2

Controls on the sedimentary rock record

H.G. Reading & B.K. Levell

2.1 Controlling factors

Sedimentation results from the interaction of the supply of sediment, its reworking and modification by physical, chemical and biological processes, and accommodation space - that is, the space available for potential sediment accumulation. In many settings reworking allows only a small proportion of the delivered sediment to be preserved. Most is removed either almost immediately by an increase in physical energy, such as that produced by a storm or tidal current, by sediment instability as with deposition on a slope, by chemical dissolution, or, over a longer period of time, by environmental changes such as channel migration or shoreline advance or retreat. The accommodation space is controlled largely by external processes such as changes in sea level, climate, tectonic movements, volcanic activity, compaction and longer-term subsidence rates which together define a depositional base level.

2.1.1 Sediment supply

The supply of sediment varies in volume, composition and grain size, as well as in the mechanism and rate of delivery. These variations are the result of the climate, basinal water chemistry and the tectonics and bedrock geology of the source area. Where the supply of land-derived (terrigenous) sediment is abundant, siliciclastic sediments predominate. Where this is low or absent, physical erosion may be effective and chemical, biochemical and biological processes have a chance to produce or modify sediments. Evaporites, carbonates, diatomites, cherts, ironstones, phosphorites and carbonaceous sediments then predominate. There is thus a fundamental distinction between terrigenous extrabasinal particles derived ultimately, if not immediately, from outside the sedimentary basin, and biochemical intra-basinal particles generated within the depositional basin. Erupting volcanoes, however, upset this general rule and may supply large volumes of intrabasinal terrigenous volcaniclastic material of unusual compositions. In addition, substantial amounts of 'intrabasinal' terrigenous sediment may be supplied, for instance, from uplifted fault blocks or pre-rift thermal domes within large basins.

TERRIGENOUS SYSTEMS

A knowledge of the source or provenance of detrital sediment adds substantially to our understanding of depositional basins. The approximate grain size and composition of the lighter fraction, the heavy mineral population, and isotopic signatures can yield invaluable information on the nature of the bedrock and weathering processes in the source area.

Each depositional system has its own immediate source area. Deep-water systems such as submarine fans are supplied from an adjacent shelf or delta whose morphology, size, tectonics and climate they reflect. Shelves are supplied from coasts or coastal plains, which may be in part deltaic; deltas are supplied from alluvial systems which themselves reflect the features of the hinterland. Thus contemporaneous depositional systems are linked together as 'systems tracts' (Sect. 2.4). The downcurrent systems are directly controlled by the sediment supplied, or not supplied, by the upcurrent systems.

The rate of sediment supply is generally controlled more by the volume of sediment available in a given time interval than by transport capacities. Rates vary by many orders of magnitude, even within those systems dominated by terrigenous sediment.

The sizes and gradients of terrigenous depositional systems are related to the grain size or calibre of the sediment (Reading & Orton, 1991). Coarse-grained or gravel-rich alluvial, deltaic and deep-sea systems are all relatively small and steep with delta plain areas of 1-100 km2 and gradients of >5 m km-1 (Table 6.1). Deep-water coarse-grained submarine fans have radii of 1-50 km and slope gradients of 20-250 m km-1 (1°-14°). Owing to the high competence required, sediment transport and deposition are largely by short-lived, but frequent, catastrophic events such as floods initiated by rain storms or slumping caused by seismic shocks.

Medium-grained or sand-rich systems tend to be intermediate in size, with moderate gradients. Delta plains have areas of 100-25 000 km2 and gradients of 5.0-0.1 m km-1. Deep-water fan systems have radii of 10-100 km and gradients of 18-6 m km-1. The range of grain sizes available means that physical processes of transport and deposition operate over a wide range of energy levels. The resulting facies tend to be well differentiated, reflecting the full range of basin processes - river flow, tidal, storm, wave and wind energy of the basin as well as its morphological pattern. These systems are therefore particularly suitable both for modelling sedimentary dynamics and for the application of process models to environmental interpretation.

Although some fine-grained mud-rich systems can be small, the majority are very large, with low gradients. Delta plains have areas of 20 000-460 000 km2 and gradients of 0.1-0.001 m km-1. Submarine fans have radii of 100-3000 km and gradients of 5-1 m km-1. The very low gradients of muddy shorelines make them very sensitive to sea-level changes caused either by tides or by longer-term rises and falls. Where sediment supply is high, the rapid deposition of mud and silt causes frequent slumping on delta slopes and submarine fans, in spite of the low gradients. On the other hand, slope aprons, where sediment accumulation is slow, are characterized by infrequent but very large slumps. The size of most mud-rich systems means that there is a considerable time lag between major changes in the controlling factors and the sedimentary response. Changes in one system may eventually affect patterns of sedimentation in the next system downcurrent, though with delay times of perhaps millions of years.

The pattern of sediment delivery to the basin is also important. Sediment may be supplied from a single point source, from multiple sources, from a linear source, from all around the basin, or from one end or the side of the basin. Such patterns may change with time, with important consequences. At one instant there may be a single point source and, in some situations, this may stay fixed for a long period. However, more commonly, sources change over time and thus over the longer, geological time periods in which rock successions accumulate, sediment sources are likely to have switched on one or more occasions. Modern siliciclastic depositional models, based on 'instantaneous' present-day examples, tend to emphasize single point sources. Ancient examples more commonly suggest multiple sources.

Volcanoes are also major contributors of sediment, yielding the whole range of grain sizes both below and above any water level and with variable composition (Sect. 2.1.10; Chapter 12).

BIOCHEMICAL AND CHEMICAL SYSTEMS

The production of biochemical sediment in lakes and the sea is controlled primarily by the nature and productivity of the biota which, in turn, depend on temperature, water chemistry, and the penetration of light into the water. In shallow water, although non-skeletal grains such as ooids, peloids and some lime mud can be important, carbonate production is primarily from the skeletal parts of animals and plants (algae) (Sect. 9.2). Thus the productivity of the 'carbonate factory' depends not only on the appropriate conditions of salinity, nutrients and temperature, but especially on light intensity. This is because some producers such as red and green algae are phototrophic. Others, such as mixotrophs (hermatypic corals and larger benthic forams), are light dependent because they use symbiont algae. Yet others, such as molluscs (especially bivalves), bryozoans, crinoids and brachiopods, are or were suspension feeders that ultimately depend on phytoplankton.

In deep basins only near surface waters penetrated by light produce significant sediment. Planktonic foraminifera and coccoliths yield calcareous material; Radiolaria, diatoms, siliciflagellates and some sponges yield siliceous material; upwelling phosphorus-rich waters yield phosphates. Productivity varies substantially (Sect. 10.2.1), with nutrient-rich waters in zones of oceanic upwelling producing 10 times as much pelagic sediment as nutrient-poor oceanic waters. However, of equal importance to productivity in determining sediment accumulation is the rate of dissolution of the particles as they descend through the water column. Calcareous particles are particularly susceptible to dissolution such that below a certain depth, the calcite compensation depth (CCD), few calcareous particles survive and the bottoms of deeper basins are covered by siliceous rather than calcareous oozes.

Evaporites are precipitated directly from sea or lake waters that have become concentrated to form brines. Their composition depends not only on the salinity of the brine but on its ionic make up, which, in lake waters, may vary considerably. Rates of deposition can be faster than for any other normal process of sedimentation, with vertical accretion up to 100 m ky-1 (Table 8.3).

2.1.2 Climate

The two main aspects of climate are temperature and precipitation, but, locally, wind regimes may also be significant. Not only are mean annual temperature and precipitation important, but also their fluctuations, both seasonal and non-seasonal, and the magnitude and frequency of extreme events.

The meteorological patterns of the Earth are primarily a...