Is this a mud volcano?

One of our Bath GS members sent this prior to this Thursday's talk about mud volcanoes.

 These occur at just one horizon, close to the top of the clay/lime bands, Lias, Lyme Regis. Locally they are known as sunstones. They outcrop near Charmouth at beach level, exposed at a very low tide.

Extra info:- The rock is of about the size and proportion of an old style galvanised steel dustbin lid. I think we see two processes here. Firstly a precipitation of carbonate within the surface sediment as a consequence of methane escape. This results in a slight dome around the vent. Secondly the flow of fluids carrying sediment from below and trickling down the slope as meanders, with the loss of gas these meanders stop in a consistent distance.

Reply:
This could be a septarian concretion - so probably composed of calcite. The radial cracks are a source of some argument - whether they have formed through dewatering / contraction of the sediment or some other process. The cracks will probably also be filled with calcite crystals, sometimes other minerals (siderite, dolomite, barite, quartz etc).

Photograph above of a somewhat similar nodule from the Carboniferous of NE England.

More thoughts: It is a septarian nodule on, I suspect, Monmouth beach. In fact it is a Birchi nodule from the Bichi bed in the Charmouth Mudstone formation and just in this area the beef calcite tends to grow in this very symmetrical fashion. Locals call them 'sunstones' and I think they feature on Ian West's site. The geochemistry behind beef formation is very complex and is, I believe, related to very rapid burial and therefore pressures in poorly consolidated muds that have a high organic/methane content and that was happening at that time.