Intertidal high energy over intermediate substrate

Long description

Intertidal very high to medium wave energy over coffee rock on a sandy beach, which is exposed and buried by sand deposition associated with wave action (e.g. storm events, cyclones, longshore drift). Often continuous with subtidal coffee rock, refer to type 101 (subtidal intermediate substrate including coffee rock).

Coffee rock is a soft, dark-brown coloured friable rock consisting of sand grains weakly cemented together by organic matter (mostly plant-based). Coffee rock formed when humic acids from decaying plant matter are washed down through sandy soils, meeting shallow aluminium-rich groundwater where it precipitates to form an indurated sand layer in the B horizon of podzol soils. These occur on quartz sand systems in the sub-tropics and tropics[1]. On high energy shorelines it has become exposed by wave action against the sandy dune escarpment, outcropping on beaches and in the shallow subtidal area. Wave action can break up the soft rock into boulders by undermining the cliff face with coffee rock blocks dropping or sliding off the cliff face[3].

As it forms a hardened layer, coffee rock can be associated with groundwater discharge via freshwater seepage or springs draining through the sand dunes and then flowing sideways where it meets the low permeability rock (see Coastal sand masses (high dunes) conceptual model). Habitat forming biota other than green algae (e.g. Ulva sp. and other freshwater tolerant filamentous algae) seldom grows in pools in the coffee rock. Crabs may occupy the crevices within the rock (Moyra McRae pers. comm.).

Special values

Provides habitat for fish and other mobile fauna at high tide on an otherwise featureless shoreline. High energy coffee rock is associated with groundwater dependent ecosystems. Following high wave energy events, dense algal cover on coffee rock can provide important food source and habitat for herbivores.

Diagnostic attributes

Inundation 'Intertidal – Lower low', 'Intertidal – Mid low', 'Intertidal – Upper low', 'Intertidal – Low undifferentiated', 'Intertidal – Lower medium', 'Intertidal – Upper-medium', 'Intertidal – Medium undifferentiated', 'Intertidal – High', 'Intertidal – Undifferentiated', 'Intertidal – High undifferentiated'

Consolidation 'Intermediate'

Energy magnitude (wave) 'Very high', 'High', 'Medium'

Qualifiers

The Naturalness qualifier is relevant as coffee rock is very soft and it is possible to scrape the surface or cut and remove it. Coffee rock is commonly encountered during beach driving where it becomes an obstacle to drivers, who in some instances have cut through coffee rock to gain beach access. Coffee rocks can be damaged by vandalism or graffiti. The Period and Trend qualifiers are also relevant as it can be alternately exposed and buried by sand after storms, cyclones, east coast lows and associated high energy wave action. (Moyra McRae pers. comm.).

Distribution

Coffee rock can be found up and down the Queensland coastline as a result of coastal processes and sea level change. High energy shorelines with coffee rock are found on K'gari (Fraser Island) and Cooloola southward, associated with high wave action along dunes of the large sand islands Bribie, Moreton and Stradbroke, and other coastal sand masses. Coffee rocks occur on sand masses further south in New South Wales (e.g. Broadwater Beach, Black Rocks), in New Zealand and overseas (e.g. South America).

The following relates to distribution of this ecosystem type within the Central Queensland mapping area:

• High energy coffee rock typically occurs on sandy beaches along the eastern shore of K'gari (Fraser Island) and Rainbow Beach. It is a remnant of a time when the K'gari (Fraser Island) sandmass extended further seaward and the coffee rock that we can see today was further inland and a part of the sandmass soil layers[2].
• May be present in type 30 (intertidal high energy over sand) as this is a rapidly changing environment, and coffee rock is exposed and covered on sandy coastlines. Coffee rock is mapped as a co-type.

Comments

Other relevant attributes could include Substrate composition, Energy source, Energy magnitude and Lithology. The Terrain slope, Terrain roughness and Terrain morphology are indicative of cliffs, platforms, pools, crevices and other spaces as potential habitat for macroalgae and other biota. Associated with Lithology is the resistance of the rock to erosive processes and a Rock Mass Index has been used to measure this[3].

Additional Information

• K'gari (Fraser Island) - Nature, culture and history (Department of Environment, Science and Innovation)
• Coastal sand masses (high dunes) - WetlandInfo

References

1. ^ Cox, M, Harbison, J, Ezzy, T, Preda, M, Brooke, B, Lee, R, Lester, J, Oberhardt, M & Laycock, J (2000), 'Coffee rock: an overview of its character and occurrence in the Pumicestone region', PASSCON 2000. Science informing catchment management : 22 and 23 November, 2000, Queensland University of Technology : extended abstracts / Pumicestone Passage and Deception Bay Catchment Conference, pp. 23-24, QUT, ed. Malcolm E. Cox.
2. ^ Queensland Government (2018), 'K'gari (Fraser Island), Great Sandy National Park - Nature, culture and history', Department of Environment and Science. [online] Available at: https://parks.des.qld.gov.au/parks/fraser/culture.html.
3. ^ a b Woodroffe, CD (2002), Coasts: form, process and evolution, Cambridge University Press.

Last updated: 12 July 2019