11 April, 2009


After returning from fieldwork, partaking in a course on "Organic Petrology and Geochemistry of Petroleum Source Rocks" together with MapleLeaf and spending some days relaxing (finally!) I want to share with you a very nice example of some oncoids from the German Middle Jurassic that I found during my diploma mapping project in the Upper Rhine Graben. The image below is a neat cross-section through one of the many oncoids I found. It's not cut nor polished but simply splitt with a well aimed blow of the rock hammer.

Bored oncoid with shell nucleus

On top of the Middle Hauptrogenstein is a layer called Mumienbank (Mummy Bed) that consist almost entirely of oncoids of up to 6 cm in diameter. The first geologists to work on this coined the term because this layer is unusually rich in gastropod shells (Nerinea sp. most commonly) as nucleus of the oncoids. The below sample is a wonderful example of the nucleus forming gastropods picked from another location within my mapping area.

Oncoids with gastropod shells as nucleus

There is one really cool outcrop of the Mumienbank in the southern half of my mapping area that should get even the most reluctant to utter some equivalent of "cool" or "wow". It is even better when standing right in front of it of course. The lower layer is one of the usual Oo-Pack-/Grainstones that are so abundant in my mapping area. In the middle is a layer of smaller oncoids of up to 2 cm diameter. In the upper layer oncoids are 2 to 5 cm in diameter.

Outcrop of the Mumienbank with upward increasing oncoid size

So what's an oncoid? How do we recognise one in the field and why does it matter?

An oncoid is in the most basic sense nothing but nodular coated grain. A nucleus that can be of diverse origin is encursted by microbes, algae or other encrusting organisms, i.e. foraminifera. They are unattached, rounded and up to several cm in size. There are both calcerous and non-calcerous nodules of more or less concentric nature and with partially overlapping laminae around a bio- or lithoclast as nucleus. In America the term pisoid is also still in use for oncoid. They are most common in limestones and marls. Best way to recognise them is size and the irregular, concentric and partially overlapping laminae around a nucleus.

So why does it matter? It matters because they are in contrast to most other carbonate grains very good indicators of palaeoenvironment, sea-level changes and depositional settings. They appear from tropical to arctic environments, depending on the encrusting organisms. Even though they can be found even in basinal deposits most fossil oncoids appear in shallow environments and peritidal carbonates. Beware - they are not exclusively marine and can be found in freshwater and saline waters in lacustrine and fluvial environments from the Precambrian until today.

So why does that matter to me when I map potential lime resources? Two reasons: First, they are very easy to recognise in the field without any special equipment. The oncolite horizons form excellent stratigraphic orientation marks in the field, indicating the very top of a several 10s of meters thick and very pure limestone succession. Second, while mapping I noticed marly clay inbetween the oncoids at least in some areas. That's undesirable considering your resource requirements and has to be considered.

There exist a wide range of oncoids that I don't have the time to detailledly explain here. I recommend taking a look into Carbonate Sedimentology by Maurice E. Tucker and V. Paul Wright (take a look at my post about Wright's Revised Classification of Limestones) or Microfacies of Carbonate Rocks by Erik Fl├╝gel. If you are more a fan of the internet take a look into the USC Sequence Stratigraphy Web.

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