Holocene Sedimentary Record and Coastal Evolution in the Makran Subduction Zone (Iran)

Abstract

The western Makran coast displays evidence of surface uplift since at least the Late Pleistocene, but it remains uncertain whether this displacement is accommodated by creep on the subduction interface, or in a series of large earthquakes. Here, we address this problem by looking at the short-term (Holocene) history of continental vertical displacements recorded in the geomorphology and sedimentary succession of the Makran beaches. In the region of Chabahar (Southern Iran), we study two bay-beaches through the description, measurement and dating of 13 sedimentary sections with a combination of radiocarbon and Optically Stimulated Luminescence (OSL) dating. Our results show that lagoonal settings dominate the early Holocene of both studied beach sections. A flooding surface associated with the Holocene maximum transgression is followed by a prograding sequence of tidal and beach deposits. Coastal progradation is evidenced in Pozm Bay, where we observe a rapid buildup of the beach ridge succession (3.5 m/years lateral propagation over the last 1950 years). Dating of Beris Beach revealed high rates of uplift, comparable to the rates obtained from the nearby Late Pleistocene marine terraces. A 3150-year-old flooding surface within the sedimentary succession of Chabahar Bay was possibly caused by rapid subsidence during an earthquake. If true, this might indicate that the Western Makran does produce large earthquakes, similar to those that have occurred further east in the Pakistani Makran.

Sea Level Change Controlled the Sedimentary Processes at the Makran Continental Margin Over the Past 13,000 yr

“…Classical models of sequence stratigraphy and more recent 3‐D seismic reflection observations of stratigraphic successions in marine environments suggest a systematic development of deep‐water sequences mainly related to relative sea level fluctuations (Posamentier & Kolla, 2003). The early Holocene was a period of rapid sea level rise that far exceeded the average uplifting rate of the Holocene, and the sea level reached the maximum transgression at approximately 6 kyr (Lambeck, 1996; Normand, Simpson, Herman, et al, 2019). However, after reaching the maximum transgression, the uplift of the coast of Makran caused the decline in the relative sea level of the Makran continental margin in the late Holocene.…”

“…High‐energy waves continue to erode the Makran coasts (Saket & Etemad‐Shahidi, 2012), and the differential erosion of resistant sandstone and soft marl is a key factor in shaping the current coastal geomorphology (Normand, Simpson, & Bahroudi, 2019). Wave action erodes faster through soft marl bedrock than through indurated sandstones, which causes the coastline to develop into deep bays and protruding headlands, which are dominated by more resistant calcareous sandstones (Normand, Simpson, Herman, et al, 2019).…”

“…After comparing their results with meteorological records and satellite and sediment trap data from the coast in 1993 and 1998, Luckge et al (2002) suggested that short‐term (few days) heavy rainfall brought large amounts of sediments to the eastern Makran continental margin, which deposited as centimeter‐thick layers on the continental slope. In addition, more small seasonal rivers in central Makran are observed to mostly drain the fine‐grained bedrock of the coastal plain (Normand, Simpson, Herman, et al, 2019), and the Sarbaz River system covers an area of ca. 20,711 km 2 and flows southward for ~313 km in the onshore Makran area (Zoleikhaei et al, 2016).…”

“…In addition, large numbers of river systems are irregularly distributed along the Makran coast ( Figure 1). These coastal rivers are dry most of the year under the arid to semiarid climate condition but activate during heavy rain episodes when flash flood events inundate the coastal plain and bring large amounts of sediments to the sea (Normand, Simpson, Herman, et al, 2019). At present, the amount of rainfall is highest during the winter monsoon (from December to March) in the Makran region, and between 1956 and 1997, the precipitation records of the Jiwani meteorological stations indicated that the average summer precipitation was 10 mm/yr and the average winter rainfall was 90 mm/yr (Luckge et al, 2001).…”

“…In other words, the sea level flooded the flat wide coastal plain of the Makran coast (approximately 20 km) in the early Holocene. A few outcrops in the coastal plain reveal the nature of the underlying bedrock, which consists predominantly of Upper Miocene gray marls, and these rocks are often deeply eroded to form badland topography (Normand, Simpson, Herman, et al, 2019). There is no doubt that the erosion of marl by waves and tides can provide a large amount of fine-grained sediments to the Makran continental margin.…”