A new macrofaunal limit in the deep biosphere revealed by extreme burrow depths in ancient sediments

Macrofauna is known to inhabit the top few 10s cm of marine sediments, with rare burrows up to two metres below the seabed. Here, we provide evidence from deep-water Permian strata for a previously unrecognised habitat up to at least 8 metres below the sediment-water interface. Infaunal organisms exploited networks of forcibly injected sand below the seabed, forming living traces and reworking sediment. This is the first record that shows sediment injections are responsible for hosting macrofaunal life metres below the contemporaneous seabed. In addition, given the widespread occurrence of thick sandy successions that accumulate in deep-water settings, macrofauna living in the deep biosphere are likely much more prevalent than considered previously. These findings should influence future sampling strategies to better constrain the depth range of infaunal animals living in modern deep-sea sands. One Sentence Summary: The living depth of infaunal macrofauna is shown to reach at least 8 metres in new habitats associated with sand injections

Slope gradient and lithology as controls on the initiation of submarine slope gullies; Insights from the North Carnarvon Basin, Offshore NW Australia

Slope-confined submarine gullies are present on many continental margins, yet the controls on their initiation and demise are poorly understood because modern or recently active systems are rarely if ever monitored, and exhumed systems, typically formed in very fine-grained successions, are poorly preserved at outcrop. We use 3D seismic reflection and borehole data from offshore NW Australia to investigate long-term (~ 40 Myr) variations in the geomorphology of Eocene-to-Miocene gullies that developed in mixed carbonate–clastic clinothems. Through time, clinoform slope gradient increases from 1.6° to 3.2°, with gullies forming when the clinoform slope exceeds 2.5°. After their inception, gullies increase in width (from 350 m to 770 m) and depth (from 37 m to 60 m). Slope steepening appears to coincide with a change from poorly cemented, fine-grained carbonate to better-cemented, coarse-grained carbonate, implying a secondary, lithological control on slope dip and, ultimately, gully formation

Confined to unconfined: Anatomy of a base of slope succession, Karoo Basin, South Africa

Two contemporaneous weakly confined deepwater systems form Unit B of the Permian Laingsburg Formation are sufficiently well-exposed to allow investigation of the down dip passage from channelized base of slope to distributive deposits over a 25 kmdip section, with strike control over some 20 km. A high resolution stratigraphy was established over a 1200 m strike section in the proximal Skeiding locality and extended regionally at a coarser scale. Analysis indicates that Unit B comprises 3 depositional sequences which, at a regional scale, thin towards the N and E. The lowstand systems tract of sequence 1 comprises weakly confined high-aspect ratio, vertically stacked channels cut into a basal frontal lobe system, and are overlain by a regionally correlated condensed hemipelagic mudstone interval (combined TST/HST) that shows evidence of remobilisation in up-dip areas The LST of sequence 2 includes two superimposed channel complexes of different styles that become deeply entrenched 5 km down dip and pass basinward into tabular, distributive lobe sandstones. Sequence 3 is marked by a regional 100 m thick levee complex related to a lower slope channel system and marks a basinward shift in facies. Channel fills in the lowstand systems tracts of all 3 sequences commonly include a thin drape of mudstone-clast conglomerate over the basal erosion surface, onlapped by thin-bedded sandstones with tractional structures that exhibit a characteristic axis to off-axis transition in facies. These deposits accumulated during periods of sediment bypass and are overlain by amalgamated structureless sandstones which thin from the axis into characteristic wings that extend laterally up to 200 m. Levees are absent in the lower two sequences and flows appear to have been only weakly confined by basal erosional keels. The resultant succession is extremely sandstone rich (90%), a much higher percentage than in underlying basin floor fan and overlying slope channel-levee complexes. The levee deposits of sequence 3 are much less sandy (30%), consistent with a more proximal, slope setting. In the 1400 m thick Karoo deepwater succession the base of slope channel sandstones of Unit B represent the maximum sandstone content and connectivity, which has predictive implications for hydrocarbon reservoir development in weakly confined deepwater channel systems

Depositional architecture and sequence stratigraphy of the Karoo basin floor to shelf edge succession, Laingsburg depocentre, South Africa

The Laingsburg depocentre of the SW Karoo Basin, South Africa preserves a well-exposed 1200 m thick succession of upper Permian strata that record the early filling of a basin during an icehouse climate. Uniformly fine-grained sandstones were derived from far-field granitic sources, possibly in Patagonia, although the coeval staging and delivery systems are not preserved. Early condensed shallow marine deposits are overlain by distal basin plain siltstone-prone turbidites and volcanic ashes. An order of magnitude increase in siliciclastic input to the basin plain is represented by up to 270 m of siltstone with thin sandstone turbidites (Vischkuil Formation). The upper Vischkuil Formation comprises three depositional sequences, each bounded by a regionally developed zone of soft sediment deformation and associated 20-45 m thick debrite that represent the initiation of a major sand delivery system. The overlying 300 m thick sandy basin-floor fan system (Unit A) is divisible into three composite sequences arranged in a progradational-aggradational-retrogradational stacking pattern, followed by up to 40 m of basin-wide hemipelagic claystone. This claystone contains Interfan A/B, a distributive lobe system that lies 10 m beneath Unit B, a sandstone-dominated succession that averages 150 m thickness and is interpreted to represent a toe of slope channelized lobe system. Unit B and the A/B interfan together comprise 4 depositional sequences in a composite sequence with an overall basinward-stepping stacking pattern, overlain by 30 m of hemipelagic claystone. The overlying 400 m thick submarine slope succession (Fort Brown Formation) is characterized by 10-120 m thick sand-prone to heterolithic packages separated by 30-70 m thick claystone units. On the largest scale the slope stratigraphy is defined by two major cycles interpreted as composite sequence sets. The lower cycle comprises lithostratigraphic Units B/C, C and D while the upper cycle includes lithostratigraphic Units D/E, E and F. In each case a sandy basal composite sequence is represented by an intraslope lobe (Units B/C and D/E respectively). The second composite sequence in each cycle (Units C and E respectively) is characterized by slope channel-levee systems with distributive lobes 20-30 km down dip. The uppermost composite sequence in each cycle (Units D and F respectively) are characterised by deeply entrenched slope valley systems. Most composite sequences comprise three sequences separated by thin (<5 m thick) claystones. Architectural style is similar at individual sequence scale for comparable positions within each composite sequence set and each composite sequence. The main control on stratigraphic development is interpreted as late icehouse glacio-eustasy but along-strike changes associated with changing shelf edge delivery systems and variable bathymetry due to differential substrate compaction complicate the resultant stratigraphy