Welcome to the Foreshore Reserve on the bank of Swan River near Peppermint Grove.
Type of EarthCache: Sedimentary feature and fossil site.
Tamala Limestone
The Tamala Limestone stretches from the Shark Bay area of the Carnarvon Basin in the north to Naturaliste Region (between Cape Naturaliste and Cape Leeuwin) in the south. The type section, as defined by Playford et al. (1975) is at Womerangee Hill on the Zuytdorp Cliffs (in the Carnarvon Basin).
The Tamala Limestone consists of medium- to coarse-grained (0.25-1.00 mm) calcarenite, composed largely of skeletal fragments (mainly foraminifers and molluscs), containing variable amounts of quartz sand. Large-scale cross-bedding is characteristic, while soil horizons and rhizoliths (calcified root structures) are common.
The formation was deposited as coastal sand dunes, and in the Perth Basin (especially in the central part), it occurs in several belts representing successive lines of late Pleistocene dunes. The most extensive is the band forming the Spearwood Dune System.
The Tamala Limestone contains several marine sub-units (called members) which are not so extensive; one of these, is the Peppermint Grove Member which we will see at Peppermint Grove.The distribution of Tamala Limestone in the Perth Region (between Guilderton and Mandurah) is shown in Figure 1.
Figure 1. Generalised geological map of the Perth region.
(Source: modified from Davidson, 1995)
Tamala Limestone at Peppermint Grove - Introduction
The Tamala Limestone at Peppermint Grove (Figure 1) is exposed at the Peppermint Grove cliffs, which is located near the Scotch College Boat Shed on the banks of the Swan River in shire of Peppermint Grove.
It is an excellent location to observe the aeolian units and the shelly marine units of the Tamala Limestone. The marine units here constitute the type section of the Peppermint Grove Member of the Tamala Limestone (Fairbridge, 1954). They can be seen to interfinger with the aeolian parts of the Tamala Limestone (Gozzard, 2007).
Sedimentary Features
The shallow marine beds at the base grade up into beach environment and are capped by aeolian deposits, which make up the bulk of the ridge. This site illustrates coastal progradation (i.e. gradual retreat of the sea) from shallow-marine environment to a beach environment. This sequence is very typical of the Pleistocene coastal dunes in the Perth region (from Guilderton to Mandurah) and in other parts of Australia (Moss et al., 1998).
The aeolian parts of the Tamala Limestone at Peppermint Grove are mainly medium- to coarse-grained (0.25-1.00 mm) calcarenite. Compositionally it consists of sand-sized (0.0625-2.00 mm) fragments of molluscs and foraminifers with substantial amounts of detrital quartz and lesser amounts of feldspar (Figure 2). The marine calcarenites of the Tamala Limestone (Peppermint Grove Member) is shell rich and contains whole shell fragments.
Figure 2. Aeolian calcarenite (with sand-sized grains) of Tamala Limestone at Peppermint Grove.
(Source: Dark Energy, 2012)
Sedimentary features are important attributes of sedimentary rocks. They occur both upon the lower and upper surfaces of beds, as well as within beds. Many of these features are formed as the sediment is deposited; hence, they contain significant amounts of information regarding the processes and conditions of deposition of sediments.
These sedimentary features include;
1. Medium-scale cross-bedding (Figure 3), which is indicative of offshore deposition in shallow water. In this environment, water currents cause ripples in the sediment to migrate, and as the lee sides (downstream sides) avalanche, dipping foreset laminae are produced (Figure 4). The orientation of the cross-beds therefore indicates the direction of the currents.
Figure 3. Medium-scale cross-bedding
(Source: modified from University of Oxford, Department of Earth Sciences, n.d.)
Figure 4. Cross-bedding showing direction of current, lee side, and foreset laminae.
(Source: modified from Sedimentary Stuctures, n.d.)
2. Ripple beds (Figure 5), which form because of the oscillating motion of waves, and therefore indicated deposition in a near-shore surf zone.
Figure 5. Ripple beds.
(Source: Datapages, 2012)
3. Planar-bedded (Figure 6) calcarenites, which indicates high-energy movement of sediment in a foreshore beach environment, which is also referred to intertidal zone, littoral zone, and seashore. It is the area that is above water at low tide and under water at high tide (the area between tidemarks).
Figure 6. Planar bedding.
(Source: About.com, 2012)
4. Cemented beach rock (Figure 7) consists of pebbles (4-64 mm) and cobbles (64-256 mm) indicating beach environment of deposition.
Figure 7. Beach rock of Tamala Limestone at Peppermint Grove.
(Source: Dark Energy, 2012)
5. Large-scale cross-bedded aeolian calcarenites. These large-scale cross-beds (Figure 8) represent the avalanching lee sides of migrating dunes through the action of wind, and indicate a major dune-building episode.
Figure 8. Large-scale cross-bedding in the Navajo Sandstone.
(Source: Roadgoer, n.d.)
6. Rhizoliths (calcified root structures as shown in Figure 9), which are formed after the sediments had been deposited through the calcification (via circulating groundwater) of plant root systems.
Figure 9. Fossiliferous rhizoliths of Tamala Limestone at Peppermint Grove.
(Source: Dark Energy, 2012)
Fauna and Age of Rocks
The fauna of the marine units of the Tamala Limestone at Peppermint Grove includes abundant foraminifers and molluscs. Kendrick (1960) has shown that the molluscan fauna is typical of a shallow-marine gulf, which occupied the site of the present-day Swan estuary, under temperatures similar to those occurring today (Playford et al., 1976). Figure 10 shows a number of molluscan fauna found in the marine units. Kendrick (1960) considered the molluscan faunas to be Middle Pleistocene in age.
Figure 10. Molluscan fauna of the marine units of Tamala Limestone at Peppermint Grove: (G1-G4) gastropods and (B1-B5) bivalves.
(Source: Dark Energy, 2012)
How to claim this EarthCache?
Send me the following;
1. The text "GC7KJW0 Foreshore Reserve - Tamala Limestone" on the first line.
2. The answers to the following questions;
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Study the sedimentary beds A, B, C, and D as shown in Figure 11 (Waypoint 01). Where applicable, determine the sedimentary structures (medium-scale cross-bedding, ripple beds, planar-bedded, and large-scale cross-bedded aeolian calcarenites) and deduce their significance and the environment of deposition. If the sedimentary structures are not clearly seen, please go further north (as shown by red arrow in Figure 11) to make your observation.
Figure 11. (Waypoint 01) Sedimentary beds A, B, C, and D at EarthCache site.
(Source: Dark Energy, 2012, modified)
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Study the sedimentary beds E and F as shown in Figure 12 (Waypoint 02). Where applicable, determine the sedimentary structures (cross-bedding, ripple beds, planar-bedded), clastic components (sand, pebbles, cobbles), and fossil content found in each of the beds and deduce their significance and the environment of deposition.
Figure 12. (Waypoint 02) Sedimentary beds E and F at EarthCache site.
(Source: Dark Energy, 2012, modified)
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Study the sedimentary beds G and H as shown in Figure 13 (Waypoint 03). Where applicable, determine the sedimentary structures (cross-bedding, ripple beds, planar-bedded), rhizoliths, and fossil content found in each of the beds and deduce their significance and the environment of deposition.
Figure 13. (Waypoint 03) Sedimentary beds G and H at EarthCache site.
(Source: Dark Energy, 2012, modified)
3. Provide a photo of yourself or a personal item to prove you have visited the site.*
Glossary
1 Aeolian – Formed by the wind, especially said of deposits such as dune sand.
2 Beach rock – A friable to well-cemented sedimentary rock, formed in the intertidal zone.
3 Calcarenite – Calcareous sandstone to sandy limestone with sand-sized (0.0625-2.00 mm) carbonate grains.
4 Cross-bedding – A term for criss-cross bedding layers in sedimentary deposits, produced by the migration of ripples or dunes on bedding surfaces.
5 Foraminifers – A group of single-celled organisms, mostly marine, that produce a calcium carbonate shell.
6 Molluscs – Invertebrates of the phylum Mollusca. Examples include clams, oysters, snails, squid, slugs, and octopus.
7 Type section – The originally described sequence of rocks that make up a named rock unit or stratigraphic unit (as type area).
References
1 About.com, 2012. Available from TQN. 2 Datapages, 2012. Available at Searchanddiscovery.com. 3 Davidson, W.A., 1995. Hydrogeology and groundwater resources of the Perth region, Western Australia, Western Australia Geological Survey, Bulletin 142. 4 Fairbridge, R.W., 1954. Quaternary eustatic data for Western Australia and adjacent states. Pan Indian Ocean Science Congress Proceedings, Geography and Oceanography Section F, p. 64-84. 5 Gozzard, J.R., 2007. Geology and landforms of the Perth Region. Western Australia Geological Survey, 126p. 6 Kendrick, G.W., 1960. The fossil mollusca of the Peppermint Grove Limestone, Swan River District of Western Australia. Western Australian Naturalist, v. 7, p. 53-66. 7 Moss, S.J., Freeman, M., George, A.D., Marshall, A., and Dunphy, J.M., 1998. The Prider Field Trip – the rock cycle and geology of the Perth metropolitan area. A Professional Development Day for High School Geology and Science Teachers. Geological Society of Australia (WA Division), Excursion Guidebook, No. 10, 72pp. 8 Playford, P.E., Cockbain, A.E., and Low, G.H., 1976. Geology of the Perth Basin Western Australia, Geological Society of Western Australia, Bulletin 124. 9 Playford, P.E., Cope, R.N., Cockbain, A.E., Low, G.H., and Lowry, D.C., 1975. Phanerozoic, in Geology of Western Australia. Western Australia Geological Survey Mem. 2, p. 223-433. 10 Roadgoer, n.d. Cross-bedding in the Navajo Sandstone. Available from Flickr. 11 Sedimentary Structures, n.d. Available from ocw.mit.edu. 12 University of Oxford, Department of Earth Sciences, n.d. Available from Earth.ox.ac.uk.
* Effective immediately from 10 June 2019, photo requirements are permitted on EarthCaches. This task is not optional, it is an addition to existing logging tasks! Logs that do not meet all requirements posed will no longer be accepted.
For additional information, visit; Geosociety.org, Geocaching.com Help Center and Geocaching.com Forum.
Finding the answers to an EarthCache can often be challenging, and many people tend to shy away from these caches because of this. However, it is my opinion that geocaching is also meant to be a fun family experience that simply aims to introduce interesting and unique locations such as this one. Flexibility on logging requirements, however, can only be applied if it can be established that you have actually taken the time to visit the site. For this reason, a proper log describing your adventure accompanied by a good number of photos would be much appreciated.