Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia

Marine Geology 159 Ž1999. 303–321 www.elsevier.nlrlocatermargeo

Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia C.D. Woodroffe ) , D. Grime School of Geosciences, UniÕersity of Wollongong, Wollongong, NSW 2522, Australia Received 4 September 1998; accepted 14 December 1998

Abstract Storms are considered to have significant impacts on the development of chenier plains, particularly through the devastation of mangrove vegetation, but also in terms of winnowing sand and shell from mudflats and forming chenier ridges. Shoal Bay, in the Beagle Gulf, northern Australia, contains a small chenier plain, which was struck by a severe tropical cyclone, Cyclone Tracy, on Christmas Day 1974 as it devastated the city of Darwin. The morphology, vegetation and stratigraphy of the plain are described. The plain is underlain by lower intertidal sand with shell hash. A radiocarbon age of 6130 years BP, indicates mangrove colonisation of this flat at the time that sea level stabilised around its present level after the postglacial transgression. Subsequently, progradation of mudflats has occurred, especially around 2300 years ago, and shells of this age are found both in growth position from within the mudflats, and incorporated into the shelly chenier ridges. A further phase of build-out, and subsequent erosion is reflected by stumps and in situ bivalves being excavated presently on the foreshore, radiocarbon dating about 900–1100 years BP. Within this context of evolution, Cyclone Tracy can be seen to have had severe consequences upon the patterning of mangroves, with extensive windthrow of Ceriops, and defoliation of Rhizophora at the western end of the plain. Recovery of the mangrove vegetation has been gradual, and is still incomplete. However, the storm had little influence on the pattern of landform development. Indeed, the aerial photographic sequence indicates a trend by which sand shoals are gradually reworked landwards, in places moving through mangrove vegetation. The study suggests that rather than pronounced regional episodes of alternative mudflat buildout and erosion, both processes occur simultaneously at different points along this foreshore. q 1999 Elsevier Science B.V. All rights reserved. Keywords: coastal morphodynamics; chenier plain; mangroves; storm; Northern Territory; Australia

1. Introduction Chenier plains are broad, prograded, suprarintertidal plains, with a series of sandy or shelly elongate chenier ridges perched on finer-grained, nearshore sediments ŽOtvos and Price, 1979.. The sequence of )

Corresponding author. Fax: q61-2422-14250; E-mail: [email protected]

single or bifurcating ridges indicates episodic ridge deposition in what are otherwise mud-dominated environments. Chenier plains are often associated with river deltas; the term derives from Louisiana, where ridges upon which the live oak Žchene ˆ . grows, occur across a broad plain to the west of the Mississippi River Delta ŽRussell and Howe, 1935.. The Louisiana cheniers, and cheniers from Suriname, are examples of bight coast chenier settings; cheniers are found in

0025-3227r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 5 - 3 2 2 7 Ž 9 9 . 0 0 0 0 6 - 7

304

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

bayhead settings ŽThompson, 1968; Otvos and Price, 1979., and may also occur in bayside settings ŽCook and Mayo, 1977; Woodroffe et al., 1983.. Cheniers are found on low gradient, low to intermediate wave-energy, microtidal to macrotidal settings in all latitudes, but are especially characteristic of muddy tropical coasts ŽAugustinus, 1989.. There are numerous chenier plains along the north Australian coast ŽChappell and Grindrod, 1984; Short, 1989.. For cheniers to occur there needs to be an abundant supply of mud, longshore transport of sand-sized sediments, and alternation between these conditions ŽOtvos and Price, 1979.. The mechanism by which they alternate has been the subject of considerable debate. In the case of the classic Louisiana chenier plain, the mechanism that has been suggested has been delta switching ŽGould and McFarlan, 1959.. Thus, when the Mississippi River discharges through distributaries to the west of the broad deltaic plain, rapid progradation of the coast by muddy sediments is experienced, and when the river is concentrated in a distributary to the east of the plain, the chenier plain does not receive the large quantities of mud, and erosional processes are thought to dominate, with winnowing of the coarse sand fraction from the eroding foreshore ŽTodd, 1968; Hoyt, 1969.. Although later studies have significantly revised the chronology of delta switching in the Mississippi delta, and indicate that more than one delta lobe may have been active at any time over much of the mid to late Holocene, the chronology of the chenier plain is, nevertheless, thought to be linked to the alternation between delta lobes ŽPenland and Suter, 1989.. Other authors have suggested other mechanisms by which chenier ridge formation, and mudflat progradation, alternate. Schofield Ž1960. proposed that changes of sea level triggered ridge formation in the Firth of Thames, New Zealand; although that interpretation is not supported by Woodroffe et al. Ž1983.. Along the coast of Suriname and Guyana, the lateral migration of mud shoals is considered to trigger episodes of ridge forming, or coastal buildout ŽAugustinius et al., 1989; Daniel, 1989.. In a detailed radiocarbon dated, palynological study in Princess Charlotte Bay, in northern Queensland, Chappell and Grindrod Ž1984. proposed that the alternation was controlled by an internal dynamic, within which both patterns of mud deposition and shellfish dynamics

are important. Rhodes Ž1982. believed that ridges were formed in the Gulf of Carpentaria during long dry periods of lower river discharge, whereas coastal mudflat progradation occurred during wetter periods. Regional climate change has been inferred to have influenced cheniers elsewhere in northern Australia ŽLees and Clements, 1987; Lees, 1992.. In almost all cases, storms are considered to be an important element in chenier plain evolution. Russell and Howe Ž1935. believed that chenier ridges were beaches moved bodily inland by storms, and later studies have reiterated this view ŽGould and McFarlan, 1959.. Storm surges coupled with high tides have been supposed to be responsible for chenier ridge formation in Sierra Leone and in the Yangtse Delta ŽAnthony, 1989; Qinshang et al., 1989.. In Australia, storms are also recognised to have devastating impacts on mangroves fringing chenier plains, which then enables storm waves to drive sand and shell landwards and deposit a ridge behind the mangroves ŽCook and Polach, 1973; Rhodes, 1982; Chappell and Grindrod, 1984.. Lees Ž1987. interpreted radiocarbon dating of part of a chenier plain at Point Stuart as indicating that ridges were deposited by every storm event over a short period in the late Holocene. However, Chappell and Grindrod Ž1984. considered it too simplistic to attribute a chenier ridge to each high energy event, and proposed a more complex internal dynamic. The exact role of storms in the formation of chenier ridges, and the response of chenier plains to individual storm events has rarely been documented. This paper describes the morphodynamics of a small chenier plain in Shoal Bay, northeast of Darwin in the Northern Territory of Australia. The plain was struck by a particularly intense storm, Cyclone Tracy, which devastated Darwin on Christmas Day in 1974. In this study, the morphology and vegetation of the chenier plain is described in detail. The stratigraphy of the plain is examined, and the chronology of Holocene development of the plain is described, adding to the initial radiocarbon dating results of Hickey Ž1981.. The pattern of change before and after Cyclone Tracy is described by mapping the plain from aerial photographs. It is shown that the storm, with a probable 100-year recurrence interval, has had an impact on the patterning of mangrove vegetation which has been slow to recover. How-

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

ever, the storm itself had relatively little geomorphological impact on the chenier plain, but a longer-term pattern of change is seen, with ongoing migration of seawardmost cheniers across the plain. The present vegetation and landform pattern incorporates most of the components of chenier plain formation, both erosional and depositional.

2. Regional setting The Shoal Bay chenier plain occurs along the southern shore of Shoal Bay, in the Beagle Gulf ŽFig. 1., north of Darwin Ž12821X S, 130859X E.. It is about 10 km long and averages around 500 m wide. The progradational plain is bounded by Lee Point to the west, and the Howard River to the east. A series of short perennial creeks dissect the plain; Buffalo Creek to the west, Micket Creek, and King Creek. A largely freshwater wetland, Leanyer Swamp, occurs between Buffalo and Micket Creeks, whereas the

305

eastern end of the plain is composed of a sandy foreshore, called Camerons Beach. The landscape of the Darwin region is dominated by an undulating surface cut across Proterozoic and flat-lying Cretaceous rocks, rising 25 to 40 m above sea level. These are considerably weathered and lateritised, and form cliffs and shore platforms with intermittent beaches along the shoreline. The macrotidal range results in strong currents which resuspend muddy sediments, and there are extensive intertidal mudflats and mangroves. Tidal range in Darwin Harbour reaches 7.8 m at springs, but appears to be rather less in Shoal Bay Žsee below.. The plains near the coast are inundated by heavy rainfall in summer, and the Leanyer Swamp area remains inundated till March–April, then dries slowly as a result of evaporation. It is a low wave energy coast, with a shallow gradient; waves of greater than 2.5 m are experienced less than 10% of the time, although waves in excess of 4 m may occur during storms ŽBlain, Bremner and Williams, 1984.. Storm surge is estimated to elevate water levels up to 2 m above HAT Žhighest astronomical tide. with a 100-year recurrence ŽHopley and Harvey, 1979.. The climate is tropical monsoonal ŽKoppen, Am. ¨ with an annual average rainfall of between 1500 and 1600 mm, and with most rainfall during the wet season, November to April. The winds are dominated by easterlies during the dry season, with less strong east to northwest winds during the wet season ŽMcAlpine, 1969.. Diurnal temperature range is from 348C to 248C October to April and from 328C to 178C May to September. Tropical cyclones may be experienced during the months November to April, and 21 severe cyclones were recorded within 150 km of Darwin during the period 1910 to 1975. The most devastating cyclone to hit Darwin was Cyclone Tracy, which struck on Christmas Day 1974; this paper examines the impact of that cyclone on the Shoal Bay chenier plain, and its subsequent recovery.

3. Methods

Fig. 1. Beagle Gulf and Shoal Bay, Northern Territory, and the track of Cyclone Tracy in 1974 Žshowing date and time of eye of storm.. Ž1. Casuarina Beach; Ž2. Lee Point; Ž3. Howard River.

The geomorphology of the plains was mapped from aerial photographs, overflights, and extensive ground observation. Surface topography was determined by levelling along selected transects which

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

306

were tied to benchmarks and related to Australian Height Datum ŽAHD., which approximates mean sea level. The interpretation of the pattern of Holocene sedimentation is based on the stratigraphy of auger holes across the ridges and mudflats, and a series of backhoe pits along the two transects from which Hickey Ž1981. had dated chenier shell. An indication of the absolute age of organic materials was obtained by radiocarbon dating. Samples were submitted to the Radiocarbon Dating Laboratory of the Australian National University ŽGupta and Polach, 1979.. Radiocarbon dates are listed in Table 1; each age is given with a standard error that is a measure of counting reliability and background levels. Dates on wood and shell are not directly comparable because shell samples generally require an ‘environmental correction’ for ocean reservoir effect Ži.e., the shells are taking carbon from seawater which has a lesser 14 Cr13 C ratio than the atmosphere.. While a value of 450 " 35 years is the correction generally applicable in Australia, and would usually be subtracted from the measured age, midden shells dated from the South Alligator River, 100 km to the east, have on a number of occasions given ages younger than this ŽWoodroffe et al., 1986.. This environmental correction does not appear appropriate for all shell species in all settings, but in this study, as in that by Hickey Ž1981., the environmental correction of 450 " 35 years has been applied to the dates. It is also important to recognise that the radiocarbon dates represent the time of death of the organ-

ism, and not the time of its deposition. Thus individual shells may have been reworked several times after their death before being deposited in a chenier ridge. Dates therefore are not always accurate indicators of the age of landform units ŽShulmeister and Head, 1993..

4. Geomorphology and vegetation of the chenier plain The chenier plain is a narrow progradational plain, on which a series of sandy, shelly chenier ridges are perched above a mudflat ŽFig. 2.. The cheniers form a series of long, low, narrow, bifurcating, well-vegetated ridges which run roughly parallel to the shoreline. The number of ridges varies from three to seven at any point. Ridges are relatively few to the west of the plain, in the Leanyer Swamp area. Eastwards from Micket Creek the chenier ridges diverge from each other, but recurve landwards where there are dissecting creeks. Their orientation would imply a west to east predominant direction to longshore drift. Surveys across the cheniers indicate that the sandy chenier ridges rise about 1 m above the surrounding mudflats, and their crests reach elevations 3.5–4.0 m above AHD ŽFig. 3.. Seaward ridges in the Camerons Beach area reach above 4.5 m. The ridges are generally 20–50 m wide, slightly steeper on the seaward side, but can reach up to 110 m wide in places. The vegetation of the chenier ridges is a vine thicket, 8-10 m tall, and comprises Acacia auriculi-

Table 1 Radiocarbon dating results from Shoal Bay Žincluding those in Hickey, 1981. ANU sample no.

Field code

Material

Conventional radiocarbon age Žyr BP.

Environmentally corrected age Žyr BP.

7719 7720 7721 7722 7723 1167 1168 1169 1170 1171

SB6r220 SB8r165 CB3 CB19 CB20 A1 A3 A5 C1 C3

Mangrove wood Shell, Austriella sordida Mangrove wood Shell, Glauconome cf rugosa Shell, Austriella sordida Shell, Arcanthocardia tuberculata Shell, Arcanthocardia tuberculata Shell, Arcanthocardia tuberculata Shell, Arcanthocardia tuberculata Shell, Arcanthocardia tuberculata

6130 " 100 2820 " 110 1090 " 60 1430 " 160 1270 " 100 1300 " 90 2800 " 110 2790 " 110 1120 " 100 1510 " 100

6130 " 100 2370 " 120 1090 " 60 980 " 165 820 " 110 850 " 100 2350 " 120 2340 " 120 670 " 110 1060 " 110

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Fig. 2. The Shoal Bay chenier plain; morphology and vegetation, and locations of surveyed and stratigraphic transects. See Fig. 1 for location.

307

308

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Fig. 3. Transects a–e, Shoal Bay chenier plain, showing distribution of vine thicket and mangrove species. See Fig. 2 for locations.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

formis, Drypetes lasiogyna, Mimusops elengi, Hibiscus tiliaceus, Dodonaea platyptera, Gyrocarpus americanus, Celtis philippensis, Exocarpus latifolus, Pittosporum moluccanum, Tarenna dallachiana, Micromelum minutm, Cordia subcordata and Sterculia quadrifida. In addition the mangroves Xylocarpus mekongensis and Lumnitzera racemosa may extend up onto the ridges, with a fringe of gnarled or coppiced AÕicennia marina. The mudflats are either covered with mangroves, or are bare of vegetation. The bare areas include areas which appear to lie beyond the present limit of tidal water penetration at an elevation of around 2.6 m AHD, or areas which previously contained mangrove and which, as will be shown below, were laid bare as a result of cyclone damage. The seaward mangrove fringe is dominated by A. marina, with a few Sonneratia alba interspersed among them. A zone of Rhizophora stylosa occurs behind this, presently sporadically across the plain. Ceriops tagal, and to a lesser extent Bruguiera exaristata form a zone behind that, and then there are still more landward areas which contain A. marina and Lumnitzera racemosa. This zonation, with AÕicennia both at seawardmost and landwardmost, is typical throughout most of northern Australia and Southeast Asia ŽMacnae, 1966, 1968; Chapman, 1976.; however, as will be demonstrated below it has been substantially modified by the storm.

5. Stratigraphy and development of the Shoal Bay chenier plain Fig. 3 shows the morphology of the plains across several transects. The chenier ridges are composed of poorly sorted, medium to coarse sand with shell fragments. Auger holes through the cheniers indicate that they are composed of fine sand and broken shell. There are roots at the surface; the sand coarsens with depth, with larger shell fragments and occasionally beachrock or calcite concretions. Whole shells of Anadara granosa and Telescopium telescopium were found, in addition to the Arcanthocardia tuberculata upon which Hickey Ž1981. reported ages. In addition, Michie Ž1984. records foraminiferal assemblages from the ridges, with Elphidium crispum,

309

Cellanthus multiloculus, Ammonia beccarii, and Peneroplis spp. and Quinqueloculina spp. abundant. The shell and foraminifera, while typical of estuarine, offshore or reefal settings, are consistent with winnowing of this coarser material from the shoals and sand banks in Shoal Bay. Fig. 4 shows two stratigraphic sections through the chenier plain at Camerons Beach, following, and resurveying the transects examined by Hickey Ž1981., but examining the stratigraphy of the mudflats in a series of backhoe pits ŽSB4-SB12.. The mudflat reaches an elevation of 3.0 m AHD across these transects, and the ridges rise to 4.0 " 0.2 m AHD. The mudflat comprises a surface light brown silty clay, which below 50 cm depth becomes a bluish-gray sandy mud with orange oxidation overprinting. There is an increasing abundance of shell fragments with depth. At around 1.0–1.5 m AHD, there is a sharp transition into a bluish-gray, or light gray sand, with abundant shell and mica fragments. This lower unit contains mangrove wood in several pits, and faint laminations can be discerned. A radiocarbon date of 6130 " 100 years BP was determined on mangrove wood in this unit in SB6, indicating mangroves fringing the shore at this point when sea level reached its present level around 6000 years ago ŽWoodroffe et al., 1987.. Towards the rear of the plain, the lateritised, pre-Holocene basement was encountered, as a pisolitic gravel, with red or yellow, oxidised mottles. In addition to shell fragments that characterise most pits, there are several places where entire shells were found. These included the mangrove gastropod Terebralia palustris, and in some sites in situ, articulate bivalves of Austriella sordida. This fining-up sequence represents deposition in the upper intertidal zone. The gritty sand corresponds to the lower intertidal zone of sand flats, which dominates the shore below 0.5 m AHD, forming a low gradient sand flat, with complex megaripple patterns discernible from aerial photographs ŽFig. 5.. The vertical sequence of sedimentary units underlying the plain is revealed in a truncated exposure on the foreshore beneath the seawardmost chenier ridge. Indeed, the shoreface of Camerons Beach at the transects comprises a narrow sandy beachface which is actively retreating over a fossil mudflat. Mud, indurated by compaction, is being actively eroded into mud balls. Within the mud

310 C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Fig. 4. Stratigraphy of transect A Žabove. and C Žbelow., Camerons Beach, Shoal Bay. Radiocarbon dates on cheniers are from Hickey Ž1981.. See Fig. 2 for locations.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321 Fig. 5. Aerial photograph of mouth of Mickets Creek in 1963 and 1983 Žq Northern Territory Government.. See Fig. 2 for location. Ža. sand flats and shoals exposed at low tide; Žb. sandy ridge extending into Micket Creek and migrating gradually landward through mangroves; Žc. areas of felled mangrove trees resulting from 1974 storm; Žd. area previously covered by Rhizophora, defoliated in 1974, and not supporting regrowth by 1983. 311

312

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

unit which is exposed between the elevations of 1.0 and 2.7 m AHD, there are numerous in situ mangrove stumps. There are also in situ beds of articulate bivalves being excavated by erosion of the beachface. These form distinct beds of Austriella sordida at 1.0–1.3 m AHD on transect C and 1.4–1.6 m AHD on transect A, Glauconome cf. rugosa at 1.7–1.9 m AHD on transect C and 2.0-2.2 m AHD on transect A, and Meretrix meretrix found only on transect A at 2.4 m AHD. We interpret this truncated sequence as representing gradual vertical accretion, through lateral buildout. At present on the sandy lower intertidal flat, muds are accumulating and young AÕicennia are pioneering across these ŽFig. 6, a.. Radiocarbon dates give an indication of the chronology of development.

Basal mangrove stumps were dated 1090 " 60 years BP. The overlying shells gave uncorrected ages of 1270 " 100 and 1430 " 160 for the lower Austriella and upper Glauconome respectively, which if environmentally corrected correspond to 820 " 110 and 980 " 165 years BP respectively. As indicated above, the appropriateness of the correction has not been universally established, and will vary according to the degree to which the shells are influenced by open ocean seawater. Where circulation is poor, the correction may be less. We believe that these results, together with those of Shulmeister and Head Ž1993., substantiate the need for a correction. It appears likely that the vertical sequence from mangroves through shell beds was completed within a century or two, around 900–1100 years ago.

Fig. 6. Aerial photograph of Camerons Beach, 1983 Žq Northern Territory Government.. See Fig. 2 for location. Ža. active mudflat accretion with colonisation by AÕicennia; Žb. erosion of shoreface exposing sub-fossil mangrove stumps and shellbeds as indicated in Fig. 4; Žc. sandy ridge migrating landward through mangroves.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Also shown in Fig. 4 are the dates on shells from within the cheniers initially reported by Hickey Ž1981.. These indicate that the landwardmost ridges on transect A contain shells of 2340 and 2350 years BP. Morphologically the ridges A3 and C4 appear continuous, and it thus seems likely that the oldest ridges in all of the Camerons Beach chenier plain, as well as further west, are of this age. This corresponds closely to the age of Austriella found in pit SB8, dated 2370 " 120 years BP, and is further evidence to support progradation of the plain commencing since 2500 years ago. There is nothing to indicate what the plain would have looked like between 6000 and 2500 years ago. Sub-fossil shells are presently being excavated from the shoreface on Camerons Beach, and appear to be incorporated into the modern beach, suggesting that the ages of ridges 680–1060 years BP may be related to the modern phase of erosion. These results provide further reinforcement of the unsuitability of shell material for dating the time of deposition of a chenier ridge ŽGould and McFarlan, 1959; Woodroffe et al., 1993..

6. Storm impact and changes over time Cyclone Tracy originated over the Arafura Sea, about 700 km northeast of Darwin ŽFig. 1., as a weak tropical low on 20 December 1974. It was first detected on 21 December when the outlying belt of rain came within range of the radar at Darwin Airport ŽLajoie, 1977.. An alert was issued at 16.00 h ŽCentral Standard Time., and with development of characteristic spiral cloud bands, it was classified as a tropical cyclone at 22.00 h ŽBureau of Meteorology, 1975.. The storm moved in a southwesterly track, and at 21.45 h on 22 December, it was located 230 km north of Darwin. It continued to move in the southwesterly direction until around 3.30 h on 24 December when the centre was 20 km west of the automatic weather station at Cape Fourcroy, Bathurst Island, which recorded wind speeds of 100 kmrh from the northeast and pressure of 993 mb. The storm after 9.30 h turned abruptly to the southeast, and a priority cyclone warning was issued advising of the expected landfall of Cyclone Tracy over Dar-

313

win on Christmas morning. At 2.30 h on 25 December, the anemometer at Darwin Airport was recording winds from the northwest at 110 kmrh, with gusts of up to 195 kmrh, and soon after it was damaged by a gust exceeding 217 kmrh. Radar recorded the landfall of Tracy at 3.15 h, 7.5 km from Darwin Airport; the eye was 12 km diameter. The city was devastated; buildings were flattened and 49 people lost their lives, with a further 16 reported as lost at sea. The storm then moved eastsoutheasterly across southern Arnhemland, and the Gulf of Carpentaria and into Queensland, degenerating rapidly into a rain depression. Cyclones with highest recorded gusts of over 200 kmrh have been estimated with a return period of around 100 years in the Darwin area; Cyclone Tracy is the most severe storm that has been recorded in the region. The cyclone coincided with high neap tides. The maximum height of storm surge recorded in Darwin Harbour was 1.6 m, though heights of up to 4 m occurred on Casuarina Beach, under the northern boundary of the eye of the storm. Had it coincided with high spring tides, the storm surge and associated flooding would have been much more devastating. As it occurred under neap tide conditions, it was the destructive winds of the cyclone which caused most damage. The devastation throughout Darwin was recorded on vertical aerial photography over the township, flown by the RAAF in the early days of January 1975. This photography only covers the westernmost part of the Shoal Bay chenier plain; nevertheless, it demonstrates the severe impact on the mangrove communities. Cyclone Tracy caused moderate to severe damage to vegetation over nearly 300 km2 in the Darwin area, with the most noticeable effects being: windthrow, crown damage Žleaves and twigs removed, sometimes also branches., boles fractured, boles leaning, or occasionally trees killed but left standing ŽStocker, 1976.. Within the mangroves, Rhizophoraceae were very sensitive to damage. C. tagal and B. exaristata were often windthrown; R. stylosa were defoliated, but in many cases remained standing. AÕicennia on the other hand, in the vicinity of Buffalo Creek suffered damage to crowns with bark removed from the upper part of the trees, but were undamaged within 50 cm of the ground, as this

314

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

part was evidently under water due to the storm surge ŽStocker, 1976.. The observations of Stocker Ž1976., and the reconstructions from aerial photography available for Shoal Bay directly after the storm, appear to reinforce studies of storm impact on mangroves elsewhere. Thus defoliation, but trees remaining in their upright position, was noted for Rhizophora sp. in Belize after Hurricane Hattie ŽStoddart, 1971.. Similarly, Cyclone Kathy in the Gulf of Carpentaria, resulted in extensive mangrove mortality especially to trees in excess of 15 m tall. Though Rhizophora did in some cases suffer windthrow, there were extensive stands that remained in place, though defoliated and killed by the storm ŽBardsley, 1984.. AÕicennia was one of the more resistant species also in the McArthur River delta ŽBardsley, 1984.. The storm clearly had far-reaching effects on the mangrove vegetation, but it may also have had an impact on the landforms associated with the Shoal Bay chenier plain. In order to assess this, mapping was undertaken of the plain, both in terms of vegetation and the position and form of shoreline and chenier ridges, from vertical aerial photographs in each of the years 1963, 1974, 1975 Žfor a part of the west of the plain., 1980, 1983 and 1989. A number of changes can be detected to both vegetation and landforms over the time-series of aerial photographs ŽFigs. 7 and 8.. In particular three features can be traced; first, the position of the shoreline; second, alterations to the distribution of vegetation, especially its destruction by Cyclone Tracy, and subsequent recovery or recolonisation; and third, changes in the size and extent of onshore sand bodies and beach deposits. Vertical aerial photography for 1963 Ž1:16,000., 1974 Ž1:25,000., 1980 Ž1:15,000., 1983 Ž1:18,000. and 1989 Ž1:22,000. was available for the entire chenier plain; in addition, limited photography taken by the RAAF in January 1975 Ž1:12,000., just 8 days after the cyclone, covers the western end of the plain, indicating the impact of the storm on that part of the plain. In 1963, the western part of the Shoal Bay chenier plain Žin the Leanyer Swamp area. was almost en-

tirely covered by mangrove vegetation ŽFig. 5.; the only exceptions were inland hypersaline mudflats and a small onshore sand deposit of 50 m2 at the extreme west of the study area. The coastline underwent minor erosion between 1963 and 1974 during which period there was net loss of 180 m2 of mangrove. Additional sand was also deposited at both the western and eastern ends of the Leanyer Swamp foreshore, increasing the area of the onshore sand deposit by 250 m2 . Cyclone Tracy appears to have had little erosional impact on the coastline. There was instead a minor increase in size of the onshore sand deposits to 330 m2 . The cyclone’s main impact was on the vegetation. The 1975 photographs show that 6.6 km2 of mangroves were felled from the mid and rear of the plain ŽFig. 7.. Little damage was sustained by the seaward communities, because these seaward mangroves were partly inundated by the surge, as well as both AÕicennia and Sonneratia being more resilient than the Rhizophoraceae. Trunks of felled trees, particularly Ceriops and Bruguiera are clearly visible, felled and lying in a southsoutheasterly direction on the 1975 photography. Vegetation regrowth by 1980 had reduced the area of visible felled trees to 3.3 km2 . The coastline had undergone minor progradation as a result of the increase in size of the onshore sand body to 710 m2 ; the eastern beach deposit expanded seaward, and began extending around the seaward mangroves into Micket Creek. In 1983, continued mangrove recovery had reduced the area of felled trees to 2.8 km2 . Coastal progradation had also continued, with onshore sand bodies increasing in size to a total of 1130 m2 . By 1989, the area of felled trees had again decreased, with only 600 m2 left bare. Minor erosion, and mangrove recolonisation had reduced the onshore sand deposits to 540 m2 ŽFig. 7.. In this western region, Cyclone Tracy does not appear to have had much impact on the landforms of the chenier plain. Coastal progradation and erosion occurred before, and has continued since the cyclone, and little disruption to this pattern of change occurred as a result of the storm. The most signifi-

Fig. 7. Maps of western section of Shoal Bay chenier plain interpreted from time-series of aerial photographs. Maps to the left are west of Mickets Creek; those to the right are east of Mickets Creek.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

315

316

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Fig. 8. Maps of eastern section of Shoal Bay chenier plain interpreted from time-series of aerial photographs. Maps to the right are of the Camerons Beach area; those to the left are west of Camerons Beach.

cant landform change was the growth of the onshore sand deposits, and the extension of the eastern beach deposit around the point and into Micket Creek ŽFig. 5, b.. However, the devastation of the mangroves was immense, and the pattern of regrowth has been only partial; several areas still remain unvegetated as a result of their windthrow in 1974.

By contrast, to the east of Micket Creek, no significant changes were observable from 1963 to 1974 ŽFig. 7.. The mudflat was well vegetated with mangroves ŽFig. 5., the only bare areas being the inland hypersaline flats, and the onshore sand deposit Ž140 m2 .. Interpretation from the 1980 photography indicates that while Cyclone Tracy had little effect

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

on the position of the coastline and the landforms of the area, it cleared a large area of mangroves, Žc 1.3 km2 ., from the seaward part of the mudflat. Again there remained a seaward fringe of mangroves that was not killed. Destruction of Ceriops was by windthrow; whereas Rhizophora remained in position but were defoliated. Much of the devastated Rhizophora area could be identified in the field 15 years after the storm, as the matted, peaty remains of fibrous roots from Rhizophora remain in the surface muds and appear to prevent further colonisation of these areas ŽFig. 5, d.. Indeed planting experiments suggested that the area is still unsuitable for Rhizophora regrowth, and it is not simply lack of propagules that accounts for the lack of recolonisation. In 1983, the only significant change was that progradation with accompanying mangrove establishment had taken place seaward of the original sand deposit, with build out of around 90 m. By 1989, coastal progradation and mangrove establishment had filled the gap in the vegetated shoreline caused by the original sand deposit and the tree felling by Cyclone Tracy. Regrowth had reduced the area of felled mangroves to 775 m2 . One small area isolated by the regrowth of mangroves around it has been reclassified as an onshore sand deposit, and has increased in area to 170 m2 . This area further emphasises that the storm had long-lasting impacts on the mangrove vegetation, but that it had little if any geomorphological impact on the landform development. Still further east of Micket Creek ŽFig. 8., there was a pattern of continued coastal progradation and mangrove colonisation, with up to 70 m being added to the coastline between 1963 and 1974. The expansion of the seawardmost sand deposit caused the total area of bare onshore sand to increase to 327 m2 . By 1980, this had undergone erosion at its western end, roughly back to its 1963 position. Conversely, the area of onshore sand deposits had increased to 504 m2 . In this area, only a small amount of mangrove defoliation resulted from Cyclone Tracy, with 270 m2 of felled trees visible in 1980, these areas being behind the seawardmost mangroves. The erosional trend continued until 1983, with the shoreline then 40–80 m landward of its 1963 position. Simultaneously there was a reduction in the area of the

317

sand deposit, to 469 m2 . By 1989, erosion had ceased, and the onshore sand area had increased to 760 m2 . The area of felled mangroves remained at only 160 m2 . Although this area underwent erosion, there seems little evidence that this resulted from Cyclone Tracy. The fact that it continued after 1980 indicates that the erosive period was caused by another longer-lasting influence. There appears to have been far less impact on the mangrove vegetation by the storm here, as winds presumably were of lesser intensity. In the Kings Creek area, at the far east of the chenier plain, little change occurred between 1963 and 1974, except for a reduction in the area of onshore beach sand, as the width of the beach decreased ŽFig. 8.. These onshore sand and beach deposits had increased significantly by 1980, reaching more than double their previous extent Ž540 m2 .. Much of this increase was due to the spread of sand inland, and the elongation of the beach deposit into Kings Creek. Only a small area of felled vegetation was visible, with felled trees observed in what were previously hypersaline flats. Between 1974 and 1980 the western section of the coastline underwent around 30 m of progradation seawards, and extended laterally 50 m east, with establishment of mangroves ŽFig. 6, a.. By 1983, continuing progradation had caused the western section of the coast to move a further 30 m seaward, and 50 m to the east. This build out continued until 1989, with 90 m more progradation and 100 m extension further east. A further 30 m2 of well vegetated mudflat had formed just offshore of the beach deposit. Little of the change in this eastern area can be attributed to Cyclone Tracy. The damage to mangroves was less extensive, not so much because of the lesser intensity of winds, as to the predominance of AÕicennia which is less susceptible to damage. Progradation patterns occurred independently of the impact of the storm.

7. Discussion Shoal Bay consists of a small chenier plain on which there is a sequence of ridges which appear to have formed in perhaps only two major phases of alternating erosion and deposition. Although man-

318

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

groves seemed to have been growing in the lower intertidal flat sands around 6000 years BP, when sea level first reached a level close to present, the phases of progradation recorded within the chenier sequence do not seem to have commenced until around 2300 years BP. In this respect, the chenier sequence is of shorter, and more recent duration than other chenier plains in the region. Chenier ridges have formed intermittently on the coast of van Diemen Gulf to the east of Darwin ŽWoodroffe et al., 1986, 1993.. An extensive sequence of chenier ridges occurs in front of Point Stuart. A broad, discontinuous ridge, marking the position of the shoreline 5000–6000 years ago, can be discriminated, and younger chenier ridges reflecting episodic coastal buildout in the last 3000 years ŽClarke et al., 1979; Lees, 1987; Woodroffe et al., 1993.. Distinct mid and late Holocene phases of chenier building are also recorded in association with the Victoria River in northwestern Australia ŽLees, 1992.. Some recent interpretations, based upon dating of cheniers, and the presumption that sediment stored within the plains is fluvial in origin, have invoked a wetter period in the monsoonal tropics of Australia in mid-Holocene ŽRhodes, 1982; Lees and Clements, 1987.. However, chenier formation at Shoal Bay does not appear to correlate with these other chenier-building phases elsewhere in the region. Moreover, the detailed study of a time-series of aerial photography and the stratigraphy of the plains and eroding foreshore at Shoal Bay lead us to question firstly whether the radiocarbon ages of shells within chenier ridges relate to the age of the ridge; secondly, whether individual storms are significant in the pattern of ridge evolution, and thirdly, whether chenier ridges record discrete events, or whether different stages in chenier plain evolution may take place contemporaneously over different parts of a single chenier plain. While shell dates on the younger sequence of chenier ridges at Shoal Bay have been recorded in the range 670–1060 years BP, shells within this age range are presently being winnowed from a series of sub-fossil shell beds exposed on the shoreface as a result of erosion. If, as we suspect, these fossil shells become incorporated into the modern beachface, and hence into the sandy ridges which are currently

being moved gradually landward, through mangrove stands, then the ages of ridges A1, C1, C2 and C3 may well reflect current processes acting on the foreshore and seawardmost ridges. A similar argument may explain the apparent widespread ridge formation 2300 years ago. Fossil Austriella of this age were revealed in pit SB8 at the rear of the chenier plain. Erosion into shell beds formed at this time may explain ages of around 2300 years BP for shells in the landwardmost ridges. The actual time of formation of the ridge may have been considerably later, and the prevalence of this age within the ridge sequences may merely reflect availability of sub-fossil shellbeds formed at around this time. These results serve to reinforce the caution that the age of shells within a ridge may not be a very precise indicator of the time of formation of the ridge. This was demonstrated in one of the earliest and most extensive chenier-dating programs, that by Gould and McFarlan Ž1959., who found a wide spread of ages on any one ridge. It has also been shown elsewhere in northern Australia where dates on shells have shown a wide age range, and discrepancies with other methods of dating shoreline progradation Žeg Woodroffe et al., 1993; Shulmeister and Head, 1993.. Although Cyclone Tracy was one of the most devastating storms witnessed in Australia, the storm itself does not appear to have had far-reaching geomorphological impacts. It was very significant in terms of its impact on mangrove vegetation. Current patterns, especially in terms of the structure and distribution of the Rhizophoraceae, are a direct result of the pattern of destruction. Regrowth of windthrown mangrove areas has been incomplete, and the role that mangroves have been able to play in influencing the migration of chenier ridges landward is a response to that mangrove vegetation damage. It is noticeable that AÕicennia was less devastated by the storm, and is more resilient to the migration of sandy chenier ridges across and through it. In contrast to the patterning of mangrove species, development of the ridges themselves has not been dominated, as best we can determine, by the impact of Cyclone Tracy in 1974. Comparison of aerial photographs demonstrates that the formation, and the gradual landward migration of chenier ridges was going on before, and has continued since the storm.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

Indeed, not only does the storm not appear to have had a particularly recognisable effect on the movement of these sand bodies, but it is also apparent that these sand bodies are regularly moved by events of considerably smaller magnitude, and have throughout this period of observation been migrating landward. The chenier ridges are thus features which appear to mobilise, though at slow rates, under moderate processes. In the case of Shoal Bay, rather than a turn-onrturn-off alternation between mudflat progradation and erosion of shoreface, with concurrent winnowing of sand and shell and its deposition in chenier ridges, all of the processes appear to be occurring, and have been observed at different points on the plain since 1963. There are several points at which mudflats are presently accreting. Thus for instance at the western end of Camerons Beach, there are muddy sediments presently forming on the lower intertidal sand flat. These thixotropic muds are being colonised by stands of AÕicennia, and a height gradation within these communities testifies that this has been a process that has operated over several years ŽFig. 6, a.. Elsewhere the shoreface is undergoing erosion. Indeed further along Camerons Beach Žas at the foot of transects A and C., the foreshore is being eroded and sub-fossil mangrove stumps and shellbeds are being exposed ŽFig. 6, b.. Radiocarbon ages on the shellbeds exposed by erosion fall into the 900–1000 years BP range which has also been recorded for the age of shell in the seawardmost ridges. At the eastern end of Camerons Beach Žas also in the case of the sandy beach that is migrating into mangroves west of Micket Creek. the present sandy foreshore is retreating landwards onto mangroves, and the ridge is still active ŽFig. 6, c.. Individual mangrove trees can be covered by sand and shell; this inundation kills Rhizophora in most cases, but AÕicennia have been observed which are still alive, and which resprout after the physical damage of ridge migration across them. The pattern of chenier plain evolution appears to reflect an internal dynamic, as indicated by Chappell and Grindrod Ž1984. for cheniers in Princess Charlotte Bay, Queensland. These authors suggested that the chenier plain had alternated between a ‘cut and recover’ mode of relatively slow buildout and a ‘rapidly prograding’ mode of mudflat formation.

319

Pulses in mud input appeared to reduce shellfish production, but changes in regional boundary conditions, which might account for these pulses, could not be identified from the sedimentary record. Rather than broad trends influenced by regional climatic change, the pattern of progradation at Shoal Bay appears to contain each of the temporal stages of chenier development at different points along the modern foreshore. Something triggered widespread development of mudflats along this shoreline around 2300 years ago, but since that time there has been a broad phase of net sediment accretion along the coast, with erosion dominating at some points, and deposition, ridge formation and landward migration at others. Storms are undoubtedly important in reshaping the coastline, and they would seem to be particularly important in restructuring the mangrove communities, which provide protection locally to the shoreline, and which presumably present a filter to the migrating chenier ridges. While the impact of Cyclone Tracy is still evident in the present distribution of mangroves, the chenier ridges themselves have changed since the storm, and their longer-term dynamics do not appear dependent solely upon such events.

8. Conclusions The Shoal Bay chenier plain contains a lower intertidal sand unit within which mangrove wood has been radiocarbon dated to around 6000 years BP, corresponding to the time that sea level stabilised around its present level in northern Australia. Progradation of the modern chenier plain does not appear to have begun until after 2500 years BP. Mudflats were deposited within which shellbeds dated around 2300 years BP have been found, and shells of this age are also found in the landwardmost chenier ridges. Shellbeds and mangrove stumps 900–1000 years old are being exposed by erosion on the modern foreshore, and material of this age appears to be incorporated into the seawardmost cheniers which are still actively migrating landwards across mangroves at several points along the foreshore. Cyclone Tracy, a particularly devastating storm, had widespread impacts on the mangrove vegetation of the chenier plain, but did not appear to exert a

320

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321

disproportionate influence on the overall pattern of geomorphological development of the plain. The pattern of mangrove vegetation has only partially recovered, but the storm does not seem to have been a major factor influencing the supply of sandy material to the foreshore or its rate of migration. Both active accretion of mudflats, with ongoing colonisation by AÕicennia, and erosion of the foreshore, and concentration of sand and shell into landward-migrating ridges, are currently occurring and can be seen to have been ongoing since 1963. Thus, each of the major processes of chenier development appear to have been going on simultaneously, driven by an internal dynamic, rather than distinct alternating episodes reflecting individual storms or regional changes in boundary conditions. Acknowledgements This research was funded by the Australian Research Council, the University of Wollongong, and by the North Australia Research Unit. We are grateful to Susan Hickey for discussions of her earlier study, to the Australian National University Radiocarbon Dating Laboratory for dating analyses, to the staff of the Western Australian Museum for bivalve identifications, and to David Martin for drawing the figures. References Anthony, E.J., 1989. Chenier plain development in Northern Sierra Leone, West Africa. Mar. Geol. 90, 297–309. Augustinus, P.G.E.F., 1989. Cheniers and chenier plains: a general introduction. Mar. Geol. 90, 219–229. Augustinius, P.G.E.F., Hazelhoff, L., Kroon, A., 1989. The chenier coast of Suriname: modern and geological development. Mar. Geol. 90, 269–281. Bardsley, K., 1984. The effects of Cyclone Kathy on mangrove vegetation. In: Bardsley, K.N., Davie, J.D.S., Woodroffe, C.D. ŽEds.., Coasts and tidal wetlands of the Australian monsoon region. North Australia Research Unit, ANU Press. pp. 167– 185. Blain, Bremner and Williams, 1984. Greater Darwin storm surge study. N.T. Dept. Lands, Darwin. Bureau of Meteorology, 1975. Cyclone Tracy. Aust. Gov. Publ. Serv., Canberra. Chapman, V.J., 1976. Mangrove Vegetation. J. Cramer, Germany, 447 pp. Chappell, J., Grindrod, J., 1984. Chenier plain formation in north-

ern Australia. In: Thom, B.G. ŽEd.., Coastal Geomorphlogy in Australia. Academic Press, Sydney pp. 197–231. Clarke, M.F., Wasson, R.J., Williams, M.A.J., 1979. Point Stuart chenier and Holocene sea levels in Northern Australia. Search 10, 90–93. Cook, P.J., Mayo, W., 1977. Sedimentology and Holocene history of a tropical estuary ŽBroad Sound, Queensland.. Bur. Min. Res. Bull. 170, 260 pp. Cook, P.J., Polach, H.A., 1973. A chenier sequence at Broad Sound, Queensland, and evidence against a Holocene high sea level. Mar. Geol. 14, 253–268. Daniel, J.R.K., 1989. The chenier plain coastal system of Guyana. Mar. Geol. 90, 283–287. Gould, H.R., McFarlan, E., 1959. Geologic history of the Chenier Plain, Southwestern Louisiana. Trans. Gulf Coast Assoc. Geol. Socs. 9, 261–270. Gupta, S.K., Polach, H.A., 1979. Radiocarbon dating practices at ANU, Radiocarbon Dating Laboratory, Research School of Pacific Studies, Australian National University, Canberra. Hickey, S.H., 1981. Preliminary investigation of stranded beach ridges, Shoal Bay, Northern Territory: a small chenier plain? N.T. Geol. Surv. Tech. Rept. GS81r1 Žunpublished.. Hopley, D., Harvey, N., 1979. Regional variations in storm surge characteristics around the Australian coast: a preliminary investigation. In: Heathcote, R.L., Thom, B.G. ŽEds.., Natural Hazards in Australia. Aust. Acad. Sci., Canberra, pp. 164–189. Hoyt, J., 1969. Chenier versus barrier, genetic and stratigraphic distinction. Am. Assoc. Petrol. Geol. Bull. 53, 299–306. Lajoie, F.A., 1977. On the direction of motion of tropical cyclone Tracy. Aust. Gov. Publ. Serv., Canberra. Lees, B.G., 1987. Age structure of the Point Stuart chenier plain: a reassessment. Search 18, 257–259. Lees, B.G., 1992. The development of a chenier sequence on the Victoria Delta, Joseph Bonaparte Gulf, northern Australia. Mar. Geol. 103, 215–224. Lees, B.G., Clements, A., 1987. Climatic implications of chenier dates in Northern Australia. Radiocarbon 29, 311–317. McAlpine, J.R., 1969. Climate of the Adelaide-Alligator area. In: R. Story et al. ŽEds.., Lands of the Adelaide-Alligator area, Northern Territory. CSIRO Land Research Series No. 25, 49–55. Macnae, W., 1966. Mangroves in eastern and southern Australia. Aust. J. Bot. 14, 67–104. Macnae, W., 1968. A general account of the fauna and flora of mangrove swamps and forests in the Indo-West-Pacific region. Adv. Mar. Biol. 6, 73–270. Michie, M.G., 1984. Holocene foraminifera from the Camerons Beach chenier plain, Shoal Bay, Northern Territory. Addendum to N.T. Geol. Surv. Tech. Rept. GS81r1 Žunpublished.. Otvos, E.G., Price, W.A., 1979. Problems of chenier genesis and terminology—an overview. Mar. Geol. 31, 251–263. Penland, S., Suter, J.R., 1989. The geomorphology of the Mississippi River Chenier Plain. Mar. Geol. 90, 231–258. Qinshang, Y., Shiyuan, X., Xusheng, X., 1989. Holocene cheniers in the Yangtze Delta, China. Mar. Geol. 90, 337–343. Rhodes, E.G., 1982. Depositional model for a chenier plain, Gulf of Carpentaria, Australia. Sedimentology 29, 201–221.

C.D. Woodroffe, D. Grime r Marine Geology 159 (1999) 303–321 Russell, R.J., Howe, H.V., 1935. Cheniers of Southwestern Louisiana. Geog. Rev. 25, 449–461. Schofield, J.C., 1960. Sea level fluctuations during the last 4000 years as recorded by a chenier plain, Firth of Thames, New Zealand. NZ J. Geol. Geophys. 3, 467–485. Short, A.D., 1989. Chenier Research on the Australian Coast. Mar. Geol. 90, 345–351. Shulmeister, J., Head, J., 1993. Aspects of the emplacement, evolution and 14 C chronology of ridges on a coastal spit, Groote Eylandt, Northern Australia. Mar. Geol. 111, 159–169. Stocker, G.C., 1976. Report on cyclone damage to natural vegetation in the Darwin area after Cyclone Tracy, 25 December 1974. Aust. Gov. Publ. Serv., Canberra. Stoddart, D.R., 1971. Coral reefs and islands and catastrophic storms. In: Steers, J.A. ŽEd.., Applied Coastal Geomorphology. Academic Press, New York, pp. 155–197. Thompson, R.W., 1968. Tidal flat sedimentation on the Colorado River delta, northwestern Gulf of California. Mem. Geol. Soc. Am. 107, 1–133.

321

Todd, T.W., 1968. Dynamic diversion: influence of longshore current-tidal flow interaction on chenier and barrier island plains. J. Sedim. Petrol. 38, 734–746. Woodroffe, C.D., Curtis, R.J., McLean, R.F., 1983. Development of a chenier plain, Firth of Thames, New Zealand. Mar. Geol. 53, 1–22. Woodroffe, C.D., Chappell, J., Thom, B.G., Wallensky, E., 1986. Geomorphological dynamics and evolution of the South Alligator tidal river and plains, Northern Territory, North Australia Research Unit Monograph, 190 pp. Woodroffe, C.D., Thom, B.G., Chappell, J., Wallensky, E., Grindrod, J., Head, J., 1987. Relative sea level in South Alligator River region, North Australia, during the Holocene. Search 18, 198–200. Woodroffe, C.D., Mulrennan, M.E., Chappell, J., 1993. Estuarine infill and coastal progradation, southern van Diemen Gulf, northern Australia. Sedim. Geol. 83, 257–275.