Controls on Facies Mosaics of Carbonate Platforms: A Case Study from the Oxfordian of the Swiss Jura

Abstract

Modern shallow-water carbonate systems commonly display a complex pattern of juxtaposed depositional environments with a patchy facies distribution (facies mo saics). On ancient carbonate platforms, the reconstruction of lateral facies distribution is often hampered not only by discontinuous outcrop but also by lack of suffi ciently high time resolution. This case study from the Oxfordian (Late Jurassic) of the Swiss Jura Mountains demonstrates a way to improve the temporal and spatial resolution for the interpretation of carbonate rocks. Sequence-stratigraphic and cyclostratigraphic analyses have been performed that provide a basis for defi ning depositional sequences, which formed through sea-level changes that were induced by the 400-, 100-and 20-kyr orbital cycles. On the 100-kyr scale, sequence boundaries are well developed and can be correlated between sections. However, identifi cation and correlation of sequences related to the 20-kyr cycle may be diffi cult if local processes overprinted the record of orbitally controlled sea-level changes. The reconstruction of facies distribution along selected time lines gives a dynamic picture of platform evolution with time steps of a few ten thousand years and helps to interpret the controlling factors such as differential subsidence, low-amplitude eustatic sea-level fl uctuations, climate and ecology of the carbonate-producing organisms. Reefs and ooid shoals developed preferentially on topographic highs and thus accentuated platform morphology. Siliciclastics were shed onto the platform during sea-level falls and increased rainfall in the hinterland; their distribution was controlled by platform morphology. Siliciclastics and associated nutrients hindered carbonate production and thus indirectly infl uenced platform morphology. In addition to these controls, random processes acted on the smaller-scale facies relationships. Sedimentation rates can be estimated for each facies type over time spans of 10-20 kyr. They can thus be compared more easily with Holocene rates than if averaged over millions of years. This study shows that the Oxfordian platform in the Swiss Jura was as complex and dynamic as Holocene carbonate systems.

Rate and synchronicity of environmental changes on a shallow carbonate platform (Late Oxfordian, Swiss Jura Mountains)

“…The slight variations through time and between the sections may be attributed to differences in the original composition of the carbonate, to differences in the water masses that furnished C and O to the carbonate-producing organisms, and to differences in the composition of the pore fluids that induced the early diagenetic stabilization of the carbonate (Van der Kooij et al, 2009). Such differences are to be expected on a structured carbonate platform where lateral and vertical facies changes were common (facies mosaics; Strasser & Védrine, 2009), where sealevel and climate changes influenced temperature and salinity of the ocean water, and where early diagenesis was compartmentalized due to local and episodic emersion and installation of fresh water lenses (as evidenced through cathodoluminescence studies of cements by Plunkett, 1997). In addition, time-averaging through bioturbation and storm reworking probably homogenized the original signals.…”

“…, 2009). Such differences are to be expected on a structured carbonate platform where lateral and vertical facies changes were common (facies mosaics; Strasser & Védrine, 2009), where sea‐level and climate changes influenced temperature and salinity of the ocean water, and where early diagenesis was compartmentalized due to local and episodic emersion and installation of fresh water lenses (as evidenced through cathodoluminescence studies of cements by Plunkett, 1997). In addition, time‐averaging through bioturbation and storm reworking probably homogenized the original signals.…”

Metre‐scale cycles in shallow water carbonate successions: Milankovitch and stochastic origins

“…As noted earlier, at the low latitudes (<30°) where shallow water carbonates thrive, astronomical insolation is dominated by ca 21 kyr precession cycles. Precession forcing (along with eccentricity) has been invoked as the driver of cyclicity observed in numerous pre‐Oligocene successions (Fischer, ; Grotzinger, ; Cozzi et al ., ; Strasser & Védrine, ; Eberli, ; Sames et al ., ). To explore the sensitivity of carbonate accumulation to astronomical insolation forcing of sea‐level, this study modelled the effect of increasing astronomical signal strength on the probability of preserving statistically significant astronomical precession cycles in preserved water depth.…”

“…This range of accumulation rates is in line with work by De Benedictis et al . (), Strasser & Védrine () and Strasser & Samankassou (). In particular, Strasser & Samankassou () evaluated accumulation rates in Upper Jurassic and Lower Cretaceous strata on short astronomical ( ca 20 kyr) timescales and defined a range of 0·07 to 0·6 mm per year ( ca 0·05 to 0·4 mm per year after accounting for a compaction factor of about 1·5; Strasser & Samankassou, ).…”

On facies belts and facies mosaics: Holocene isolated platforms, South China Sea

“…In carbonate systems through geological time, patterns of sediment accumulation can be highly variable, such that they commonly are described as 'complex' (e.g. LaPorte, 1967;Pratt & James, 1986;Wright, 1986;Grotzinger, 1989;Satterley, 1996;Lehrmann et al, 1998;Burgess & Wright, 2003;Ma et al, 2009;Strasser & V edrine, 2012;Alnazghah et al, 2013). In consideration of the genesis of the accumulations that build carbonate stratigraphic systems, spatial patterns of carbonate deposits in many studies are interpreted to be the net stratigraphic product of one of two distinct (planform) end-member classes, facies belts and facies mosaics (e.g.…”