The intrinsic effect of shape on the retrogradation motif and timing of drowning of carbonate patch reef systems (Lower Frasnian, Bugle Gap, Canning Basin, Western Australia)

Abstract

The evolution and architecture of a set of retreating Lower Frasnian patch reef outcrops in the Canning Basin of Western Australia were evaluated, and their depositional and stratigraphic contacts spatially recorded using digital surveying tools. The geological data, together with high-resolution digital elevation models, were assembled in three-dimensional visualization and modelling software and subsequently used for building two-dimensional surface models and three-dimensional volumetric models. Numerical data on geometry and shape were extracted from these models and used to quantitatively assess the retrogradation motif of patch reef development. The development of the patch reefs comprises three stages. During stages 1 and 2, the patch reefs exhibited an overall retrogradational escarpment-type configuration displayed by, on average, 60°steep reef-margin walls that lacked the support of coeval slope deposits. The subdivision between stages 1 and 2 is based on minor backstepping reducing less than 10% of the platformtop area. The onset of stage 3 is recognized by stromatolite development fringing reef-margin walls. During stage 3 an aggrading accretionary reefmargin developed, comprising allochthonous and autochthonous slope deposits. Both types of slope deposit onlap the previous stages and are distributed unevenly with allochthonous slope deposits being noticeably absent around the smaller and more elongate patch reefs. The variation in distribution of slope sediment type can be explained by the amount, linked to platform size, of platform-top shedding. Small patch reefs were unable to fill the available accommodation adjacent to escarpments with allochthonous slope sediments and were thus encroached by autochthonous slope sediments. The variation, which cannot be explained by the size difference in the platform-top factory, has been related to the difference in perimeter length. For patch reefs with similar platform-top production areas, a more elongate patch reef inherits a longer perimeter and a proportionally smaller volume of allochthonous slope sediment per margin length will be transported to the flanks. Thus, the more elongate patch reef intrinsically contained more sites within which autochthonous slope sediments developed. Digital outcrop modelling and numerical evaluation of the evolution of the patch reefs revealed the major differences in retrogradation motif. The quantified variations in progressive decline of platform-top area with height were confirmed by hypothetical decline curves for ellipse-shaped carbonate systems for which aspect ratio (ratio between length and width) varied. This Sedimentology (2010) 57, 956-984 mathematical model demonstrates that the progressive decline of the production area is highly sensitive to shape and can be used to numerically assess and predict the relative timing of drowning, i.e. when the platform-top production area becomes nil, of retrogradational isolated carbonate platforms that are controlled by high accommodation. Wider implications can be surmised...

Seismic geomorphology and growth architecture of a Miocene barrier reef, Browse Basin, NW-Australia

“…Rather the effect could be a result of tidal streams. Adams and Hasler (2010) provided a theory, in which the aggradational potential of a reef platform is dependant on the ability to produce slope sediment. This ability is in turn, dependant on the ratio between the sediment-producing area and length of reef perimeter.…”

Sedimentary evolution and sequence stratigraphy of Ediacaran high‐grade phosphorite–dolomite–shale successions of the Bocaina Formation (Corumbá Group), Central Brazil: Implications for the Neoproterozoic phosphogenic event

“…Hanging, encrusting bioconstructions chaotically bound to diverse rock fragments and carbonate slumps observed in Fa4 are also compatible with a shallow downslope environment, because these chaotic olistolith boundstone associations are commonplace in modern (Playton et al ., 2010, and references therein; Duce et al ., 2016) and ancient forereef carbonate slopes (e.g. Adams & Hasler, 2010; Jiang et al ., 2011; Corkeron & Slezak, 2020), being formed by gravitational collapse and re‐sedimentation of shallow forereef fragments in downslope grooves and block complexes (cf. Playton et al ., 2010).…”

Shoal-water dynamics and coastal biozones in a sheltered-island setting: Upper Devonian Pillara Limestone (Western Australia)

“…The growth, shape, evolution and relation to syndepositional fractures in the main Frasnian reefs are increasingly better understood (George et al . ; Frost & Kerans ), as is the development and palaeoecology of lagoonal patch reefs (Adams & Hasler ).…”