Interrelations between framework-building and encrusting skeletal organisms and microbes: more-refined growth history of Lower Devonian bindstones

The framework‐building stromatoporoid Stachyodes, the encrusting calcimicrobe Rothpletzella and encrusting Graticula‐like red algae are major contributors to red algal–calcimicrobial–stromatoporoid bindstones in the Lower Devonian Elmside Formation of the Yass Basin, New South Wales, Australia. The distribution and accumulation patterns of encrusting organisms within the red algal–calcimicrobial–stromatoporoid bindstones observed by optical microscopy and SEM imply a biotic interrelationship between skeletal organisms and microbes that reflects environmental changes. Rothpletzella is characterized by prostrate filaments with frequent branching and a high angle of bifurcation. Filaments of Graticula‐like red algae exhibit rare branching and a relatively low angle of bifurcation. In addition, they are prostrate at the base before becoming erect. Both Rothpletzella and the red algae successively encrust the surfaces of skeletal frameworks, but exhibit different distributions. Rothpletzella and other calcimicrobes cover both the lower and upper surfaces of frameworks, whereas red algae are limited to the upper surfaces. Their individual distributions are thus significantly influenced by the frameworks formed by the thin, laminar stromatoporoid Stachyodes, which create different microenvironments as by‐products. The limited distribution of the red algae was probably related to light levels or phototropism. Upper framework surfaces are variously encrusted by calcimicrobes and the red algae to form thick crusts with varying accumulation patterns. Micritization around the algal thalli, covering of calcimicrobes such as Wetheredella, and microbial micrites between algal thalli all suggest interruptions of algal growth that correspond to episodes of harsh environmental conditions. Transitions from Graticula‐like red algae to Rothpletzella reflect periods of deteriorating environmental change for skeletal organisms, which resulted in the predominance of microbial growth. In contrast, a resurgence of red algae on calcimicrobes suggests improved environmental change. Repeated accumulation patterns between Graticula‐like red algae and Rothpletzella indicate changing habitat environments and competitive relations within skeletal organisms and microbes. These relationships provide insight into understanding how skeletal organisms and microbes utilized space and how they interrelated with each other to produce Devonian reefal limestones.     

Sedimentary response to a tectonically induced sea-level fall in a shallow back-arc basin: the Mulichinco Formation (Lower Cretaceous), Neuquén Basin, Argentina

This study examines the sedimentary response to a tectonically driven relative sea‐level fall that occurred in the Neuquén Basin, west‐central Argentina, during the late Early Valanginian (Early Cretaceous). At this time the basin lay behind the emergent Andean magmatic arc to the west. Following the relative sea‐level fall, sedimentation was limited to the central part of the Neuquén Basin, with the deposition of a predominantly clastic, continental to shallow marine wedge on top of basinal black shales. This lowstand wedge is called the Mulichinco Formation and consists of a third‐order sequence that lasted about 2 Myr and contains high frequency lowstand, transgressive, and highstand deposits. Significant variations in facies, depositional architecture, and internal organization of the sequence occur along depositional strike. These variations are attributed mainly to tectonic and topographic controls upon sediment flux, basin gradient, fault tilting, and shifting of the depocentre through time. These controls were ultimately related to asymmetrically distributed tectonic activity that was greater towards the magmatic arc in the west. The superposition of fluvial deposits directly upon offshore facies provides unequivocal evidence for a sequence boundary at the base of the Mulichinco Formation. However, the Mulichinco sequence boundary is marked by shallow, low erosional relief and widespread fluvial deposition. The surface lacks prominent valleys traditionally associated with sequence boundaries. This non‐erosive sequence boundary geometry is attributed to the ramp‐type geometry of the basin and/or rapid uplift that limited stratigraphic adjustment to base‐level fall. Significant along‐strike facies changes and a low‐relief sequence boundary are attributes that may be common in tectonically active, semi‐enclosed basins (e.g. shallow back‐arc basins, foreland basins).     

TOPOGRAPHIC AND ISOSTATIC CORRECTIONS TO GRAVITY SURVEYS IN MOUNTAINOUS AREAS*

In mountainous areas it is an inadequate procedure to reduce gravity observations by merely subtracting the effect of an infinite flat slab of material between the station and sea-level, and adding a terrain correction. A programme is described which directly computes the effect of masses above sea-level, and mass-deficiencies below it; as well as the effect of compensating masses under the Pratt and Airy isostatic systems. As an example, the method has been applied to a regional gravity survey of Papua and New Guinea where it is seen to remove the usually high correlation of the Bouguer anomaly with local topography.     

AIR AND SAND MOVEMENTS TO THE LEE OF DUNES1

The large and extensive transverse and barchane dunes of coastal South West Africa are strongly oriented under the influence of predominantly southerly winds. During periods of strong winds (40–50 miles/h) deposition occurs on the lee slope in three ways: (1) sand is blown over the crest of the dune and falls on the lee slope; (2) rapid deposition near the dune crest results in periodic slumps and slides down the lee slope; (3) eddy currents developed to the lee of the dune pick up sand from the surface downwind from the dune and transport it to the lee slope. The size and strength of the lee eddy is surprising. With winds in the 40–50 miles/h range frequent gusts lift fine sand from the downwind surface to a height of several feet. Less frequently sand is picked up from a low position on the lee slope and redeposited higher on the slope. The addition of material to the lee slope by the eddy is much less volumetrically than the contribution directly over the dune crest from the windward direction; however, with strong winds the removal and transportation of sand from the area downwind of the lee slope back to the lee slope appears to be important in the deflation of this surface. The width of the area influenced by the lee eddy during strong winds is about equal to the height of the dune. Observations in low dunes from 1 to 20 ft. high at Sapelo Island, Ga., U.S.A., confirm the presence of a well developed eddy to the lee of these dunes during strong and moderate winds (20–50 miles/h).     

The effects of fluctuating base level on the structure of alluvial fan and associated fan delta deposits: an example from the Tertiary of the Betic Cordillera, Spain

The Villanueva Complex is a lacustrine alluvial fan - fan delta system which accumulated on the northern margin of the Guadix Basin during the Pliocene. Five transitional zones can be distinguished from proximal to distal areas: proximal alluvial fan, mid-fan, fan fringe (transition zone), proximal fan delta and distal fan delta (lacustrine). This paper focuses on sedimentation in the fan fringe and the proximal fan delta areas, where the effects of fluctuations in the base level (lacustrine level) are more easily observed. The stratigraphical succession is here characterized by an alternation of fine lacustrine sediments (mudstones and siltstones) and gravels. The gravels appear as isolated channels, stacked channels, lenses and sheets. The isolated channels, which have a V-shaped profile and better developed wings towards the top, appear in the fan fringe zone and in the fan delta. The stacked channels originate in the fan fringe zone and evolve laterally and distally to isolated channels. The lenticular gravel bodies (lenses) are well represented throughout the fan delta and present small channels at the base, indicating a radial flow pattern. Finally, the sheets are characteristic of the fan fringe zone. The first episodes of channel incision occurred at lowstand lake level. The channels and lenses developed in rising lake level conditions, and sheet deposit took place at highstand lake level. Although the final form of the lithosome was heavily controlled by the fluctuations in lake level, it depended on the relative proportions of gravel and silt sedimentation. The gravel-siltstone interface therefore represents an equilibrium surface between the proportions of sedimentation of these lithologies. The bodies mentioned above are organized in coarsening and thickening upwards sequences tens of metres thick, in which a distal to proximal evolution can be observed from isolated channels to lenses or from stacked channels to sheets. The building and thickness of these sequences were controlled by fluctuations in the lake level. An initial fall in base level caused lengthening of the channels and entrenchment from distal to proximal areas, and a continuous increase in supply due to erosion in the drainage basin installed on the subaerial fan. When the lake level rose, lobes were formed at the channel endings and overbank processes were made possible, thus generating radial channels, whose levees were formed by amalgamation with the levees of the main lens channels. At the same time, sheets were formed in proximal areas, where the available amount of sediment was greater.     

Policy narratives, landholder engagement, and oil palm expansion on the Malaysian and Indonesian frontiers

Over recent decades a structural transformation has affected agriculture in the frontier areas of Malaysian Borneo and Outer Island Indonesia with the rapid conversion of agricultural lands, fallows, and formerly forested areas into oil palm. These frontiers have similar positions in the international political economy of oil palm and have complementary resource endowments. In both cases, state planners face the common challenges of finding a disciplined labour force, delivering land for estate development, maintaining local legitimacy, and dealing with local contestation. Yet there are significant differences in systems of governance and policy frameworks regarding land, shifting capacity of state actors to facilitate the transformation of these agrarian frontiers, and changing degrees of local, national and international contestation. Considering the generic and the specific elements at play in each case, this paper argues that analogous policy narratives have shaped the ways in which landholders have been engaged in the process of oil palm expansion in Malaysia and Indonesia. In both cases, with the shift from state-led to neoliberal governance approaches to agricultural development, the 'frontier' has been created and transformed through policy narratives that facilitate the conversion of whole landscapes into oil palm. This has been achieved by obscuring indigenous forms of agriculture and land tenure, while creating reserves of available 'state' or 'idle' customary land, and counterpoising smallholder 'marginality' and 'backwardness' to the modernity of contemporary estate agriculture.