in the figures below were probably old point bars that became isolated from the bank as the water made it's way around it. In time, these bars became stable and then grass & shrubs started to grow on it. Now this is east of the Rockies and the rivers in the Central Plains and north of that are mostly meandering and most of them drain into the Mississippi River. Here are my observations:

The 1st meandering stream that I paid most attention to was a huge river which was closer to being straight than meandering like Fig. 1, but it still had the noticeable point bars. This stream also had bars within the channel that had shrubs/trees growing on it. So these bars were stable and weren't being eroded off enough which allowed plants to grow on them. As i mentioned earlier, these bars were probably created by the water moving around existing point bars.

The other streams that i noticed were highly meandering streams. Whilst some had bars within the channel (Fig. 2), others did not (Fig. 3).

Then getting excited to see the Rockies again, I decided to keep my camera on me on the flight to SFO. Here's what I noticed on the flight:

Red beds of possibly the Chugwater Formation (a sandstone deposited during the Triassic period of about 250-200my) in Wyoming.

This highly meandering stream seen (in WY) where the floodplain (green) is well developed is lying in a valley.

And so here are the Rockies in Wyoming.

First time seeing the Great Salt Lake in Utah. This very arid lake is surrounded by evaporites (white) and longshore currents are creating groin-like structures in the evaporites if you look closer to the bottom (lake portion) of the picture.

First time seeing Lake Tahoe that is right on the border of Nevada & California. I wouldn't have known it was that lake if it wasn't for the nice lady sitting next to me. First thing I noticed was that it was smaller than the Great Salt Lake and was set between the mountains (Sierra Nevadas).

Low cloud hanging over a bridge (not sure which one) in the San Francisco area.

The rest of the pictures are taken once i got off from the plane.

This is the view of the the bay in Milbrae, California and the run way where the planes land at the San Francisco airport. If you look closer at the bay area in between the run way and the land, you'll see that there is a little channel in between the algae. Photo taken at 1pm.

Here's a close up view of that channel.

Here's that same area at 3pm. The water has risen and is flooding over the algae.

Same view with plane landing :D

Same view at 4pm. The algae is all flooded!

Same area at 6pm with the grassy area flooded! The bay water rose quite a bit throughout the course of the day.

The following 2 pics are close up views of the channel inlets

A variety of birds were feeding on the insects (?) within the algae and I caught a close up. My guess is these are Marbled Godwits but I am no bird watcher.

These are a type of Brodiaea (?) found close to the channel. I saw these abundantly growing around the bay area.

At the Ocean Beach (at the Pacific Ocean) in western San Francisco. This is a rocky beach where cliffs are found near the ocean as you see here. Those rocky islands seen off in the distance is the Seal Rocks where sea lions used to roost at.

Now there are only sea gulls and pelicans perking on the island.

And then there are sea caves that have been created by the sea eroding away that sandstone cliff that is part of the Santa Cruz mountains.

Highly eroded cliff face that's overlooking the Ocean Beach.

A platform jutting out into the Pacific Ocean at Muir Beach north of the San Francisco Bay.

Little lagoon hidden away behind the Muir Beach.

Then I came across an authentic Sri Lankan restaurant (Kadupul). I was dying to get some good Sri Lankan food since there is none anyway within or in the vicinity of Kansas.

Had some delicious and spicy lamprice (or lamprie). A dutch influenced dish which consisted of rice cooked in stock with fried fish, poached egg, fried brinjals, a fish cutlet (the brown ball seen in the background), caramelized onions (aka seeni sambol), and fried plantains, all on a banana leaf. It was DELICHE!

To taste a wood apple drink again topped with ice cream was heavenly!

And Krishna ended up having a Sri Lankan biriyani. It was a tad spicier than what we expected! Was made of fried rice with cashews, raisens, a poached egg, and a variety of vegetables.

On the plane back to Kansas, not having a camera, I made more drawings of fluvial (river) geology.

Braided streams are more common within a mountainous range. This is what I observed which I believe were in the Sierra Nevadas. There were of course meandering rivers like in the following picture.

Here's an attempt on a panoramic view from my phone camera of a salty lake (Great Salt lake again?) possibly near the Colorado/Utah border.

And so my observations ended there as night befell the rest of the journey back.

I have to give a lot of credit to my boyfriend for sponsoring my trip to San Francisco as I observed and learned a lot during the trip.

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.

Saturday, July 9, 2011

Fanglomerates - deceptive but not if you take a closer look

The new word I learned today is Fanglomerate. I think it's quite strange where I came across it. I'm currently reading a book about stromatolites (since after all my thesis work is about them), and one of the chapter's is explaining the physicochemobiogeo (hope that's a word! should i add nano?? ;D) changes that had occurred to the 3rd rock from the sun that caused the right environment for these organo-sedimentary structures to form. Suddenly the author decides to throw out the word fanglomerates as part of continental deposits created around 2.6-2.0 by. So ofcourse my 1st reaction is that it's a conglomerate.....but why term fanglomerate? So unlike flakestone, there's plenty of descriptions and definitions about fanglomerates. Made me feel stupid of course! Didn't see the break down of the word to fanglomerate.....

So to sum up what i've read about fanglomerates: these are conglomerates (my gut instinct has never failed me!) but those that are usually coalesced to form an alluvial fan. Thus they would mostly be found at mountain foothills where the mostly coarse (large) material eroded off the mountains mixed with finer material to form the fan at it's base or into a mountain lake and then was cemented and solidified into a rock.

Here's the American Geologica Institute definition:

So my question is, can a delta or deep water alluvial fan be called a fanglomerate too?

And I have to give credit to a geologist Callan Bentley where he has a good picture of a fanglomerate that is found at a contact metamorphism near the Sierra Nevadas! Check out his blog & pictures:

Now i understand why my professor goes off on a tangent every time she lectures! I went off on the biggest tangent yet enough to blog, whilst reading about stromatolites! :( Back to work!

Wednesday, June 29, 2011

Flakestones - a self-explanatory word

I ran into the word flakestone as I was doing some reading on the Trucial Coast tidal flats. And even though the word is pretty self explanatory I wanted to find out what it actually meant and how it could form. So as usual I checked the internet's most reliable for definitions - google, Schlumberger's oilfield glossary, and Not getting anywhere, I decided to do some research and this is what I have come up with.

As the name suggests, these are rocks made from flakes and may have different compositions. They may be of dolostones/dolomites (flake-breccias) origin (Fairchild, 1980) or
they may be of mudstone (mud-flake breccias) origin (Fairchild, 1980, Pickering, 2005).

I have so far, mostly come up with them being formed in marine settings; be it shallow or deep, but that maybe because I am biased towards reading more about marine carbonates.

In the shallow marine, these flakes are usually related to dessication cracks where after forming the cracks these flakes can be found within those cracks that are more susceptible to erosion as they get transported to those 'depressions'. (See figure below)

Another creation of the flakes may be due to sliding or slumping of the material as found in the more deeper marine settings. And whence formed into a rock, it's called a flakestone.

Flakestone portion as part of a flake pocket of dolostone flakes (Adapted from Fairchild, 1980)

My initial idea was that these flakestones were similar to conglomerates. In a manner, they are a type of breccia more than conglomerate since they may form due to slides and slumps and are more pointy at both the edges.

I hope one day that I will come across a flakestone and thus be no longer confused!


Fairchild, I. J., 1980. Sedimentation and origin of a Late Precambrian 'Dolomite' from Scotland. Journal of Sedimentary Petrology, v. 50, n. 2, p. 0423-0446.

Pickering, K. T., Corregidor, J., 2005. Mass-Transport Complexes (MTCs) and Tectonic Control on Basin-Floor Submarine Fans, Middle Eocene, South Spanish Pyrenees. Journal of Sedimentary Research, v. 75, n. 5, p. 761-783.