Kansas Geological Survey, Subsurface Geology 12, p. 41-42
The University of Kansas
Inundation of a carbonate platform during transgression apparently does not lead to an immediate onset of rapid production of carbonate sediment. Stated another way, carbonate-sediment production does not reach its fun potential for a finite period (Schlager, 1981). Thus carbonate-sediment accumulation tends to lag behind the relative rate of sea-level rise, producing a deepening sequence (Read et al., 1986). The time between initial inundation and onset of rapid sediment accumulation is "lag time." This interval is critical in our understanding of shoaling-upward platform cycles. Without lag time, carbonate sedimentation would either remain at sea level or, if accumulation rate is less than the relative rate of sea-level rise, a continuously deepening sequence would result. Lag time is also essential to Ginsburg's (197 1) autogenic cycles; when carbonate sediment has built to sea level across a platform, thereby destroying its source of sediment, there follows a finite period of transgression before rapid sediment accumulation begins progradation.
Despite lag time's profound effect on the character of shoaling-upward cycles (cf. Read et al., 1986, p. 108), the causative processes are poorly understood. In modeling of cycles, the duration of lag time is typically assigned by trial and error to produce reasonable cycles.
Experience with modern shallow-water carbonate sedimentation in south Florida suggests that lag time is caused by extensive winnowing of sediments deposited above local storm-wave base. Regardless of in situ rates of production, the geologic record of sediment depends on net accumulation, that is, production at a rate in excess of removal. When rise in sea level places the bottom below the reach of storm waves, rapid sedimentation can occur. The accumulation rate will be rapid if the bottom remains within the euphotic zone, a common case where terrigenous input is not excessive. Alternatively, construction of barriers may dampen storm waves below the threshold of erosion, permitting carbonate accumulation in quite shallow water.
In the south Florida shelf margin, seaward of the Florida Keys, the inshore zone is dominated by rock bottoms or veneers of skeletal sediment too thin to support sea grass and infauna. Sediment does not accumulate in this zone despite significant populations of sediment-producing organisms. Depths at the outer edge of this zone, where it becomes covered with sediment, are uniformly 3 m (10 ft) along the entire length of the Florida Keys. This depth apparently represents the limit of winnowing by local waves, developed behind the shoals at the platform edge (Enos, 1977). In this setting, then, lag time is the interval from initial flooding until water depth reaches 3 in (10 ft), about 4,000 yrs in the present transgression (Scholl et al., 1969; Robbin, 1984). Obviously lag time has several controls; the most prominent are rate of sea-level rise and local hydrographic setting, primarily fetch, which determines storm-wave base.
In Florida Bay, a shelf lagoon restricted by the Florida Keys and muddy shoals, sediment accumulation in shallow basins is restricted to depths greater than about 1.8 m (6 ft). Wave base is controlled by fetch within sub-basins between the restricting mud banks. Fetch is of the same order of magnitude throughout the bay, thus significant sediment accumulation begins at about the same depth. The last 1.8 in (6 ft) of the Holocene rise of sea level required about 3,600 yrs (Scholl et al., 1969) lag time in Florida Bay.
In this concept of lag time, a key element is the control of substrate on ecologic community and thereby on production of carbonate sediment (Enos, 1977). Hard substrates restrict the fauna essentially to encrusting and boring organisms. Mobile, coarse-grained substrates also have impoverished faunas. A soft substrate of lime mud, such as accumulates below storm-wave base, supports a burrowing infauna, calcareous algae, and a sea-grass community with profuse epibionts. Thus the accumulation of mud initiates a quantum jump in sediment production (Stockman et al., 1967; Bosence et al., 1985; Nelson and Ginsburg, 1986; Bosence, 1989). This is not a "chicken and egg" dilemma; a stable substrate is a prerequisite to colonization by green algae and especially grass. A shelly substrate that is not reworked for long periods may be stabilized and support sediment-producing populations.
The sedimentary record of the delay mechanism is the familiar "basal lag" of reworked shells and pebbles overlain by finer-grained sediment. This deepening interval is followed by a shallowing sequence as carbonate production exerts its ability to outstrip most rates of sea-level rise.
Recognition of the cause of lag time does not immediately produce numbers for modeling of carbonate cycles. It does allow intelligent estimates and should lead to better quantitative models where the hydrographic setting and relative rates of sea-level rise can be approximated.
Bosence, D. W. J., 1989, Biogenic carbonate production in Florida Bay: Bulletin of Marine Science, v. 44, no. 1, p. 419-433
Bosence, D. W. J., Royston, R. J., and Quine, M. L., 1985, Sedimentology and budget of a recent carbonate mound, Florida Keys: Sedimentology, v. 32, p. 317-343
Enos, Paul, 1977, Holocene sediment accumulations of the south Florida shelf margin; in, Quaternary Sedimentation in South Florida, P. Enos and R. D. Perkins (eds.): Geological Society of America, Memoir 147, p. 1-130
Ginsburg, R. N., 1971, Landward movement of carbonate mud--new model for regressive cycles in carbonates (abs.): American Association of Petroleum Geologists, v. 55, no. 2, p. 340
Nelson, J. E., and Ginsburg, R. N., 1986, Calcium-carbonate production by epibionts on Thalassia in Florida Bay: Journal of Sedimentary Petrology, v. 56, no. 5, p. 622-628
Read, J. F., Grotzinger, J. P., Bova, J. A., and Koerscher, W. F., 1986, Models for generation of carbonate cycles: Geology, v. 14, p. 107-110
Robbin, D. M., 1984, A new Holocene sea-level curve for the upper Florida Keys and Florida reef tract; in, Environments of South Florida, P. Gleason (ed.): Miami Geological Society, Memoir 1, p. 437-458
Schlager, W., 1981, The paradox of drowned reefs and carbonate platforms: Geological Society of America, Bulletin, v. 92, no. 4, p. 197-211
Scholl, D. W., Craighead, F. C., Sr., and Stuiver, M., 1969, Florida submergence curve revised--its relation to coastal-sedimentation rates: Science, v. 163, no. 3867, p. 562-564
Stockman, K. W., Ginsburg, R. N., and Shinn, E. A., 1967, The production of lime mud by algae in south Florida: Journal of Sedimentary Petrology, v. 37, no. 2, p. 633-648
Kansas Geological Survey
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Web version May 11, 2010. Original publication date 1989.