FACIES MODELS
The recognition that certain modern sedimentary environments were characterised by a distinctive set of
sedimentary facies led to the development of ‘standard facies models’. Once constructed, these facies
models were then available for application to ancient sedimentary successions. Thus once the facies of an
ancient sedimentary succession have been evaluated, they can be related to the appropriate facies model to
suggest a depositional environment. This is a highly predictive tool, which creates a model for the
sedimentary environment. Such a model can be used, for example in hydrocarbon exploration, to suggest
the form and nature of reservoir rocks such as sand bodies.
For the various sedimentary environments that we discuss, we will look at the various facies models that
have been created. Many of these are described in books such as Reading (1996).
TRANSGRESSIONS AND REGRESSIONS
6.1. Transgression
The term transgression simply means increase in the area of the sea; that is a landward migration of the
coastline. Transgression is not necessarily related to global (eustatic) sea level rise. It can be brought about
by many factors, these include:-
i. Sediment supply unable to keep up with basin subsidence.
ii. High wave energy of a coastline leading to coastline erosion.
iii. Local tectonic forcing.
iv. Eustatic sea-level rise.
Note that transgression does not necessarily mean an increase in water depth offshore (consider case ii
above).
6.2. Regression
This is the opposite of transgression and means a decrease in the area of the sea, or seaward migration of
the coastline. The term is generally used to indicate the case where sedimentation rates exceed the rate of
basin subsidence and the coastline progrades (that is moves seawards). Note that at any particularly time
some parts of the coastline maybe undergoing transgression, while other parts maybe undergoing
regression.
6.3. Forced Regression
The term forced regression is used to describe situations where regression is brought about by external
forcing, specifically a fall in sea level.
7. CYCLE AND EVENT STRATIGRAPHY
Cycles and event beds are common in outcrops of many sedimentary successions. We will consider some
of the controls here.
7.1. Cycles versus Rhythms
Rhythms or rhythmic bedding consists of repetitions of two lithologies or couplets (e.g., AB AB AB).
Cycles consist of more than two lithologies and may be symmetrical (e.g., ABCBABCBA) or asymmetric
(ABC ABC ABC); boundaries between cycle lithologies tend to be more gradational and often show
distinctive fining or coarsening upward trends. The term cyclothem is often used for cycles, particulalry in
the Carboniferous.
7.2. Autogenic versus Allogenic Cycles
Autogenic cycles are controlled primarily by processes operating within the depositional environment.
These might include channel switching and delta lobe migration. Individual beds and bedsets generally
show low lateral continuity.
Allogenic cycles are controlled mainly be variations external to the system. These include global sea-level
variations, tectonic processes in the sedimentary source area and climatic changes. Beds and bedsets are
generally show good lateral continuity. Global (eustatic) sea-level changes and climate may be controlled
by Milankovitch cycles.
