Falling-stage systems tract in tropical carbonate rocks - Perspectives in Carbonate Geology - page 298

Falling-stage systems tract in tropical carbonate rocks

WOLFGANG SCHLAGER* and GEORG M.D. WARRLICH †
* Vrije Universiteit/Earth and Life Sciences, 1081HV Amsterdam, Netherlands
(E-mail: [e-mail])
† Petroleum Development Oman, DSC 72, P.O. Box 81, Muscat 113, Sultanate of Oman
(E-mail: [e-mail])

ABSTRACT

The standard model of sequence stratigraphy postulates that the falling limb of a
sea-level cycle leaves no depositional record on the shelf and upper slope - highstand
and lowstand systems tracts are separated by a hiatus and erosional unconformity.
There are now many examples of sediment accumulations formed during relative sea-
level fall, here referred to as 'falling-stage systems tract'. This study focuses on tropical
carbonates. Numerical modelling is used to identify key parameters for the develop-
ment of the falling-stage systems tract and the standard-model anatomy, and to deter-
mine their stability domains in this parameter space. Well-documented case studies
are used to compare model results and fi eld observations. Key parameters are the rates
of sea-level fall, erosion and carbonate production with slope angle as a modifying fac-
tor. The falling-stage systems tract is favoured by low rates of fall, low rates of erosion,
high rates of carbonate production, and low slope angle. The effects of subaerial and
marine erosion are similar. The two variables are linearly correlated in a wide range
of conditions and the sum of subaerial and marine erosion rates can be plotted on
one axis after appropriate conversion. Numerical reconnaissance and fi eld observation
indicate that the geologically probable space for the falling-stage systems tract is large
and the phenomenon should be common in tropical carbonates.

Keywords Falling-stage systems tract, sequence stratigraphy, tropical carbonates,
stratigraphic modelling.

INTRODUCTION shelf and upper slope deposits (Hunt & Tucker,

1992, 1993; Nummedal et al ., 1992; Posamentier &
Allen, 1999; with more case studies in Hunt &
Gawthorpe, 2000). Reviews by Plint & Nummedal
(2000) and Posamentier & Morris (2000) concluded

The concept of sequence stratigraphy as
introduced in the open literature includes a con-
sistent, detailed model of the depositional ana-
tomy of sequences, here referred to as the 'standard
model' (Mitchum et al ., 1977; Vail et al ., 1977;
Vail, 1987; Van Wagoner et al ., 1987, 1988; Jervey,
1988 Posamentier & Vail, 1988; Posamentier
et al ., 1988; Sarg, 1988). The standard model of
sequence stratigraphy (STM) assumes that the
lowstand systems tract is separated from the pre-
ceding highstand tract by an erosional unconfor-
mity that extends continuously from the highstand
shoreline to the lowstand minimum of sea level
(Fig. 1). A depositional record of sea-level fall
exists only on the lower slope and basin fl oor in
the form of turbidite fans and wedges of slumps
and debris fl ows.
Subsequent fi eld studies produced many exam-
ples where the falling sea level also left a record in
the form of downstepping or downward-migrating

Transgressive surface
LST Slope fan

Floor fan

Incised valley

HST

Fig. 1. Standard model of sequence stratigraphy in the
vicinity of the sequence boundary (SB). Highstand sys-
tems tract (HST) beneath the SB is dissected by an incised
valley; elsewhere erosion of HST is minor as indicated by
the absence of truncated timelines. During sea-level fall
sediment accumulated only on lower slope and basin fl oor
(grey shading). On upper slope, no falling-stage systems
tract is present and lowstand systems tract (LST) abuts
against clinoforms of preceding highstand tract. After Vail
(1987), modifi ed.