From the Field: Hudson Bay’s Ancient Treasures

Posted: September 13, 2011 - 16:07 , by David Rudkin

July 25

The Arctic high pressure system that has brought such an improvement in the weather is still with us, heralding perfect conditions for a trek to the most spectacular stretch of geology along this entire coastline! Today we’re heading down to what my colleague, Graham Young, has called the “Ancient Shore” (the name he has also conferred on his erudite, evocative, and always entertaining blog: http://ancientshore.com/about/). Here, through a serendipitous series of natural processes, the rocky shoreline of an Ordovician island is magnificently exposed along the rugged coast of Hudson Bay. It’s a stunning juxtaposition – a modern shore upon an ancient one – and fossils preserved in Upper Ordovician rocks here afford us unparalleled opportunities to study the diversity and ecology of life in a shallow coastal sea 445 million years ago. This is the place that first drew us to Churchill 15 years ago, and to which we eagerly return as often as possible.

Hiking over a ridge, the rocky ground is covered in bright green lowlying plants.

The crew hikes in across the prominent ridge of Proterozoic (2500 million-year-old) quartzites that formed the backbone of an island archipelago in Late Ordovician time, about 445 million years ago.

A view out over the water with the dark rock shorline in the foreground.

Cresting the ridge brings the subarctic waters of modern Hudson Bay into view. Almost half-a-billion years earlier the vista may have looked much the same, had any complex land-based animals been around to see it! But the Ordovician world was very different biologically, geographically, and in many other ways. The ancient continent of Laurentia (which forms the core of what we call North America today) was rotated clockwise 90 degrees astride the equator, and the highest global sea levels ever recorded brought warm marine waters teeming with invertebrate animals and simple aquatic plants to the very centre of Laurentia.

The crew stands on ancient rock formations.

Graham (facing) briefs the troops. Behind him rise the cliffs of ancient quartzite that would have formed a bleak, rocky island in the Ordovician sea. He, Sean (left) and Debbie (middle) stand on buff-coloured Ordovician dolostones formed from sediments deposited just off-shore of the island. On the right, Matt steps onto a grey quartzite boulder that eroded from the cliff and tumbled into the sea during Ordovician time. It, like thousands of others here, was enveloped by the accumulating Ordovician sediments. Fossils in the dolostones record the life that flourished 445 million years ago in shallow, warm waters along this boulder-strewn island shore.

My own reason for re-visiting the site is to observe the effects of active shoreline erosion on some particularly interesting fossils – complex trackways made by large trilobites as they foraged on the shallow Ordovician sea floor. We’ve been documenting and analyzing these trace fossils since 1998, but surf, sand, and ice take a relentless toll on the relatively soft carbonate host rocks. Even the most prominent traces are being slowly ground away. In 2002 we made latex moulds of a few critically important examples – this year we find some of those traces have all but disappeared.

A close up of a shallow patern engraved in the rock surface.

A large trilobite trackway in Upper Ordovician dolostone – these faint trace fossils are exposed to intense erosion in the intertidal zone and will eventually disappear altogether.

Gray rockes piled along the shoreline, partially covered in algae.

Part of the rocky "ancient shore" showing the very tough Proterozoic quartzite boulders standing in relief amongst younger and softer Ordovician dolostones. At high tide, this whole section is covered by the shallow waters of Hudson Bay, mimicking the scene as it would have appeared 445 million years ago. We can even find hints of broad wave ripples in the Ordovician rocks ... just like those seen in modern sands that now mantle the outcrop in places.

Close-ups of shallow rock formations in ancient rock.

Left - A colonial coral fossil, one of many hundreds here that have been mapped and studied by Graham Young and his colleague Bob Elias (University of Manitoba) in a long-term project to document the diversity and distribution of corals along the ancient shore. Right - Part of a recently exposed head shield (cephalon) of the trilobite Isotelus rex. In 2003, we named and described this species based on a complete specimen found a few tens of metres away … at almost 70 cm long, it still stands as the world's largest known trilobite.

We continue our morning traverse of this extraordinary site along the low tide line, crossing the exposed Ordovician boulder field and then a lengthy stretch covered by loose sand and more recently eroded quartzite blocks and cobbles.

Crew walking along the waters edge on a sandy shoreline.

At the end of the embayment there is a broad, horizontal bench of quartzite that may well have been a wave-cut platform 445 million years ago – now it provides a brief rest stop before the long walk back, this time hugging the base of the exquisitely sculpted cliffs. Occasionally we run across narrow vertical fissures in the dense grey quartzite where remnant patches of fossiliferous Ordovician dolostone still shelter tenaciously.

Laying on the warm granite beach, the crew takes a break looking out over the water.

The crew pauses for a breather on the quartzite "bench" – veins of pale quartz fill secondary fracture systems.

Two close up shots of rock formations surronded by granite.

A pair of vertical fissures in the cliff face containing fossiliferous Ordovician sediments and rounded fragments of quartzite ... little outliers of more extensive deposits that have been eroded away.

We reluctantly wrap up our long hike along the “ancient shore” knowing full well that we will have to return in future years to continue the work here. The remainder of the day is taken up with a sweep of Cape Merry at the mouth of the Churchill River, continued sorting, labeling and wrapping of fossils, and a late evening drive to check a sampling site on the old rocket range south of the Churchill Northern Studies Centre. It’s been another very full day.

An manmade structure rising out of the landscape.

The "new" battery at Cape Merry (part of the Prince of Wales Fort National Historic Site), constructed by the Hudson’s Bay Company in 1747 and rebuilt from the original stone blocks just over 200 years later (1959-60).

The crew works at a long table inside the lab.

Sorting, labeling, and wrapping fossils in the old Churchill Northern Studies Centre lab facility (photo (c) Graham Young, The Manitoba Museum).

 

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