Page 61 - Carbonate Sedimentology and Sequence Stratigraphy

52 WOLFGANG SCHLAGER

The karst hypothesis has a conceptual basis in what one karst weathering (Fig. 2.34) suggest that extrapolation of the
may call the “edge effect of dissolution”: a fluid runs off laboratory experiments to natural settings needs to proceed
the edge of a horizontal surface faster than from the center with caution.
of this surface. In his dissolution experiments on limestone The karst interpretation of raised rims on Cretaceous plat-
blocks, Purdy (1974) observed that – as the rate of acid rain forms in the Pacific has created a lively debate. The hypoth-
was gradually reduced – , the acid film on the limestone was esis was advanced by Winterer et al.(1995), Winterer (1998),
pierced first at the edge. This reduced the etching time at Van Waasbergen and Winterer (1993) and Purdy and Win-
the edge and produced elevated rims on orginally flat sur- terer (2001). It faces the problem that there is practically no
faces of homogeneous limestone. A second principle that isotopic evidence of meteoric diagenesis. This would imply
has been invoked but has not been demonstrated in this par- that during millions of years of exposure, carbonate rocks
ticular application, is case hardening: extra cementation of were extensively dissolved but no meteoric cements precip-
the steep outer cliffs of limestone islands by evaporation of itated in these formations – an assumption that runs counter
mineral-bearing water. to extensive observations on Quaternary rocks (Jenkyns and
Observational support for the karst hypothesis is the dis- Wilson, 1999). Erosional morphology is apparent in seismic
covery of Purdy and Winterer (2001) that on Pacific atolls lines and sea-floor maps. However, similar patterns can be
lagoon depth positively correlates with rainfall. observed in the overlying pelagics and the stratigraphy and
Geologic field observations provided a number of exam- facies of the pelagic caps indicate extremely low sedimen-
ples of karst islands with raised rims and low centers (see tation rates and numerous hiatuses – in agreement with the
Purdy and Winterer, 2001 for summary). The problem with widespread evidence for current-enhancement by sharp to-
most of them is the lack of detailed maps of the limestone pography (see Figs 2.26, 2.25).
facies. Thus, it is generally impossible to determine whether In summary, I conclude that the formation of raised rims
the ridges were carved from homogeneous limestone as in by differential growth can be demonstrated by a number of
the laboratory experiments of Purdy (1974), or whether karst examples from the Holocene and the distant past. The the-
dissolution merely accentuated concentric patterns of depo- oretical underpinning for this phenomenon is the measur-
sitional facies. An example of the latter is the well-mapped ably higher growth potential of the rim and the disposition
island of Makatea (Fig. 3.25; Montaggioni et al., 1985). Maps of reefs to grow upward rather than shift laterally to more
and sections leave little doubt that karst weathering has sig- favorable locations. Regarding the karst hypothesis, I think
nificantly enhanced the relief between rim and lagoon but differential lowering of karst surfaces during lowstands has
the data also show that what now constitutes the morpho- been shown to at least significantly enhance the relief be-
logic rim was originally a barrier reef with its debris apron. tween rim and lagoon. Purdy (1974) has demonstrated that
Purdy and Winterer (2001) offer the raised Miocene lime- in laboratory experiments raised rims can be created on hor-
stones of Tuvutha (Fiji) as a particularly convincing example izontal surfaces solely by dissolution without lateral hetero-
of concentric ridges carved from homogeneous limestone. geneities in the limestone. Field observations in support of
Indeed the entire limestone cap was mapped as one forma- this extreme form of the karst hypothesis are scant and not
tion, mainly composed of algae and foraminifers (Ladd and compelling. However, the discussion should be continued.
Hoffmeister, 1945). However, the mapping was done in the It forms an important part of the broader issue of the effects
1920’s, before modern reef research had demonstrated that and vestiges of exposure in carbonate rocks.
the geologic record of most reefs is a pile of rubble. Conse- Constructional “empty buckets” observed in outcrop or
quently, Ladd and Hoffmeister (1945) emphasized the lack seismic data indicate that the platform was stressed by a rel-
of outcrops or samples with in-situ framework of corals. ative sea-level rise that closely approached the growth po-
They did report coral fragments to occur regularly (Ladd tential of the factory. It remains open, of course, if the sea-
and Hoffmeister, 1945, p. 216-217). Furthermore, the authors level rise was exceptionally fast or if the platform’s growth
interpret the vertical walls on the seaward side of the ridges potential was reduced by environmental factors.
as sea-cliffs formed during the gradual uplift of the island. The bottomline of the discussion is that both hypotheses,
Thus, the present data do not exclude the possibility that selective construction and karst weathering, can be counted
the ridges are remnants of platform margin facies just like on to produce raised rims and deep lagoons. In fact, at times
on Makatea. of large and rapid sea-level fluctuations both processes are
The dissolution experiments of Purdy (1974) were per- likely to alternate. The record of the Late Quaternary in-
formed on tight limestones (erroneously referred to as “mar- dicates that the rates of sea-level rise during deglaciations
ble” by Schlager, 2003, p. 451). Thus, the experiments approached the growth potential of healthy modern reefs.
demonstrate that a saucer shape is created if the acid runs off Constructional empty buckets are a likely result. On the
the edges rather quickly but collects in the center of the sam- other hand, differential karst weathering almost certainly
ple surface and continues to lower the surface there. In con- occurred during glacial lowstands. Both conditions must
trast, much of the dissolution caused by rainwater on young have alternated repeatedly in the past 700 ky, creating a com-
carbonate rocks takes place below the surface because the posite of constructional and karst effects. One task before us
rocks are highly porous and permeable. Examples of well- is to discriminate between the constructional and the karst
preserved depositional morphology after nearly 100 ky of hypothesis in the distant geologic past and determine the