Michener's sweeping South African saga; The Covenant, by James A. Michener. New York: Random House Inc. $15.95.

In 1835 an event of such cataclysmic importance occurred in South Africa that it has reverberated throughout that racially troubled land ever since, giving rise to a new term in the English language: the Great Trek.

A tough, God-fearing, exclusively Bible-reading people fled English domination by ox wagon. Like the Israelites, they took refuge in the wilderness and entered into a covenant with God as His "chosen people." It was a covenant so exclusive that in seeking their own freedom the Boers, or Afrikaners, denied it to others.

In his latest work, "The Covenant," James Michener reveals how the seeds of discord sown by this religious exclusivity have today grown into plantations of racial trouble.

A prodigious author of prodigiously long novels ("Hawaii," "Centennial," "The Source") Michener's "Covenant" is yet another blockbuster -- a near 900-page novel on South Africa that encompasses all of South African history and spans a period of 500 years. It is this vast narrative sweep of almost epic proportions which helps explain why the Great Trek culminated in the tumultuous headon collision between the northward advancing Boers (the latter-day Israelites) and the southward penetrating Africans (the Canaanites), and in the Boer War at the turn of the century, when white fought white.

For plot and historic continuity Michener has conjured up three distinct but imaginary families and mingled them with the outstanding historic figures of their times. They are the Nxumalos, the Van Doorns, and the Saltwoods, representing respectively the African, Afrikaans, and English streams flowing into South Africa's muddy social and political river.

Michener follows their experiences generation upon generation through the Trek, the African border wars, the Boer War, the founding of the South African state, and the consolidation of white rule on Calvinist, racially purified principles.

Over several hundred years their descendants make contact, and thrive through the contact, only to become adversaries as contact subsequently gives way to conflict.

By ironic twists, Detleef Van Doorn, whose earliest South African ancestor married a Malay slave, burrows his way into the civil service and ultimately surfaces as leading architect of a battery of Afrikaner racist laws that ensures white survival and domination. Prof. Daniel Nxumalo, whose forefathers provide loyal companionship and domestic service to generations of Van Doorns, becomes ensnared in the Van Doorn-devised system and is imprisoned for opposing it. Laura Saltwood, the liberal Englishwoman, whose antecedents fought shoulder to shoulder with the Van Doorns to hold off advancing Xhosa tribesmen, is politically banned for trying to alleviate the hardships of apartheid victims.

While Michener's prose is frequently as flat and colorless as the veld, his research is impressive. In fact he overdoes it -- great dollops of history that tend to clog the flow of the story. Yet it is his surefooted grasp of what South Africa is and has been that gives this book extraordinary authority and conviction.

"The Covenant" also has a scope to it that avoids the too often one-dimensional view of South Africa. Olive Schreiner, Laurens Van der Post, and Alan Paton have taken up the cudgels for the Afrikaner, the Bushman, and the African, respectively. What Michener has set out to do is to embrace all South Africa's people and follow them through all time, and do it without favor to any one particular group. For the whites, in particular -- both English and Afrikaans, who constantly complain the outside world misunderstands them -- Michener has brought understanding. At times almost a fondness.

It is precisely because Michener has been so scrupulously evenhanded throughout this book that his dismay in its conclusion at the excesses of apartheid, of an ideology gone beserk, is so chillingly credible.