Basin Research - Volume 14, Issue 1, 2002

ABSTRACT This paper investigates syn‐rift stratigraphic architecture and facies relationships along a 7 km long strike section towards the tip of a major, basin‐bounding normal fault segment (Thal Fault) in the Suez Rift, Egypt. In this location, the fault is composed of two precursor fault strands, Gushea and Abu Ideimat, linked by a jog or transfer fault. We document a Miocene syn‐rift succession, deposited more than c. 5.5 Myr after rift initiation, that is composed of a range of carbonate‐clastic facies associated with coarse‐grained deltaic, shoreface and offshore depositional systems. Key regionally correlatable stratal surfaces within this succession define time equivalent stratal units that exhibit variability in thickness and architecture, related to the interplay of both regional and local controls, in particular, the evolution of two, small‐scale (<6 km long) precursor fault strands (Gushea and Abu Ideimat). Integration of structural and stratigraphic data indicates that the boundary (relay ramp) between these two fault strands was a relative high during much of the rift event, with hard‐linkage and considerable displacement accumulation not occurring until at least c. 7.5 Myr after rift initiation. This is because: (i) the preserved stratigraphy is thinner in the hanging wall of the strand boundary; (ii) a eustatic sea‐level fall with an amplitude of 100 m generated more than 25 m of incision at the strand boundary, a region that has a final fault displacement of c. 600 m; and (iii) the fault strand boundary persisted as a footwall low and transport pathway for coarse‐grained deltas entering the basin. This study indicates that variability in stratal thickness and stratigraphic architecture towards the tip of the Thal Fault was related to the linkage history of two small‐scale (c. 6 km long) precursor fault segments. We suggest that similar, small‐scale stratal variability may occur repeatedly along the entire length of major basin‐bounding fault segments due to the process of fault growth by the linkage of smaller scale precursor strands.

ABSTRACT The Alkyonides half‐graben is separated from the Gerania Range to the south by active faults whose offshore traces are mapped in detail. The East Alkyonides and Psatha Faults have well‐defined, Holocene‐active tip zones and cannot be extrapolated from the onshore Skinos Fault into a single continuous surface trace. During the late Quaternary, catchments draining the step‐faulted range front have supplied sediment to alluvial fans along a subsiding marine ramp margin in the hangingwall of the Skinos Fault, to shelf ledge fans on the uplifting footwall to the East Alkyonides Fault and to the Alepochori submarine fan in the hangingwall of the latter. During late Pleistocene lowstand times (c. 70–12 ka), sediment was deposited in Lake Corinth as fan deltas on the subsiding Skinos shelf ramp which acted as a sediment trap for the adjacent 360 m deep submarine basin plain. At the same time, the uplifting eastern shelf ledge was exposed, eroded and bypassed in favour of deposition on the Alepochori submarine fan. During Holocene times, the Skinos bajada was first the site of stability and soil formation, and then of substantial deposition before modern marine erosion cut a prominent cliffline. The uplifting eastern shelf ledge has developed substantial Holocene fan lobe depositional sequences as sediment‐laden underflows have traversed it via outlet channels. We estimate mean Holocene displacement rates towards the tip of the Psatha Fault in the range 0.7–0.8 mm year⁻¹. Raised Holocene coastal notches indicate that this may be further partitioned into about 0.2 mm year⁻¹ of footwall uplift and hence 0.5–0.6 mm year⁻¹ of hangingwall subsidence. Holocene displacement rates towards the tip of the active East Alkyonides Fault are in the range 0.2–0.3 mm year⁻¹. Any uplift of the West Alkyonides Fault footwall is not keeping pace with subsidence of the Skinos Fault hangingwall, as revealed by lowstand shelf fan deltas which show internal clinoforms indicative of aggradational deposition in response to relative base‐level rise due to active hangingwall subsidence along the Skinos Fault. Total subsidence here during the last 58 kyr lowstand interval of Lake Corinth was some 20 m, indicating a reduced net displacement rate compared to estimates of late Holocene (< 2000 bp) activity from onshore palaeoseismology. This discrepancy may be due to the competition between uplift on the West Alkyonides Fault and subsidence on the onshore Skinos Fault, or may reflect unsteady rates of Skinos Fault displacement over tens of thousands of years.

ABSTRACT This study combines stratigraphic evidence with geodynamic modelling to demonstrate that a forebulge played an identifiable role in Cenomanian–Turonian erosion and sediment accumulation in the North American Western Interior basin. The early to middle Turonian forebulge migrated progressively eastwards, and by the upper middle Turonian acted as a ‘backstop’ against which barrier islands formed in the axial basin.

This paper focuses on the progressive migration of an unconformity on the forebulge. The lengthwise orogen‐parallel orientation and time‐transgressive orogen‐normal migration of the forebulge unconformity are characteristics that differentiate it from unconformities developed on reactivated basement structures. We present a conceptual model in which the unconformity formed as the seafloor was uplifted by forebulge‐related flexure to a water depth at which submarine bypass and erosion occurred. A numerical model that describes forebulge migration in response to load dispersal by erosion of the orogenic front and sedimentation into the foredeep indicates that the distance from the thrust front to the forebulge is within reasonable bounds established using a flexural rigidity of 3×10²⁴ Nm.

We identify architecturally similar, coeval unconformities from Montana to New Mexico, and interpret the similar distance from the thrust front to a point where each unconformity dissipates as indicative of a uniform lithospheric flexural response along the orogenic front. Here we ascribe cratonward (west‐to‐east) forebulge migration to erosional load redistribution, whereas orogen‐parallel (north–south) stratigraphic climb of the forebulge unconformities developed in response to depocentre migration. Inherited lithospheric inhomogeneities may have allowed the forebulge in central Colorado to crest farther from the orogen than to the north and south.

ABSTRACT The Pan‐African Gariep Belt in south‐western Namibia and westernmost South Africa provides an excellent window into the interplay between tectonic and climatic changes during the Neoproterozoic era. Recently acquired chemostratigraphic data from cap carbonate sequences above glaciogenic diamictite horizons, together with U–Pb zircon and Pb–Pb carbonate ages, indicate sedimentation in the Gariep Basin from continental break‐up around 770 Ma to basin closure and continent collision around 545 Ma. The basin is subdivided into an eastern failed rift graben and a western half graben that evolved into an oceanic basin between the Kalahari and the Rio de la Plata cratons. Three megasequences are distinguished in the external, para‐autochthonous part of the belt (Port Nolloth Zone): an early continental, predominantly siliciclastic, sag rift megasequence (M1), a passive continental margin, carbonate‐rich megasequence (M2), and a syn‐orogenic carbonate and flysch megasequence (M3). Two glaciogenic diamictite horizons at the end of M1 and M2 are recognized and they are correlated with the global ∼750 Ma Sturtian and ∼580 Ma Marinoan glaciations, respectively. While the former is restricted to proximal continental rift shoulders, the latter extends into the oceanic realm which marks the internal part of the belt (Marmora Terrane). Only the younger diamictite is associated with iron formation. The sequence of regressive and transgressive stages recorded by the sediment fill does not reflect simply the tectonic evolution from rifting to drifting and eventual basin closure, but is strongly controlled by severe climatically induced sea‐level changes that were either competing with or reinforcing tectonically induced sea‐level changes.

ABSTRACT Inter‐ and intrabasinal correlation of Neoproterozoic carbonate successions and associated glaciogenic deposits from the Pan‐African Gariep Belt and the Kango inlier in the Saldania Belt (Namibia and South Africa) is proposed on the basis of new δ¹³CCarb and ⁸⁷Sr/⁸⁶SrI data. Highly positive δ¹³CCarb values (as much as + 8.65‰) and low ⁸⁷Sr/⁸⁶SrI ratios (0.7071–0.7077) were obtained on carbonate successions of the Hilda Subgroup between an older and a younger diamictite in the Port Nolloth Group (Gariep Belt). These results are in agreement with data elsewhere that suggest deposition between the global ∼ 750 Ma Sturtian and ∼ 580 Ma Marinoan glaciations. Considerably lower, positive δ¹³CCarb values (up to + 1.01‰) and higher ⁸⁷Sr/⁸⁶SrI ratios (0.7082–0.7085) mark the carbonates (Bloeddrif Member, Holgat Formation) on top of the Numees Formation diamictite and support a correlation of this diamictite with the Marinoan glaciation.

In the southern extension of the Gariep Belt, correlation of the Widouw Formation limestone (Gifberg Group) with the Bloeddrif Member carbonates is proposed based on similar isotopic characteristics. In the Kango inlier of the Saldania Belt, two carbonate‐bearing members exist in the Matjies River Formation. The lower one of these two (Nooitgedacht Member) compares well with the Hilda Subgroup, whereas the upper one (Kombuis Member) shows strong similarities to the Bloeddrif Member carbonates. This implies that all younger stratigraphic units of the Kango inlier are not correlatives of the Port Nolloth Group, as previously assumed, but syn‐ to post‐orogenic with respect to the 540–580 Ma Pan‐African orogeny.