ALBERT

Description: Although conceptual models of carbonate systems typically assume a dominance of external forcing and linear behaviour to generate metre-scale carbonate parasequences, there is no reason to preclude autocyclic and non-linear behaviour in such systems. Component parts of the carbonate system represented in this numerical forward model are entirely deterministic, but several parts are non-linear and exhibit complex interactions. Onshore sediment transport during relative sea-level rise generates autocyclic quasi-periodic shallowing upward parasequences but model behaviour is sufficiently complex that water depth evolution and parasequence thickness distributions are not predictable in any detail. The model shows sensitive dependence on initial conditions, resulting in divergence of two model cases, despite only a small difference in starting topography. Divergence in water-depth history at one point takes [~] 10 ka, and for the whole model grid takes [~] 100 ka. Fischer plots from the two cases show that divergence leads to entirely different parasequence thickness evolution in each case. Chaotic behaviour is a specific type of sensitive dependence, and calculation of trajectory divergence in a 3-D pseudo-phase space indicates that water depth evolution is not truly chaotic. If sensitive dependence, divergence and complex processes generating random products turn out to be common in real carbonate systems, predictions should be limited to elements of the system unaffected by these phenomena, or limited to cases where an element of periodic external forcing over-rides their affects. These results also suggest that increasingly complex and sophisticated stratigraphic forward models are not necessarily going to lead directly to more deterministic predictive power, although they may well be useful sources of statistical data on carbonate strata.

Description: Continental SE Asia is the site of an extensive Cretaceous–Paleocene regional unconformity that extends from Indochina to Java, covering an area of c. 5 600 000 km2. The unconformity has previously been related to microcontinental collision at the Java margin that halted subduction of Tethyan oceanic lithosphere in the Late Cretaceous. However, given the disparity in size between the accreted continental fragments and area of the unconformity, together with lack of evidence for requisite crustal shortening and thickening, the unconformity is unlikely to have resulted from collisional tectonics alone. Instead, mapping of the spatial extent of the mid–Late Cretaceous subduction zone and the Cretaceous–Paleocene unconformity suggests that the unconformity could be a consequence of subduction-driven mantle processes. Cessation of subduction, descent of a northward dipping slab into the mantle, and consequent uplift and denudation of a sediment-filled Late Jurassic and Early Cretaceous dynamic topographic low help explain the extent and timing of the unconformity. Sediments started to accumulate above the unconformity from the Middle Eocene when subduction recommenced beneath Sundaland.

Description: Current classifications of carbonate platforms use depositional gradient as the main criterion for separating systems into two end-member types, ramps and flat-topped platforms (FTPs). However, many examples do not conform to this simple classification. To investigate why this is and to better understand probable controls on platform development, we have used a series of 2D numerical forward model runs to investigate how sediment production, diffusional sediment transport, and other controls such as tectonic subsidence, antecedent topography, and relative sea-level oscillation interact to determine platform geometry. Modeling results reaffirm that rates of down-dip sediment transport relative to rates of autochthonous production are a critical factor in maintaining a ramp profile in stable cratonic settings under a constant rate of relative sea-level rise. Type of carbonate production versus water-depth curve, for example euphotic versus oligophotic, is not a significant control in our model cases. Both euphotic and oligophotic production profiles produce FTPs when diffusion coefficients are low relative to production rates, and ramps when diffusion coefficients are relatively high. These results suggest a continuum of platform types, ranging from transport-dominated, low-gradient systems at one end of the spectrum, to in situ accumulation dominated systems at the other. A system may be transport-dominated because high-energy processes are able to break down and transport even bound sediment, or because carbonate factories produce only sediment that is easily transportable under even low-energy conditions. Time evolution is also probably important. Initially low gradient systems will, in the absence of sufficiently high sediment transport rates, tend to evolve towards high-gradient flat-topped steep-margined platforms. Many observed or inferred platform geometries are therefore likely to be transient forms, and this could complicate interpretation. Investigating how basin bathymetry and style of subsidence control platform geometry suggests that, in transport-dominated systems, strata simply drape the underlying topography, and that pre-existing breaks of slope and differential fault subsidence are a stronger control on platform geometry in in situ accumulation dominated systems. Rotational subsidence tends to create transport-dominated systems during rotation as the topographic gradient increases and transport rate increases and outpaces in situ production rate. Relative sea-level oscillations tend to move the locus of sediment production laterally along any slope present on the platform, distributing the sediment accumulation across the whole width of the platform, suppressing progradation and steepening, and so favoring development of low-gradient systems. Based on all these results, we suggest that a simple cutoff classification into ramp and flat-topped platform types can still be useful in some circumstances, but a more meaningful approach may be to describe and predict platform strata in terms of a multiple-dimension platform parameter space containing a continuum of geometries controlled by sediment production, sediment diffusion coefficient, antecedent topography, differential subsidence effects, relative sea-level oscillations and perhaps other as yet unappreciated controls.

Description: Identifying order or pattern in strata on the basis of qualitative interpretation forms the basis for much current sedimentological and sequence stratigraphic analysis. Order can be usefully defined as some arrangement of facies or unit thickness that has a discernable trend or pattern that is unlikely to occur by chance because it requires some particular systematic process to form. Coarsening, fining, thickening, or thinning-upward trends, and arrangement of strata into cycles are examples of order. Qualitative interpretations of order often demonstrate little more than an implicit assumption of order. This paper defines a robust yet simple-to-apply quantitative method to identify order in strata and to indicate when order cannot be reliably demonstrated. The method is based on two calculated metrics, the Markov metric m derived from analysis of a vertical facies succession, and the runs metric r derived from analysis of observed thicknesses of stratal units. Most importantly, both metrics can be compared with equivalent metrics calculated for disordered strata composed of many randomly shuffled versions of the same lithological units. Probability values can then be calculated from the comparison between observed and randomly shuffled cases, and these p values indicate the degree of evidence present for order in the observed strata. Several test examples using synthetic strata show that the m and r values can define and identify different degrees and types of stratal order, and that the metrics are robust for both stationary and non-stationary successions with a range of different lengths and numbers of distinct facies. Analysis of four outcrop examples, two siliciclastic and two carbonate, demonstrate that ordered facies successions and thickness trends may be less common than typically assumed; none of the four examples analyzed show trends in thickness, and only the examples from the Book Cliffs, which represent a bedset scale composite of observations, show evidence for facies order. The examples demonstrate how a quantitative analysis can lead to better understanding of strata, either ordered or disordered, and can provide better insight into the validity of current stratigraphic interpretations and models. Absence of order in many of the analyzed 1D vertical successions may also indicate that we need to focus more on longer-term trends and analysis of 2D and 3D stratal geometries.