Mason Dykstra

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.

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.

Deep-Water Tidal Sedimentology

Tides are well-documented in modern deep-water environment, especially around areas of topography on the seafloor such as ocean ridges and continental slopes. Recognition of deep-water tidal deposits in the ancient has lagged far behind, however, with very few examples in the published literature. This paper presents a review of the current state of knowledge about both modern deep-water tidal sediments and ancient deep-water tidal deposits, including new data on tidalites from the Cretaceous Wheeler Gorge channel-levee complex (California), and the Cretaceous Cajiloa submarine canyon (Mexico). In both of these settings detailed analysis of laminae thickness trends revealed cyclicities with frequencies characteristic of tidal deposits. Recognition criteria for ancient deep-water tidal deposits include statistically significant cyclicities within thin successions (tidalites are unlikely to be very thick) in combination with mud-couplets, mud-bounded ripples, ripples with reactivation surfaces, and, more rarely, bi-directional ripple sets. Typical tidalites successions include cyclically thickening and thinning laminae (5–40 cm thick), and rippled intervals (5–20 cm thick) that exhibit large energy asymmetries (mud drapes) and an overall increase then decrease in ripple size, often arranged in cycles. Although tides are common in deep-water environments, settings where deposits may be preserved are relatively rare. Such settings must not be subject to erosive turbidity currents yet require a relatively steady sediment supply and local accommodation space. Abandoned meander bends, the backsides of levees, topographic lows on the surface of submarine landslides, and abandoned plunge pools all potentially fit this category. This paper documents a tidalite succession that is preserved within a topographic low above a submarine landslide deposit.

Ichnology of Late Cretaceous Turbidites from the Rosario Formation, Baja California, Mexico

The Late Cretaceous Canyon San Fernando channel-levee system in the Rosario Formation of Mexico contains a diverse assemblage of trace fossils that is fully documented for the first time. The aim of this study is to describe the trace fossils present and document their palaeoenvironmental range across a turbidite-channel-levee system, from the coarse-grained channel axes within the channel belt to the most distal parts of the external levee. Trace fossil distributions are strongly controlled by palaeoenvironments and physical and chemical conditions within the turbidite system. Large, deep-tier deposit-feeding macrofauna are most abundant in and close to the channel axes (i.e., organisms forming Ophiomorpha rudis and Scolicia), and smaller, deposit-feeding and gardening organisms are more abundant in more distal terrace and external levee environments. The distribution of shallow-tier ichnotaxa (e.g., graphoglyptids) is restricted to levee and terrace/overbank settings. It is not known whether this represents the true ecological range of the trace-making organisms, or the limit of the suitable taphonomic conditions required for their preservation. Further studies of this kind will help shed further light on the biological, physical, and chemical controls on trace fossil assemblages across a variety of turbidite depositional systems throughout the Phanerozoic. Integrated ichnological and palaeoenvironmental analysis of deep marine clastic sediments has an important potential role to play in the analysis and characterization of hydrocarbon reservoirs in the subsurface.