William & Mary geologists Lauren Visokay ’18, Moussa Dia ’18, and Cece Hurtado ’17 enjoying the beach at Qantab, Oman.
Qantab is a village at the eastern edge of the Muscat metropolitan area, it’s hemmed in by steep rocky hills, and flanked by a broad strand that faces out to the Gulf of Oman. It is one of my favorite spots in Oman, and I recently visited Qantab with my research students on a Spring Break trip to study the Semail ophiolite. After nearly a week of field work in the mountains, our plan for Qantab involved relaxing on the beach and playing in the surf. However, in Oman, geology is all around and even while bobbing in the surf it is hard not to appreciate the spectacular scenery onshore.
The village of Qantab, Oman with the waters of the Gulf of Oman visible in the distance. The craggy and steep hills are underlain by Cretaceous ophiolite.
Just up the road, about a kilometer from Qantab’s beach is a geological triple point; it’s a spot where three of Oman’s major geologic units meet. But just what is a geological triple point?
Simplified bedrock geologic map of the area near Qantab, Oman illustrating a geological triple point. Image base from Google Earth, geology based on field observations and modified from Le Metour et al. (1986).
To appreciate the significance of a geological triple point, we must first understand the concept of a geologic contact. Simply put the world is composed of rocks, with many flavors of igneous, sedimentary, and metamorphic rocks: the boundary between different rock types is a geologic contact.
There are 4 major types of geologic contacts:
- Stratigraphic – a contact between sedimentary layers in which there has been continuous deposition.
- Igneous – a contact between an igneous rock and the rock into (or onto) which it intrudes or extrudes.
- Tectonic – a contact in which the rocks have been juxtaposed together along faults or shear zones.
- Erosional – a contact in which older rocks were eroded prior to the formation of the younger rocks, the contact is an unconformity.
Types of geologic contacts: Stratigraphic contacts from Goblin Valley, Utah. Igneous contacts from Capitol Reef National Park, Utah. Tectonic contact from Cedar Canyon, Utah. Erosional contact from near Salina, Utah. Click on the image for a bigger view.
Learning to recognize different geologic contacts in the field is a basic, but challenging, skill. On a map, a geologic contact appears as a line, but it is important to realize that geologic contacts are three-dimensional surfaces.
South of Qantab, three geologic units are in contact at one location and form a geological triple point. Geological triple points are significant because much can be inferred about the geologic history of a region at these special locations.
The geologic units at this triple point include: 1) Triassic carbonate rocks, these sedimentary rocks were deposited in a shallow tropical sea ~230 million years ago – today, they underlie a precipitous ridge with the strata tilted between 40˚ and 50˚ towards the north, 2) Cretaceous ophiolite, these dense ultramafic rocks were once part of the Earth’s mantle, they were later emplaced onto the Arabian continental margin – today these rocks underlie steep and craggy brown hills, 3) Paleogene sedimentary rocks that includes a sequence of tawny-colored conglomerate, sandstone, and limestone, these rocks were deposited between 55 to 35 million years ago and underlie broad hills and rounded valleys.
Let’s first examine the contact between the ophiolite and the Paleogene sedimentary rocks, it’s well-exposed in a road cut south of Qantab. Notice the ophiolite is exposed below the overlying sedimentary rocks. Just above contact there are clasts of the underlying ophiolite in the conglomerate, indicating that the contact between the ophiolite and the sedimentary rocks is erosional.
Unconformable contact between the Cretaceous ophiolite and overlying Paleogene strata exposed in roadcut to the south of Qantab. The Paleogene conglomerates contain clasts of the ophiolite.
Let’s frame up the geologic history at this spot – the ophiolite formed, was emplaced and uplifted, later it was eroded and exposed only to be buried by layers of coarse gravel and sand during the Paleogene. In recent times, erosion has sculpted the landscape and the ophiolite is exposed once again.
The unconformity between the ophiolite and overlying strata continues to the south until it is truncated at the contact with the Triassic carbonates – so the unconformity must be the older of the two contacts.
But what type of contact separates the Triassic rocks from the other units? At places this contact involves ophiolite sitting atop the Triassic strata, and at other places it is the Paleogene strata atop the Triassic strata—it’s one continuous contact. Both the igneous layering in the ophiolite and bedding in the Paleogene strata are truncated at this contact. These observations are consistent with a tectonic (fault) contact.
Cross section of the geological triple point near Qantab, Oman based on surface observations.
The Qantab area, and Oman in general, are exceptional places to see and study geology. No doubt we’ll return to this spot on next year’s Rock Music Oman study abroad program. I’m going to end with a few questions, and I look forward to some juicy discussion regarding these questions.
- What type of fault is exposed near Qantab?
- When did faulting occur in the Qantab area?
- On the geologic map the Paleogene strata dip between 15˚ and 33˚, why do they appear to dip less steeply on the cross section?
- During a recent lab mapping exercise in my 2017 Earth Structure & Dynamics course, many students drew geological quadruple points! Should geological quadruple points be common? What’s the 3D geometry of a geological quadruple point? How might a geological quadruple point form?