Tuesday, 1 October 2013

The Woodburn sands of time

I’ve been spending some time working on a project in the lower reaches of the Richmond River Valley. This project got thinking about the stratigraphy and depositional history of that area. Particularly about a unit of unconsolidated sand called the Woodburn Sands (Drury 1982). In some ways this post follows on from a couple of posts that touched on the subject of sea level changes during the Quaternary.

To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.

Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.

The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.

Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand.  These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.

Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.

Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:

...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...

As such, I have now changed my mind! The Broadwater Sandrock member is not the best name after all. It seems that 'B-horizon of the humus podzol' is indeed one of the best ones. Humicrete is another good one. Well, it seems that the diversity of names will probably continue, but we can remove the one I thought the simplest (Broadwater Sandrock member) from the list!


*Den Exter, P.M. 1974. The Coastal Morphology and & Late Quaternary Evolution of the Camden Haven District, NSW. Australia. PhD thesis. University of New England, Armidale.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.


  1. Interesting to me as an amateur as I have been puzzling over two features notable at Wooli. Layers as described at Diggers occur at Jones' Point. They are now very exposed being the most seaward land in the vicinity. The other feature is the indurated sand cliff of about 1 meter height which is present along Wooli River and associated creeks which is currently being everywhere undercut causing large trees to be continually falling into the river. (A feature also of Bonville Creek near Coffs Harbour) Towards the river mouth this indurated sand underlies the sandhills but does not appear above high water mark. This naturally makes me wonder about the times of sea level changes.

    1. Hi Grant, I'm really an amateur too... despite me having this blog.

      I think that I remember seeing a photograph of just such a feature a little way up the Wooli River. I can't remember where I saw it though. I've not been to Wooli for a fair while. Your observations of possible features preserving evidence of a previous higher sea level seems quite reasonable to me. Thanks for letting me know about a few more. I'll have to check those out if I get the opportunity.

    2. Rod, humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" by Bruce Druery. Where the accumulation of cations and organic matter as a B-horizon occurs above the water table in a high dune, the sand is usually reddish brown due to the iron. Where the organic matter accumulates AT the water table, the humicrete is usuall dark brown or black although the organic carbon content may be quite low (1-2 %).
      The best sea level indicators in the Wooli region are on the southern side of Angourie Point where lower beach pebbles indicate a Last Interglacial Pleistocene sea level about 3 m above present. Very nice outcrops of beach cobbles within humicrete also occur along Jerusalem Creek near Black Rocks south of Evans Head, and are part of Macauleys Lead, an erosional disconformity within the huge Qp barrier to the north of Woombah.

    3. Hi Anonymous,

      Upon rereading my post it is apparent that I implied that Coffee Rock is not diagenetic. As you have rightly pointed out, Coffee rock is diagenetic. This is obviously important for stratigraphy too. I have added a postscript to the post including some of your comment.

      Thanks for making things clearer.