Modern to Ancient Barrier Island Dimensional Comparisons: Implications for Analog Selection and Paleomorphodynamics

Julia S Mulhern,John M Martin,Cari L Johnson

doi:10.3389/feart.2019.00109

Julia S Mulhern, John M Martin + Show 1 more

Open Access

https://doi.org/10.3389/feart.2019.00109

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Abstract

Ancient barrier islands are poorly understood relative to other clastic depositional environments, despite being prominent features along modern coastlines and important for understanding transgressive shoreline deposits. A new dataset of ancient barrier island dimensions (n=83 examples) addresses this knowledge gap with a quantitative analysis of barrier island sand body dimensions including thickness (vertical), length (shore-parallel direction), and width (shore-perpendicular direction). This dataset of barrier island deposits was compared to planform measurements made for modern islands (n=274), to investigate possible scaling relationships and other aspects of modern to ancient linkages. These measurements are nuanced and challenging to perform, and first-pass comparisons show that modern barrier islands should not be used as direct analogs for ancient systems. Nevertheless, results emphasize key depositional and preservation processes, and the dimensional differences between deposits formed over geologic versus modern time scales. Using the methods outlined herein, barrier island deposits appear to be 2-5x longer (p50 modern = 10.7 km; p50 ancient = 20.0 km), and 6-15x wider (p50 modern = 1.2 km; p50 ancient = 7.3 km) than modern barrier islands. We interpret the results to indicate that ancient barrier islands are time-transgressive deposits recording vertical amalgamation, and barrier island growth by lateral accretion, and progradation. When comparing single barrier islands, thickness measurements do not vary systemically between modern and ancient examples, suggesting that local accommodation dictates barrier island thickness as a preservation control. Gross length, width, and thickness measurements are too coarse for robust paleomorphodynamic calculations, therefore more detailed sub-environment analysis (e.g., upper shoreface delineation), with improved facies models, is required before rigorous quantifications can be generated. However, these initial comparisons do show scaling trends between length and width which could be leveraged, with caution, in the interim. As sea levels continue to rise, understanding barrier island motion and preservation will be central to predicting coastal change. Keywords: paleomorphodynamics, barrier island, scaling relationships, accommodation, shallow marine, dimension prediction, modern analog, reservoir, transgressive

Highlights

Barrier islands are elongate coastal sand bodies which comprise 10% of the world’s coastlines (Hoyt, 1967; Oertel, 1985; Stutz and Pilkey, 2011)
Modern analog dimensions should not be directly applied to ancient interpretations and predictions, and caution should be used when comparing between ancient examples
First-order comparison of modern and ancient barrier island dimensions shows that ancient barrier island deposits are wider and longer than modern barrier islands, recording lateral and shoreperpendicular motion through time

Summary

Introduction

Barrier islands are elongate coastal sand bodies which comprise 10% of the world’s coastlines (Hoyt, 1967; Oertel, 1985; Stutz and Pilkey, 2011). Well-known examples from the U.S Atlantic and Gulf coasts as well as the Dutch Wadden Sea form the basis of facies models that are used to interpret ancient barrier island deposits (Davies, 1978; Barwis and Hayes, 1979; Reinson, 1979; McCubbin, 1982). While barrier island deposits have been interpreted for over 80 years, their dimensions have not been rigorously quantified The dimensions of preserved barrier island deposits lend insight into transgressive processes on siliciclastic coastlines (McCubbin, 1982; Cooper et al, 2018a; Jones et al, 2018). Barrier islands commonly form the thickest sandstone units within a transgressive succession, quantifying the range of preserved barrier island dimensions could improve predictions for subsurface hydrocarbon reservoirs (Davies et al, 1971; Reinson, 1992; Reynolds, 1999). Barrier islands will become an increasingly important hydrocarbon reservoir type as exploration expands beyond regressive sequences (Hampson et al, 2004)

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