Tracing sedimentary uncoformities across the marine – continental decoupled zone: an experimental 3D study

MILANA, JUAN PABLO; TIETZE, KRAUS W. ; MUGAS LOBOS, ANA CECILIA

Belém, Pará

Congreso; 3rd Latinamerican Congress of Sedimentology; 2003

Universidade Federal do Pará

One of the main challenges of the sequence stratigraphy is the descyphering of the way coastal marine and proximal alluvial unconformities encroaches within the mixed sedimentary deposits in between. On one side, it has been widely recognized that the base level effect tends to decrease to the upstream direction until complete dissapearance. On the other hand, models focused on the processes at proximal alluvial systems indicate that the effects of controls like hydrology, sedimentary supply and (mountain) tectonism tend to dissapear in the downstream direction. As a result, several authors have mentioned the existence of a "decoupled zone" where the effects of controls advancing from both directions may encroach. The study of the sedimentary dynamics on this decoupled zone and the answer to the question about correlating regional marine unconformities into the continental strata, were the main objetive for the present study. There are several sequence depositional models that connect the fluvial-alluvial cyclic behavior to a hypothetical fluctuating base level, and a study of the decoupled zone would through light on this end.                 In order to find answers and test those sequence stratigraphical models, a preliminary set of experiments including the effect of baselevel on the downstream end of a sedimentary wedge and of the hydrology on the upstream end were designed. Although having physical limitations for such a model, a decoupled zone was generated between a proximal area were hydrology controlled accumulation and a distal area where baselevel dominated the scenario. Hydrological-baselevel cycles resembled glacioesutatic cycles, which in theory show a maximum discharge peak (melt water pulses, as detected in the Mexico Gulf) during sea-level rises and low discharge stages when the sea-level falls. In theory, base level rises because discharge increase (melt-out of continental ice) while base level falls when discharges are minimal, due to the take of water to feed continental ice growth. Variations to this type cycle as in its shape and duration of each sub-cycle were studied.                 The experimental facility used was the main flume of the Sedimentary Laboratory of the Marburg University, a tiltable glass-walled channel, 16 m long, 1.2 m wide and 0.9 m deep. To simulate a basin margin to induce accumulation and the generation of an alluvial wedge, a ramp consisting of three articulated segments made of plywood was inserted. Details on ramp dimensions, scaling procedure and the experimental protocol are given by Milana and Tietze (2002). The depositional area of the ramp was divided in two segments of different slopes but prior to the initiation of baselevel control, a natural sedimentary wedge was produced by scaled streams, leaving visible only the upper part of the ramp ("bedrock" sloping at 0.16) and the flume floor, sloping at 0.01, enough to avoid ponding at the end of the sedimentary wedge. Like in the other experiments mentioned, sediment used was well sorted natural fine and medium sand, sediment supply was always 1kg/min. and discharges applied varied from 0.14 and 0.7 liters/min. Base level was chaged from 0 to 11 cm, at different rates. Resulting cycles would be varied as different combinations of discharge, baselevel and cycle duration can be tested, so only variations resembling glacioeustatic cycles were studied and cycle duration was initially chosen at 1 hour, as it was observed in previous experiments that systems were achieving a certain stabilty of autocyclic processes. High resolution maps were produced for each stage and sometimes maps were surveyed for substages to image the fast changes that occur after an external control si modified, lika a base level fall. Mapping (usually a 24-hs long procedure for a n unprocessed survey) and availability of dry sediment limited a number of cycles tested to four different.                 The first cycle is a simple square-like wave, that is baselevel is high or low with rapid changes from one to other and discharge is simply low or high. The second and third cycles are sinusoidal-like waves achieved using a three-step discharge function and 4 baselevel substages (rising, high, falling and lowstand), and they differ on the duration of each substage, which was 20 and 40 minutes for each one. They gave the opportunity to see the effects of the velocity of change on the sedimentary wedge. The fourth cycle is a simple 2-fold cycle with a diminished sediment supply. Main results show that there is not  connection between coastal dynamics and proximal alluvial dynamics. Although coastal scarp erosion is much faster during high discharge stages than during low-discharge stages, the effects of retrocedent erosion does not extends until the proximal alluvial realms. On the other hand, at proximal reaches, discharge (within a constant sediment supply) defines the basic sedimentary processes acting at the limit between the bedrock and the sedimentary wedge. However, even during the maximum rates of proximal incision due to increased discharges, the erosion never affected the dynamics at the coastal area dominated by the baselevel. We proved that it is perfectly possible to record accumulation and onlap at the coastal-distal alluvial realms, while an important erosion is occurring at the purely fluvial-alluvial settings. On the other hand, the experiments indicate that is perfectly possible that a major base level fall occur causing extensive coastal erosion, while at the neighboring alluvial realm, accumulation is onlapping the basin margin. Notice that, if this dynamics is possible in a sedimentary wedge of 5-7 m long, where the decoupled zone is usually 1 m, it is perfectly possible a real-scale basin would record our finding: that is incorrect to tie marine and continental unconformities, without a certain physical connection.                 Most of the findings about the dynamics at the upstream portion of the sedimentary wedge are similar to what already was summarized by Milana & Tietze (2002) as experimental variables were the same. On the other hand, the coastal dynamics, that is, coastal scarp erosion, shoreline progradation and retrogradation are quite similar to experimental findings of Van Heijst et al. (2001). However, in our case, shelf slope has been much more gentle as the shoreline advanced and retreated over naturally deposited sediments and not an artificial ramp as in the referred experiments. As a result, deltaic backstepping lobes were more developed spatially and showing a significant difference between a slow and a rapid rise of baselevel. During rapid rises, the river mouth has not time to move laterally accross the strandplain, generating a series of concatenated backstepping lobes, while during slow rises the delta lobes are repositioned all across the flume width and as a result all the drowned shelf shows regularly distributed backstepping lobes.                 These experiments suggest, that fluvial /alluvial dynamics may be completely independent from baselevel dynamics. However, it is important to realize that conditions can be such that the effect of baselevel can advance headwaters until the basin margin in some cases as for instance reduced fluvial systems or low rates of sediment supply, that enhance backwater erosion. Therefore, while our findings indicate one possible scenario, others might be completely different due to a contrasting value of controlling variables. Thus, it is possible that an eustatic unconformity can be physically connected to an alluvial one, as well as the opposed alternative indicating we need to be critic at the moment of tracing sedimentary surfaces across a basin.