Juan Pablo Milana

Largest wind ripples on Earth

Unique wind ripples attaining heights to 2.3 m, wavelengths to 43 m, and a crest maximum grain size of 19 mm occur on the Argentine Puna Plateau at ~4000 m altitude. These are the largest ripples reported on Earth, comparable only to Mars counterparts. They form in the presence of high proportions of low-density pumice clasts (0.91 g/cm 3 ), although crests are exclusively composed of varnished, normal-density clasts (2.43 g/cm 3 ). Mature ripple profiles are partly excavated on bedrock, so they form by a combination of deflation, winnowing of finer grains, minor wind drift of fine gravel, and lagging of clasts >4 cm. The large ripple size appears to be related to strong winds, dense saltation layers, and a long time for evolution. Ripple sizes are smaller on obstacles, as compared to flat terrain; there is a lack of correlation between clast size, wavelength, and the extreme ripple size (in spite of the thin atmosphere), all of which suggest that while small-scale gravel ripples may form according to a reptation model, their evolution into large-scale types may relate to aerodynamic instabilities originating at the saltation curtain–air interface.

A depositional record of deglaciation in a paleofjord (Late Carboniferous [Pennsylvanian] of San Juan Province, Argentina): The role of catastrophic sedimentation

The combination of high sediment supply rates and ample accommodation within fjords provides high-resolution records of deglaciation. Ancient fjord fills provide the potential for three-dimensional views of the evolution of depositional environments in response to changing sediment supply and base level through the deglacial process. We describe the fill (Jejenes Formation) of a well-exposed Late Carboniferous (Pennsylvanian) glacial valley and its tributaries; these deposits record the progressive marine flooding and infilling of topography by glacially derived sediments. The geometry of the valley and its tributaries is controlled by the underlying lower Paleozoic lithologies: a deep valley with steep sides exists where the bedrock is massive limestones, and a broader, shallower valley exists where the bedrock is generally a fine-grained olistostrome. The valleys are locally floored with diamictites, including both in situ tillites and remobilized diamictites. In the trunk valley these are locally overlain by a small, shallow-water delta. The major part of the valley fill consists of a ≤150-m-thick mudstone-dominated succession (probably generated by plumes of glacial outwash) containing numerous dropstones that decrease in abundance down the fjord. The mudstones contain numerous thin sandstone and conglomerate turbidites that were supplied laterally via subaqueous gravel fans feeding in from the tributary valleys, each of which has a distinctive clast suite related to the local subcrop. The entire succession is overlain by >300 m of sandy turbidites, of which the upper part includes large mass-transport complexes. Intercalated within the succession in the trunk valley are structureless, graded, silty mudstones lacking dropstones but with abundant large wood fragments. Close to the steep western margin of the trunk valley, each of these massive mudstones is underlain by a slump or debris flow, locally containing meter-scale blocks of Ordovician limestone from the valley side. We interpret these as a consequence of rockfalls from the steeper valley sides, triggering debris flows on the subaqueous fjord slopes. We suggest that large solitary waves were generated as the rockfalls entered the water, traveling along the fjord and stripping vegetation from the shoreline. Large amounts of mud and silt were thrown into suspension during these events and subsequently settled from suspension to form the structureless graded beds. These deposits, and other mass-flow deposits within the succession, emphasize the potential importance of catastrophic sedimentation within deglacial successions.

Deglacial and postglacial sedimentary architecture in a deeply incised paleovalley-paleofjord—The Pennsylvanian (late Carboniferous) Jejenes Formation, San Juan, Argentina

Quebrada de las Lajas, near San Juan, Argentina, preserves an early Pennsylvanian deglacial-postglacial succession in a highly confined paleofjord setting. The sedimentary succession records four distinct stages in the evolution of the valley fill. Stage 1 is represented by the deposits of subglacial diamictites, ice-contact deltas, and related deep-water lacustrine environments, including several subaqueous channels. Stage 2 records a glacio-eustatic marine transgression, and a slow-down of the clastic supply into the paleofjord. Stage 3 records a sandy, confined turbidite environment. Stage 4 consists of a coarse-grained delta, which represents a significant rejuvenation of the sedimentary system in the paleofjord. The transition from stage 1 to stage 2 was abrupt and basinwide, and has proven to be a good regional correlation marker. In this paleovalley, the glacio-eustatic rise probably caused floating and consequent rapid melting of the valley glacier. This resulted in a paleovalley-wide turbidite event that is up to 5 m thick and indicates an overall waning character. The rapid sediment emplacement in this event and resulting loading of the paleovalley sediments may have helped cause widespread mass-transport events at the stage 1–2 boundary. All stages show evidence of mass-transport–related deposits, but stage 1 records the most widespread mass transport, with a large spectrum of processes represented, including coherently slumped material, rafted blocks, and completely mixed debris-flow deposits. These mass-transport deposits range from a few meters to over 50 m thick, and up to hundreds of meters wide and long. Most of the large mass-transport deposits in the lower part of the fjord fill occurred at the transition to stage 2, implicating a rise in relative sea level as a possible trigger for the slope failures. Additionally, stage 3 deposits were affected by several thrust-sense dislocations that have large offsets relative to the scale of the sedimentary succession (tens to over 50 m vertically and hundreds of meters laterally); strata associated with the dislocations exhibit growth, indicating very early movement, and they are interpreted as the frontal ramp zones of mass-transport deposits. The abundance and size of mass-transport deposits in stages 1 and 3 helped control sediment pathways throughout the paleofjord.

Solar cycles recorded in Carboniferous glacimarine rhythmites (Western Argentina): relationships between climate and sedimentary environment

Abstract A Carboniferous glacial episode has been widely known in the western part of Argentina since ca. 1910. In some sections it is associated with rhythmic sedimentation. One of those rhythmite successions is interpreted as composed by varves (annually deposited sedimentary cycles) accumulated in a glacimarine environment. It is further analyzed as a time series searching for cyclic signals. Several localities show varve-like successions, but only a few of them are not disturbed by reworking of bottom currents. The Rio Francia section (San Juan, Argentina) is well-suited for varve-counting as no current-produced structures are recognized through the rhythmite interval, and outcrops are good enough for counting directly in the field. Five varve-types which show differences mainly in thickness, grain size and dropstone frequency are recognized. Internally, they are mostly symmetric, distinguishing them from traditional normally graded varves, a feature that has also been observed in Holocene glacimarine varves. The different varve types indicate varying positions with respect to the ice front. Because of disturbances or lack of well defined textural or colour contrasts, only the three varve types with intermediate thicknesses were counted. Three units including 1028, 690 and 465 varves were measured and then analyzed as time series in order to detect cyclic signals. Some differences in the quality of the cyclic signals can be seen in spectral analyses, depending on the main type of the varve that composes the unit. However, a peak around 12 yrs can be seen in most of the spectra, and is interpreted as the sunspot-cycle record. A secondary peak near 24–26 yr also may be related to this phenomenon (Hale cycle). Four major cycles, recognized in the field but not in the spectra, that represent major glacial advances and retreats, can also be related to a solar periodicity of 2500–3000 yr. The recognition of sunspot cycles in these Carboniferous varves is not unusual as they are found in other ancient varve successions. Random processes, typified by the distribution of dropstones, played an important role in disturbing the climatic signal in this sedimentary environment. The random process of sediment transport and deposition by ice rafting disturbs the cyclic climatic signal mainly in proximal varve-successions. Increased water motion and distance to the source of sediment toward the open sea tend to produce more homogeneous very fine-grained distal varves, eventually causing the loss of varve-like appearance. The remaining varves located at intermediate positions with respect to the glacio-related sediment source seem to be the best suited for recording climatic cyclicity, as shown by spectral analyses.

Mass-transport and slope accommodation: Implications for turbidite sandstone reservoirs

Mass-transport events are virtually ubiquitous on the modern continental slope and are also frequent in the stratigraphic record, but the potential they create for stratigraphic trapping within the sea-floor topography is not generally appreciated. Given the abundance of mass-transport deposits (MTDs), we should expect that many turbidite systems are so affected. The MTDs may be very large (volumes > 103 km3 [∼250 mi3], areas > 104 km2 [∼6250 mi2], thicknesses > 102 m [∼330 ft]), and they extensively remold sea-floor topography on the continental slope and rise. Turbidity currents are highly sensitive to topography; thus, turbidite reservoir distribution and geometry on the slope and rise are often significantly affected by subjacent MTDs or their slide scars. Turbidites may be captured within slide scars and on the trailing edges, margins, and rugose upper surfaces of MTDs; developed in accommodation when the mass movement comes to rest; or subsequently resulting from compaction or creep. The filling of such accommodation depends on the properties of the turbidity currents, their depositional gradient, and how they interact with basin floor topography. The scale of accommodation on top of MTDs is determined largely by the dynamics of the initial mass flow and internal structure of the final deposit, and it typically has a limited range of length scales. We present interpretations of a range of previously published and original case studies to illustrate the range of accommodation styles associated with MTD-related topography within the evacuated space of the slide scar, around and on top of the deposits themselves. In fact, several well-known deep-water outcrops probably represent examples of sedimentation influenced by MTDs. Hydrocarbon reservoirs in many slope settings may be controlled by the accommodation related to MTD topography. At the exploration scale, entire shelf margin and slope depositional systems may be contained within the scars evacuated on the upper slope by mass failure, whereas at the production scale, the rugosity on the top of MTDs creates widespread potential for stratigraphic trapping. The location, geometry, and property distribution of such reservoirs are closely controlled by the interaction of turbidity currents with the topography; thus, an understanding of these processes and their impact on slope stratigraphy is vital to reservoir prediction.

Sequence Stratigraphy in Alluvial Settings: A Flume-Based Model with Applications to Outcrop and Seismic Data

A simple theoretical model for generating proximal alluvial depositional sequences was developed from flume experiments and illustrated by seismic reflection geometries. During periods of low discharge, the transport system is inefficient, and the sediment is mainly deposited in proximal areas; at extreme conditions of hydrological deficit, maximum backfilling occurs. The rise of discharge begins to retransport sediment stored in proximal areas during low-efficiency periods. At maximum discharge, the maximum degree of incision is reached, concomitant with a progradation of coarse material toward the basin center. In this model, a transit cycle of the knickpoint occurs, caused only by transport efficiency changes and not by base-level change. A dominantly fining-upward sequence ending with a short coarsening-upward interval is thus deposited, bounded by truncation surfaces well developed at the basin margins. Flume experiments were designed to explore this model. Two runs were analyzed that differed in the rate of volume growth of the water-sediment system of the flume, with the first run at no volume growth and the second run at 0.38 L/min. These runs simulated different rates of generation of accommodation space. A single grain size (1 mm) was used to simplify the analysis and to avoid high-relief bed forms. During each cycle, four main behaviors occur. (1) The rise of discharge produces an immediate proximal erosion and progradation of the sedimentary wedge. (2) Within the interval of high discharge, a maximum erosion forms the truncation surface, and soon after that slow deposition at high regime occurs in proximal areas (minor onlap). (3) A fall of discharge produces a rapid onlap on the simulated basin margin, the sedimentary wedge moves upstream, and a surface of nondeposition or an interval of very low rate of accumulation is formed distally. (4) During the low-discharge interval, onlap proceeds together with the progradation over the nondeposition surface (a downlap surface) of the sediment wedge formed at the basin margin. These behaviors were observed in both runs, but the timing, degree of progradation and retrogradation, and the relief of stratal geometries differ slightly; however, the higher rate of volume growth (more accommodation space) led to less proximal erosion and to better downlap development. Application of the model is demonstrated with a natural example in a Neogene basin showing well-defined truncations in seismic lines at the unfaulted margin, and major proximal incisions in the active part of the basin. Downlap stratal geometry occurs as predicted by the model. Nondepositional surfaces or intervals deposited at low rates were located close to where the analog model predicted. A method for correlating these sequences is proposed. Using the global water cycle as a link, times of maximum alluvial incision (due to the cycle of peak discharge) can be correlated to the time of fastest eustatic rise and marine onlaps; however, when water is stored on the continents and not delivered to the oceans, hydrological deficits might produce alluvial onlaps.

Application of Radio Echo Sounding at the arid Andes of Argentina: the Agua Negra Glacier

Abstract The results of ground-based Radio Echo Sounding (RES) of the Agua Negra Glacier of the arid Andes of Argentina are analyzed. The glacier (30°15′S, 69°50′W) extends from 5.200 to 4600 m in altitude, and presents a smooth and convex upward surface without evident crevasses. Most potential crevasses seem to be sealed by the high rate of melting–freezing due to extreme differences between positive (diurnal) and negative (night) energy peaks. Seismic methods suggest the existence of a thick unit of debris or altered rock at the base of the glacier. Higher than normal seismic velocities indicate a very compact ice system (ice+voids), very dense near the glacier terminus. An impulse transmitter of 12 MHz was used for the RES survey, with 4 m half-length antennas, and an antenna separation of 30 m. Three profiles were surveyed and isolated measurements were taken in addition, covering about a tenth of the total glacier surface. Very clear bottom echoes occurred in most cases, indicating a maximum ice thickness of 50 to 55 m and an ice volume beneath the surveyed area of 2.0×10 6 m 3 . The total glacier volume is estimated as 10 times higher, representing an important water resource for this region. It is also possible that an older ice body lies beneath the debris that forms the present glacier base. The multistorey structure of local glaciers may be an inheritance of past glacial cycles, modulating the evolution of a complex glaciolithic system. These preliminary results, suggest the RES method works well on local glaciers, provided one take into account problems such as debris saturation, low-scale relief producing scattered echoes and low ice thickness. Its application should contribute to studies of the water reserves in this largely glacier-dependent arid region.

The largest wind ripples on earth: REPLY

We thank [de Silva (2010)][1] for the opportunity to discuss the origin of the unique megaripples (MR) found on the Puna Plateau. Forman and Krohling were invited as co-authors of this Reply, because we produced new data supplied here. De Silva suggests that the genetic relationship inferred by [

The sieve lobe paradigm: Observations of active deposition

Sieve deposits were once considered to be one of the building blocks in alluvial fan stratigraphy. Later reinterpretation of sieve lobes as debris-flow deposits, favored because no visual records of active sieve deposition had been reported, undermined their significance, divided opinions, and left this issue unresolved. Here I document active deposition of sieve lobes in natural settings, in support of the original model. Sieve deposition can easily occur in natural settings such as proglacial outwash fans, small arid alluvial fans, or perennial streams when there is a scarcity of fine material, significant bedload, high slope, permeable ground, and discharges moderate enough to allow infiltration. The only hydrodynamic requirement for sieve deposition is a high rate of water loss promoted by permeable bed sediments. Under some circumstances alluvial fans can be built almost entirely of sieve deposits, as shown here. One effect of the rapid extraction of water is the creation of sigmoidal fan profiles. A gradation from sieve deposition to sheetflood occurs if sediment becomes progressively less permeable or if water flow increases, overcoming bed permeability. Sieve deposition is a universal depositional process based simply on infiltration, and it explains matrix-poor clast-supported gravels, while alternative hypotheses, such as matrix winnowing of debris flows used to dispute the sieve model, still need to be proven by observations in nature.

A model of the Glaciar Horcones Inferior surge, Aconcagua region, Argentina

The deformation, resulting from a surge in 1985, of Glaciar Horcones Inferior is analyzed using structural geological models. During the surge, previously continuous debris cover was deformed by the formation of regularly separated and rotated ice blocks, suggesting a system of linked rotational extensional faults. Block tilting was measured from photographs taken shortly after the surge, showing rotation of the debris-covered surface. Fault inclination was assumed to be coincident with the debris-free side of the block. Glacier advance during the surge was obtained by comparing pre-surge aerial photographs with the position of maximum advance after the surge. Glacier thinning was estimated from the debris surface average lowering (relief generated at lateral scarps coincident with shear zones) and ice thickness measurements after surge termination. Three independent sets of information, geometry of the deformation (i.e. depth of detachment, fault traces, fault spacing, block rotation), glacier thinning and net advance, limit possible interpretations. Surface geometry suggests a domino-style or a linked planar rotational extensional fault system. In the observed configuration, however, these models can only explain a 12-13% extension. Glacier thinning suggests 30% local extension, and total glacier advance implies 16% minimum extension, which does not account for some frontal compression, as observed. A linked curved rotational extensional fault model fits the data well, implying a significant degree of internal deformation within each block. This model satisfactorily explains the observed deformation produced by the surge. It may also explain some modes of fast glacier flow, since the observed style of block tilting is present in other glaciers with high relief.