May 13. Jefferson formation

After hearing about all the organic material in the Marcellus Shale, you may start to think that there was a lot of life in the Devonian, dying in the oxygen poor water and falling apart and getting trapped in sediment to become oil and gas millions of years later. On the whole, you’d be right. The Devonian – mostly middle to late Devonian – is one of the six primary oil source systems on earth, and by far the largest one in the Paleozoic era. It depends on who’s making the estimate, but Devonian source rocks worldwide may contain as much as 60 billion barrels. For comparison, Cretaceous source rocks might hold as much as 500 billion barrels. 

When I called the depositional environment of the Marcellus Shale a stagnant sea, I more or less mean that literally. Organics in the rock generate hydrogen sulfide gas – the rotten egg smell associated with a lot of oil and gas.

Out here in Montana, you may recall that the Devonian rocks rest directly upon the Cambrian rocks, and all the Ordovician and Silurian rocks have been eroded away. If there were any early and middle Devonian rocks, they were eroded away too – the oldest Devonian, generally here in western Montana, is Late Devonian in age. The most prominent rock unit is called the Jefferson Formation, named in 1893 for exposures along the Jefferson River, near the three forks of the Missouri in southwestern Montana. It consists of massive dolomite – calcium magnesium carbonate, like limestone except for the magnesium in the mineral’s crystal structure. It’s anywhere from 200 to 800 feet thick, and there are some interesting things in it that make it a geology student trying to map the rocks happy to see it.

For one thing, there's a really prominent member at the top, called the Birdbear. It's usually pretty distinctive. Then there are conglomerates in places, rocks with really large grains, as much as several inches long. But these pebbles are made of the same kind of rock, dolomite, as the rest of the formation. And they are long, but flat – maybe a half-inch thick but 4 or 5 inches long. These things are called intraformational conglomerate, which means that the pebbles came out of the formation itself, rather than being washed in from some eroding land source. They’re also called flat-pebble conglomerates, for the shape of the pebbles. They probably originated when storms ripped up bits of the sea floor that had started to lithify from sediment into solid rock. The stuff the deep storm waves tore up was a lot more solid than loose sediment, but not as solid as hard rock. So the waves tore flat slabs of semi-solid carbonate off the sea floor, and when things calmed down, those chunks fell back into the sediment and were incorporated into it. It can make for a distinctive, and rather pretty rock. But there are intraformational conglomerates in lots of carbonates, so that alone doesn’t tell the geology student that he or she is in the Jefferson formation.

The Jefferson formation is often black. That may seem surprising for a limestone or dolomite, but it’s actually fairly common for the impurities in limestone to make it gray, and more and more impurities – especially if those impurities are organic matter – can indeed make it black.

In the deep Williston Basin of eastern Montana and western North Dakota, the Jefferson formation is an important reservoir for oil, and even here in western Montana if you whack a piece of the rock with a hammer, you often get a distinct rotten egg smell – hydrogen sulfide, the decay product of organic material. That’s a pretty good clue that you’ve found the Jefferson, but it doesn’t ALWAYS have that smell.

If the Jefferson started out as a limestone, and mineral rich waters percolated through and converted it to dolomite – and that’s a reasonable possible way for dolomite to form – you might expect it to have fossils. And it does. You can find little horn corals, and in some places there are these little white things that look a lot like short strands of spaghetti. Both the corals and the spaghetti-looking stuff are silicified – turned to quartz, which stands out in marked contrast to the black dolomite. The spaghetti stuff is probably the remnants of algal mats – the things that made stromatolites back in the Precambrian, some of the earliest large evidence for life on earth. Those mats might have gotten torn up in the same storms that made the flat-pebble conglomerates, and they probably became silicified because they preferentially were replaced by silica from the water, just as petrified wood becomes silicified but the surrounding rock does not, at least not necessarily.

One of my professors at Indiana University’s geologic field station here in Montana, Lee Suttner, called the “spaghetti rock” one of your few geologic “friends” – if you found it in a dolomite, you were definitely in the Jefferson. If the dolomite was black, and smelled like rotten eggs, and had flat-pebble conglomerate, and was between the Cambrian rocks and what you expected to be above it, well then, you really had it nailed. Put it on the map with great confidence.

—Richard I. Gibson

Resources:
USGS – Jefferson formation
Flat-pebble conglomerate photo