159-6 An Indirect Indicator of Deep-seated Hypogene Karst: Sag Syncline Networks with Patterns that Reflect the Geometries of Fault and Fracture Conduits for Hypogene Fluids in Underlying Bedrock, Western Desert, Egypt

Session: Hypogene Speleogenesis: An Overview of Research in Memory of Dr. Alexander Klimchouk

Presenting Author:

Authors:

Abstract:

Where hypogene karst is too deep-seated to have explorable hypogene caves, indirect indicators become critically important for recognizing hypogene speleogenesis at depth. Exposures of Eocene limestone in the Western Desert of Egypt offer strong indirect evidence for hypogene karst at depth, likely generated by ascending aggressive fluids associated with crustal extension and magmatism in Egypt during Red Sea rift initiation at the Oligocene-Miocene boundary.

Over an area of ~8000 km², we have mapped an extensive network of narrow non-tectonic synclines that formed as 200-400 meter-wide downwarps in otherwise flat-lying limestone. Limb dips are shallow, and doubly plunging hinges form multiple basin closures along syncline lengths. Isolated circular to elliptical structural basins are also common. Synclines and basins are all at the same scale, with neither larger structures nor parasitic structures, and are best described as non-tectonic sag synclines. In many areas, flat-topped ridges 20-40 m wide and 2-5 m high also occur along trend between and at the ends of synclines and as networks with the same patterns as nearby synclines. Bedding in the ridges is horizontal, and limestone within the ridges contains thin calcite veins and displays a distinctive honeycomb structure. Synclines and ridges of different orientations branch and merge along their lengths, forming a network, but the patterns are not random.

The network displays two distinct patterns in different areas: 1) synclines and ridges that outline polygons 700-2000 m across and 2) narrow N-S zones of synclines spaced 5-10 km apart and connected by WNW-ESE to NW-SE trending shallow synclines and ridges that traverse the panels between the N-S zones in a “Lazy S” pattern. We suggest these two patterns reflect the geometries of faults and fractures in rocks beneath the limestones: 1) polygonal faults in underlying shales and 2) reactivated N–S, left-lateral basement faults that are largely blind at the current level of erosion. These served as conduits to convey fluids upward into the limestones, triggering dissolution at depth, an inherited pattern of sag above, and ridges with veins and honeycomb structure where deep dissolution was not extensive enough to cause sag. The unique patterns and characteristics of this network of synclines are applicable elsewhere as an indirect indicator of deep-seated hypogene karst (doi.org/10.3389/feart.2021.678565).

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7066

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.